forks of the eccentric rod were of steel, and the length of the eccentric
rod was the distance between the centre of the crank axle and the centre of
the valve shaft; but in modern engines the use of the link motion is
universal. The valve lever in locomotives is usually longer than the
eccentric lever, to increase the travel of the valve, if levers are
employed; but it is better to connect the valve rod to the link of the link
motion without the intervention of levers. The pins of the eccentric lever
in the old engines used to wear quickly; Stephenson used to put a ferule of
brass on these pins, which being loose, and acting like a roller,
facilitated the throwing in and out of gear, and when worn could easily be
replaced, so that there was no material derangement of the motion of the
valve from play in this situation.
518. _Q._--What is the arrangement of a starting lever?
_A._--The starting lever travels between two iron segments, and can be
fixed in any desired position. This is done by a small catch or bell crank,
jointed to the bottom of the handle at the end of the lever, and coming up
by the side of the handle, but pressed out from it by a spring. The smaller
arm of this bell crank is jointed to a bolt, which shoots into notches,
made in one of the segments between which the lever moves. By pressing the
bell crank against the handle of the lever the bolt is withdrawn, and the
lever may be shifted to any other point, when, the spring being released,
the bolt flies into the nearest notch.
519. _Q._--In what way does the starting handle act on the machinery of the
engine to set it in motion?
_A._--Its whole action lies in raising or depressing the link of the link
motion relatively with the valve rod. If the valve rod be attached to the
middle of the link, the valve will derive no motion from, it at all, and
the engine will stop. If the attachment be slipped to one end of the link
the engine will go ahead, and if slipped to the other end it will go
astern. The starting handle merely achieves this change of position.
520. _Q._--Will you explain the operation of setting the valve of a
locomotive?
_A._--In setting the valves of locomotives, place the crank in the position
answerable to the end of the stroke of the piston, and draw a straight
line, representing the centre line of the cylinder, through the centres of
the crank shaft and crank pin. From the centre of the shaft describe a
circle with the diameter equal to the throw of the valve; another circle to
represent the crank shaft; and a third circle to represent the path of the
crank pin. From the centre of the crank shaft, draw a line perpendicular to
the centre line of the cylinder and crank shaft, and draw another
perpendicular at a distance from the first equal to the amount of the lap
and the lead of the valve: the points in which this line intersects the
circle of the eccentric are the points in which the centre of the eccentric
should be placed for the forward and reverse motions. When the eccentric
rod is attached directly to the valve, the radius of the eccentric, which
precedes the crank in its revolution, forms with the crank an obtuse angle;
but when, by the intervention of levers, the valve has a motion, opposed to
that of the eccentric rod, the angle contained by the crank and the radius
of the eccentric must be acute, and the eccentric must follow the crank: in
other words, with a direct attachment to the valve the eccentric is set
_more_ than one fourth of a revolution in advance of the crank, and with an
indirect attachment the eccentric is set _less_ than one fourth of a circle
behind the crank. If the valve were without lead or lap the eccentric would
be exactly one fourth of a circle in advance of the crank or behind the
crank, according to the nature of the valve connection; but as the valve
would thus cover the port by the amount of the lap and lead, the eccentric
must be set forward so as to open the port to the extent of the lap and
lead, and this is effected by the plan just described.
521. _Q._--In the event of the eccentrics slipping round upon the shaft,
which you stated sometimes happens, is it necessary to perform the
operation of setting the valve as you have just described it?
_A._--If the eccentrics shift upon the shaft, they may be easily refixed by
setting the valve open the amount of the lead, setting the crank at the end
of the stroke, and bringing round the eccentric upon the shaft till the
eccentric rod gears with the valve. It would often be troublesome in
practice to get access to the valve for the purpose of setting it, and this
may be dispensed with if the amount of lap on the valve and the length of
the eccentric rod be known. To this end draw upon a board two straight
lines at right angles to one another, and from their point of intersection
as a centre describe two circles, one representing the circle of the
eccentric, the other the crank shaft; draw a straight line parallel to one
of the diameters, and distant from it the amount of the lap and the lead:
the points in which his parallel intersects the circle of the eccentric are
the positions of the forward and backward eccentrics. Through these points
draw straight lines from the centre of the circle, and mark the
intersection of these lines with the circle of the crank shaft; measure
with a pair of compasses the chord of the arc intercepted between either of
these points, and the diameter which is at right angles with the crank, and
the diameters being first marked on the shaft itself, then by transferring
with the compasses the distance found in the diagram, and marking the
point, the eccentric may at any time be adjusted without difficulty.
[Illustration: Fig. 45.]
522. _Q._--Will you describe the structure and arrangement of the feed
pumps of locomotive engines?
_A._--The feed pumps of locomotives are generally made of brass, but the
plungers are sometimes made of iron, and are generally attached to the
piston, cross head, though in Stephenson's engines they are worked by rods
attached to eyes on the eccentric hoops. There is a ball valve, fig. 45,
between the pump and the tender, and two usually in the pipe leading from
the pump to the boiler, besides a cock close to the boiler, by which the
pump may be shut off from the boiler in case of any accident to the valves.
The ball valves are guided by four branches, which rise vertically, and
join together at the top in a hemispherical form. The shocks of the ball
against this cap have in some cases broken it after one week's work, from
the top of the cage having been flat, and the branches not having had their
junction at the top properly filleted. These valve guards are attached in
different ways to the pipes; when one occurs at the junction of two pieces
of pipe it has a flange, which along with the flanges of the pipes and that
of the valve seat are held together by a union joint. It is sometimes
formed with a thread at the under end, and screwed into the pipe. The balls
are cast hollow to lessen the shock, as well as to save the metal. In some
cases where the feed pump plunger has been attached to the cross head, the
piston rod has been bent by the strain; and that must in all cases occur,
if the communication between the pump and boiler be closed when the engine
is started, and there be no escape valve for the water.
523. _Q._--Are none but ball valves used in the feed pump?
_A._--Spindle valves have in some cases been used instead of ball valves,
but they are more subject to derangement; but piston valves, so contrived
as to shut a portion of water in the cage when about to close, might be
adopted with a great diminution of the shock. Slide valves might be
applied, and would probably be found preferable to any of the expedients at
present in use. In all spindle valves opened and shut rapidly, it is
advisable to have the lower surface conical, to take off the shock of the
water; and a large lift of the valve should be prevented, else much of the
water during the return stroke of the pump will flow out before the valve
shuts.
524. _Q._--At what part of the boiler is the feed water admitted?
_A._--The feed pipe of most locomotive engines enters the boiler near the
bottom and about the middle of its length. In Stephenson's engine the water
is let in at the smoke box end of the boiler, a little below the water
level; by this means the heat is more fully extracted from the escaping
smoke, but the arrangement is of questionable applicability to engines of
which the steam dome and steam pipe are at the smoke box end, as in that
case the entering cold water would condense the steam.
525. _Q._--How are the pipes connecting the tender and locomotive
constructed, so as to allow of play between the engine and tender without
leakage?
_A._--The pipes connecting the tender with the pumps should allow access to
the valves and free motion to the engine and tender. This end is attained
by the use of ball and socket joints; and, to allow some end play, one
piece of the pipe slides into the other like a telescope, and is kept tight
by means of a stuffing box. Any pipe joint between the engine and tender
must be made in this fashion.
526. _Q._--Have you any suggestion to make respecting the arrangement of
the feed pump?
_A._--It would be a material improvement if a feed pump was to be set in
the tender and worked by means of a small engine, such as that now used in
steam vessels for feeding the boilers. The present action of the feed pumps
of locomotives is precarious, as, if the valves leak in the slightest
degree, the steam or boiling water from the boiler will prevent the pumps
from drawing. It appears expedient, therefore, that at least one pump
should be far from the boiler and should be set among the feed water, so
that it will only have to force. If a pump was arranged in the manner
suggested, the boiler could still be fed regularly, though the locomotive
was standing still; but it would be prudent to have the existing pumps
still wrought in the usual way by the engine, in case of derangement of the
other, or in case the pump in the tender might freeze.
527. _Q._--Will you explain the construction of locomotive wheels?
_A._--The wheels of a locomotive are always made of malleable iron. The
driving wheels are made larger to increase the speed; the bearing wheels
also are easier on the road when large. In the goods engines the driving
wheels are smaller than in the passenger engines, and are generally coupled
together. Wheels are made with much variety in their constructive details:
sometimes they are made with cast iron naves, with the spokes and rim of
wrought iron; but in the best modern wheels the nave is formed of the ends
of the spokes welded together at the centre. When cast iron naves are
adopted, the spokes are forged out of flat bars with T-formed heads, and
are arranged radially in the founder's mould, the cast iron, when fluid,
being poured among them. The ends of the T heads are then welded together
to constitute the periphery of the wheel or inner tire; and little
wedge-form pieces are inserted where there is any deficiency of iron. In
some cases the arms are hollow, though of wrought iron; the tire of wrought
iron, and the nave of cast iron; and the spokes are turned where they are
fitted into the nave, and are secured in their sockets by means of cutters.
Hawthorn makes his wheels with cast iron naves and wrought iron rims and
arms; but instead of welding the arms together, he makes palms on their
outer end, which are attached by rivets to the rim. These rivets, however,
unless very carefully formed, are apt to work loose; and it would probably
be found an improvement if the palms were to be slightly indented into the
rim, in cases in which the palms do not meet each other at the ends. When
the rim is turned it is ready for the tire, which is now made of steel.
528. _Q._--How do you find the length of bar necessary for forming a tire?
_A._--To find the proper length of bar requisite for the formation of a
hoop of any given diameter, add the thickness of the bar to the required
diameter, and the corresponding circumference in the table of
circumferences of circles is the length of the bar. If the iron be bent
edgewise the breadth of the bar must be added to the diameter, for it is
the thickness of the bar measured radially that is to be taken into
consideration. In the tires of railway wheels, which have a flange on one
edge, it is necessary to add not only the thickness of the tire, but also
two thirds of the depth of the flange; generally, however, the tire bars
are sent from the forge so curved that the plain edge of the tire is
concave, and the flange edge convex, while the side which is afterward to
be bent into contact with the cylindrical surface of the wheel is a plane.
In this case the addition of the diameter of two thirds of the depth of the
flange is unnecessary, for the curving of the flange edge has the effect of
increasing the real length of the bar. When the tire is thus curved, it is
only necessary to add the thickness of the hoop to the diameter, and then
to find the circumference from a table; or the same result will be obtained
by multiplying the diameter thus increased by the thickness of the hoop by
3.1416.
529. _Q._--How are the tires attached to the wheels?
_A._--The materials for wheel tires are first swaged separately, and then
welded together under the heavy hammer at the steel works; after which they
are bent to the circle, welded, and turned to certain gauges. The tire is
now heated to redness in a circular furnace; during the time it is getting
hot, the iron wheel, turned to the right diameter, is bolted down upon a
face plate or surface; the tire expands with the heat, and when at a cherry
red, it is dropped over the wheel, for which it was previously too small,
and it is also hastily bolted down to the surface plate; the whole mass is
then quickly immersed by a swing crane in a tank of water five feet deep,
and hauled up and down till nearly cold; the tires are not afterward
tempered. The tire is attached to the rim with rivets having countersunk
heads, and the wheel is then fixed on its axle.
530. _Q._--Is it necessary to have the whole tire of steel?
_A._--It is not indispensable that the whole tire should be of steel; but a
dovetail groove, turned out of the tire at the place where it bears most on
the rail, and fitted with a band of steel, will suffice. This band may be
put in in pieces, and the expedient appears to be the best way of repairing
a worn tire; but particular care must be taken to attach these pieces very
securely to the tire by rivets, else in the rapid revolution of the wheel
the steel may be thrown out by the centrifugal force. In aid of such
attachment the steel, after being introduced, is well hammered, which
expands it sideways until it fills the dovetail groove.
531. _Q._--Is any arrangement adopted to facilitate the passage of the
locomotive round curves?
_A._--The tire is turned somewhat conical, to facilitate the passage of the
engine round curves--the diameter of the outer wheel being virtually
increased by the centrifugal force of the engine, and that of the inner
wheel being correspondingly diminished, whereby the curve is passed without
the resistance which would otherwise arise from the inequality of the
spaces passed over by wheels of the same diameter fixed upon the same axle.
The rails, moreover, are not set quite upright, but are slightly inclined
inward, in consequence of which the wheels must be either conical or
slightly dished, to bear fairly upon the rails. One benefit of inclining
the rails in this way, and coning the tires, is that the flange of the
wheels is less liable to bear against the sides of the rail, and with the
same view the flanges of all the wheels are made with large fillets in the
corners. Wheels have been placed loose upon the axle, but they have less
stability, and are not now much used. Nevertheless this plan appears to be
a good one if properly worked out.
532. _Q._--Are any precautions taken to prevent engines from being thrown
off the rails by obstructions left upon the line?
_A_.--In most engines a bar is strongly attached to the front of the
carriage on each side, and projects perpendicularly downward to within a
short distance of the rail, to clear away stones or other obstructions that
might occasion accidents if the engine ran over them.
CHAPTER IX.
STEAM NAVIGATION.
* * * * *
RESISTANCE OF VESSELS IN WATER.
533. _Q._--How do you determine the resistance encountered by a vessel
moving in water?
_A._--The resistance experienced by vessels moving in water varies as the
square of the velocity of their motion, or nearly so; and the power
necessary to impart an increased velocity varies nearly as the cube of such
increased velocity. To double the velocity of a steam vessel, therefore,
will require four times the amount of tractive force, and as that
quadrupled force must act through twice the distance in the same time, an
engine capable of exerting eight times the original power will be
required.[1]
534. _Q._--In the case of a board moving in water in the manner of a paddle
float, or in the case of moving water impinging on a stationary board, what
will be the pressure produced by the impact?
_A._--The pressure produced upon a flat board, by striking water at right
angles to the surface of the board, will be equal to the weight of a column
of water having the surface struck as a base, and for its altitude twice
the height due to the velocity with which the board moves through the
water. If the board strike the water obliquely, the resistance will be
less, but no very reliable law has yet been discovered to determine its
amount.
535. _Q._--Will not the resistance of a vessel in moving through the water
be much less than that of a flat board of the area of the cross section?
_A._--It will be very much less, as is manifest from the comparatively
small area of paddle board, and the small area of the circle described by
the screw, relatively with the area of the immersed midship section of the
vessel. The absolute speed of a vessel, with any given amount of power,
will depend very much upon her shape.
536. _Q._--In what way is it that the shape of a vessel influences her
speed, since the vessels of the same sectional area must manifestly put in
motion a column of water of the same magnitude, and with the same velocity?
_A._--A vessel will not strike the water with the same velocity when the
bow lines are sharp as when they are otherwise; for a very sharp bow has
the effect of enabling the vessel to move through a great distance, while
the particles of water are moved aside but a small distance, or in other
words, it causes the velocity with which the water is moved to be very
small relatively with the velocity of the vessel; and as the resistance
increases as the square of the velocity with which the water is moved, it
is conceivable enough in what way a sharp bow may diminish the resistance.
537. _Q._--Is the whole power expended in the propulsion of a vessel
consumed in moving aside the water to enable the vessel to pass?
_A._--By no means; only a portion, and in well-formed vessels only a small
portion, of the power is thus consumed. In the majority of cases, the
greater part of the power is expended in overcoming the friction of the
water upon the bottom of the vessel; and the problem chiefly claiming
consideration is, in what way we may diminish the friction.
538. _Q._--Does the resistance produced by this friction increase with the
velocity?
_A._--It increases nearly as the square of the velocity. At two nautical
miles per hour, the thrust necessary to overcome the friction varies as the
1.823 power of the velocity; and at eight nautical miles per hour, the
thrust necessary to overcome the friction varies as the 1.713 power of the
velocity. It is hardly proper, perhaps, to call this resistance by the name
of friction; it is partly, perhaps mainly, due to the viscidity or adhesion
of the water.
539. _Q._--Perhaps at high velocities this resistance may become less?
_A_.--That appears very probable. It may happen that at high velocities the
adhesion is overcome, so that the water is dragged off the vessel, and the
friction thereafter follows the law which obtains in the case of solid
bodies. But any such conclusion is mere speculation, since no experiments
illustrative of this question have yet been made.
540. _Q._--Will a vessel experience more resistance in moving in salt water
than in moving in fresh?
_A._--If the immersion be the same in both cases a vessel will experience
more resistance in moving in salt water than in moving in fresh, on account
of the greater density of salt water; but as the notation is proportionably
greater in the salt water the resistance will be the same with the same
weight carried.
541. _Q._--Discarding for the present the subject of friction, and looking
merely to the question of bow and stern resistance, in what manner should
the hull of a vessel be formed so as to make these resistances a minimum?
_A._--The hull should be so formed that the water, instead of being away
driven forcibly from the bow, is opened gradually, so that every particle
of water may be moved aside slowly at first, and then faster, like the ball
of a pendulum, until it reaches the position of the midship frame, at which
point it will have come to a state of rest, and then again, like a
returning pendulum, vibrate back in the same way, until it comes to rest at
the stern. It is not difficult to describe mechanically the line which the
water should pursue. If an endless web of paper be put into uniform motion,
and a pendulum carrying a pencil or brush be hung in front of it, then such
pendulum will trace on the paper the proper water line of the ship, or the
line which the water should pursue in order that no power may be lost
except that which is lost in friction. It is found, however, in practice,
that vessels formed with water lines on this principle are not much
superior to ordinary vessels in the facility with which they pass through
the water: and this points to the conclusion that in ordinary vessels of
good form, the amount of power consumed in overcoming the resistance due to
the wave at the bow and the partial vacuity at the stern is not so great as
has heretofore been supposed, and that, in fact, the main resistance is
that due to the friction.
[1] This statement supposes that there is no difference of level between
the water at the bow and the water at the stern. In the experiments on the
steamer Pelican, the resistance was found to vary, as the 2.28th power of
the velocity, but the deviation from the recognized law was imputed to a
difference in the level of the water at the bow and stern.
EXPERIMENTS ON THE RESISTANCE OF VESSELS.
542. _Q._--Have experiments been made to determine the resistance which
steam vessels experience in moving through the waters?
_A._--Experiments have been made both to determine the relative resistance
of different classes of vessels, and also the absolute resistance in pounds
or tons. The first experiments made upon this subject were conducted by
Messrs. Boulton and Watt, and they have been numerous, long continued, and
carefully performed. These experiments were made upon paddle vessels.
543. _Q._--Will you recount the chief results of these experiments?
_A._--The purpose of the experiments was to establish a coefficient of
performance, which with any given class of vessel would enable the speed,
which would be obtained with any given power, to be readily predicted. This
coefficient was obtained by multiplying the cube of the velocity of the
vessels experimented upon, in miles per hour, by the sectional area of the
immersed midship section in square feet, and dividing by the numbers of
nominal horses power, and this coefficient will be large in the proportion
of the goodness of the shape of the vessel.
544. _Q._--How many experiments were made altogether?
_A._--There were five different sets of experiments on five different
classes of vessels. The first set of experiments was made in 1828, upon the
vessels Caledonia, Diana, Eclipse, Kingshead, Moordyke, and Eagle-vessels
of a similar form and all with square bilges and flat floors; and the
result was to establish the number 925 as the coefficient of performance of
such vessels. The second set of experiments was made upon the superior
vessels Venus, Swiftsure, Dasher, Arrow, Spitfire, Fury, Albion, Queen,
Dart, Hawk, Margaret, and Hero-all vessels having flat floors and round
bilges, where the coefficient became 1160. The third set of experiments was
made upon the vessels Lightning, Meteor, James Watt, Cinderella, Navy
Meteor, Crocodile, Watersprite, Thetis, Dolphin, Wizard, Escape, and
Dragon-all vessels with rising floors and round bilges, and the coefficient
of performance was found to be 1430. The fourth set of experiments was made
in 1834, upon the vessels Magnet, Dart, Eclipse, Flamer, Firefly, Ferret,
and Monarch, when the coefficient of performance was found to be 1580. The
fifth set of experiments was made upon the Red Rover, City of Canterbury,
Herne, Queen, and Prince of Wales, and in the case of those vessels the
coefficient rose to 2550. The velocity of any of these vessels, with any
power or sectional area, may be ascertained by multiplying the coefficient
of its class by the nominal horse power, dividing by the sectional area in
square feet, and extracting the cube root of the quotient, which will be
the velocity in miles per hour; or the number of nominal horse power
requisite for the accomplishment of any required speed may be ascertained
by multiplying the cube of the required velocity in miles per hour, by the
sectional area in square feet, and dividing by the coefficient: the
quotient is the number of nominal horse power requisite to realize the
speed.
545. _Q._--Seeing, however, that the nominal power does not represent an
invariable amount of dynamical efficiency, would it not be better to make
the comparison with reference to the actual power?
_A._--In the whole of the experiments recited, except in the case of one or
two of the last, the pressure of steam in the boiler varied between 2-3/4
lbs. and 4 lbs. per square inch, and the effective pressure on the piston
varied between 11 lbs. and 13 lbs. per square inch, so that the average
ratio of the nominal to the actual power may be easily computed; but it
will be preferable to state the nominal power of some of the vessels, and
their actual power as ascertained by experiment.
546. _Q._--Then state this.
_A._--Of the Eclipse, the nominal power was 76, and the actual power 144.4
horses; of the Arrow, the nominal power was 60, and the actual 119.5;
Spitfire, nominal 40, actual 64; Fury, nominal 40, actual 65.6; Albion,
nominal 80, actual 135.4; Dart, nominal 100, actual 152.4; Hawk, nominal
40, actual 73; Hero, nominal 100, actual 171.4; Meteor, nominal 100, actual
160; James Watt, nominal 120, actual 204; Watersprite, nominal 76, actual
157.6; Dolphin, nominal 140, actual 238; Dragon, nominal 80, actual 131;
Magnet, nominal 140, actual 238; Dart, nominal 120, actual 237; Flamer,
nominal 120, actual 234; Firefly, nominal 52, actual 86.6; Ferret, nominal
52, actual 88; Monarch, nominal 200, actual 378. In the case of swift
vessels of modern construction, such as the Red Rover, Herne, Queen, and
Prince of Wales, the coefficient appears to be about 2550; but in these
vessels there is a still greater excess of the actual over the nominal
power than in the case of the vessels previously enumerated, and the
increase in the coefficient is consequent upon the increased pressure of
the steam in the boiler, as well as the superior form of the ship. The
nominal power of the Red Rover, Herne, and City of Canterbury is, in each
case, 120 horses, but the actual power of the Red Rover is 294, of the
Herne 354, and of the City of Canterbury 306, and in some vessels the
excess is still greater; so that with such variations it becomes necessary
to adopt a coefficient derived from the introduction of the actual instead
of the nominal power.
547. _Q._--What will be the average difference between the nominal and
actual powers in the several classes of vessels you have mentioned and the
respective coefficients when corrected for the actual power?
_A._--In the first class of vessels experimented upon, the actual power
was about 1.6 times greater
than the nominal power; in the second class, 1.67 times greater; in the
third class, 1.7 times
greater; and in the fourth, 1.96 times greater; while in such vessels as
the Red Rover and City of
Canterbury, it is 2.65 times greater; so that if we adopt the actual
instead of the nominal power in
fixing the coefficients, we shall have 554 as the first coefficient, 694
as the second, 832 for the
third, and 806 for the fourth, instead of 925, 1160, 1430, and 1580 as
previously specified; while
for such vessels as the Red Rover, Herne, Queen, and Prince of Wales, we
shall have 962 instead of
2550. These smaller coefficients, then, express the relative merits of
the different vessels without
reference to any difference of efficacy in the engines, and it appears
preferable, with such a
variable excess of the actual over the nominal power, to employ them
instead of those first referred
to. From the circumstance of the third of the new coefficients being
greater than the fourth, it
appears that the superior result in the fourth set of experiments arose
altogether from a greater
excess of the actual over the nominal power.
548. _Q._--These experiments, you have already stated, were all made on
paddle vessels. Have similar coefficients of performance been obtained in
the case of screw vessels?
_A._--The coefficients of a greater number of screw vessels have been
obtained and recorded, but it would occupy too much time to enumerate them
here. The coefficient of performance of the Fairy is 464.8; of the Rattler
676.8; and of the Frankfort 792.3. This coefficient, however, refers to
nautical and not to statute miles. If reduced to statute miles for the
purpose of comparison with the previous experiments, the coefficients will
respectively become 703, 1033, and 1212; which indicate that the
performance of screw vessels is equal to the performance of paddle vessels,
but some of the superiority of the result may be imputed to the superior
size of the screw vessels.
INFLUENCE OF THE SIZE OF VESSELS UPON THEIR SPEED.
549. _Q._--Will large vessels attain a greater speed than small, supposing
each to be furnished with the same proportionate power?
_A._--It is well known that large vessels furnished with the same
proportionate power, will attain a greater speed than small vessels, as
appears from the rule usual in yacht races of allowing a certain part of
the distance to be run to vessels which are of inferior size. The velocity
attained by a large vessel will be greater than the velocity attained by a
small vessel of the same mould and the same proportionate power, in the
proportion of the square roots of the linear dimensions of the vessels. A
vessel therefore with four times the sectional area and four times the
power of a smaller symmetrical vessel, and consequently of twice the
length, will have its speed increased in the proportion of the square root
of 1 to the square root of 2, or 1.4 times.
550. _Q._--Will you further illustrate this doctrine by an example?
_A._--The screw steamer Fairy, if enlarged to three times the size while
retaining the same form, would have twenty-seven times the capacity, nine
times the sectional area, and nine times the power. The length of such a
vessel would be 434 feet; her breadth 63 feet 4-1/2 inches; her draught of
water 16-1/2 feet; her area of immersed section 729 square feet; and her
nominal power 1080 horses. Now as the lengths of the Fairy and of the new
vessel are in the proportion of 1 to 3, the speeds will be in the
proportion of the square root of 1 to the square root of 3; or, in other
words, the speed of the large vessel will be 1.73 times greater than the
speed of the small vessel. If therefore the speed of the Fairy be 13 knots,
the speed of the new vessel will be 22.49 knots, although the proportion of
power to sectional area, which is supposed to be the measure of the
resistance, is in both cases precisely the same. If the speed of the Fairy
herself had to be increased to 22.29 knots, the power would have to be
increased in the proportion of the cube of 13 to the cube of 22.49, or 5.2
times, which makes the power necessary to propel the Fairy at that speed
equal to 624 nominal horses power.
STRUCTURE AND OPERATION OF PADDLE WHEELS.
551. _Q._--Will you describe the configuration and mode of action of the
paddle wheels in general use?
_A._--There are two kinds of paddle wheels in extensive use, the one being
the ordinary radial wheel, in which the floats are fixed on arms radiating
from the centre; and the other the feathering wheel, in which each float is
hung upon a centre, and is so governed by suitable mechanism as to be
always kept in nearly the vertical position. In the radial wheel there is
some loss of power from oblique action, whereas in the feathering wheel
there is little or no loss from this cause; but in every kind of paddle
there is a loss of power from the recession of the water from the float
boards, or the _slip_ as it is commonly called; and this loss is the
necessary condition of the resistance for the propulsion of the vessel
being created in a fluid. The slip is expressed by the difference between
the speed of the wheel and the speed of the vessel, and the larger this
difference is the greater the loss of power from slip must be--the
consumption of steam in the engine being proportionate to the velocity of
the wheel, and the useful effect being proportionate to the speed of the
ship.
552. _Q._--The resistance necessary for propulsion will not be situated at
the circumference of the wheel?
_A._--In the feathering wheel, where every part of any one immerged float
moves forward with the same horizontal velocity, the pressure or resistance
may be supposed to be concentrated in the centre of the float; whereas, in
the common radial wheel this cannot be the case, for as the outer edge of
the float moves more rapidly than the edge nearest the centre of the wheel,
the outer part of the float is the most effectual in propulsion. The point
at which the outer and inner portions of the float just balance one another
in propelling effect, is called the _centre of pressure_; and if all the
resistances were concentrated in this point, they would have the same
effect as before in resisting the rotation of the wheel. The resistance
upon any one moving float board totally immersed in the water will, when
the vessel is at rest, obviously vary as the square of its distance from
the centre of motion--the resistance of a fluid varying with the square of
the velocity; but, except when the wheel is sunk to the axle or altogether
immersed in the water, it is impossible, under ordinary circumstances, for
one float to be totally immersed without others being immersed partially,
whereby the arc described by the extremity of the paddle arm will become
greater than the arc described by the inner edge of the float; and
consequently the resistance upon any part of the float will increase in a
higher ratio than the square of its distance from the centre of motion--the
position of the centre of pressure being at the same time correspondingly
affected. In the feathering wheel the position of the centre of pressure of
the entering and emerging floats is continually changing from the lower
edge of the float--where it is when the float is entering or leaving the
water--to the centre of the float, which is its position when the float is
wholly immerged; but in the radial wheel the centre of pressure can never
rise so high as the centre of the float.
553. _Q._--All this relates to the action of the paddle when the vessel is
at rest: will you explain its action when the vessel is in motion?
_A._--When the wheel of a coach rolls along the ground, any point of its
periphery describes in the air a curve which is termed a cycloid; any point
within the periphery traces a prolate or protracted cycloid, and any point
exterior to the periphery traces a curtate or contracted cycloid--the
prolate cycloid partaking more of the nature of a straight line, and the
curtate cycloid more of the nature of a circle. The action of a paddle
wheel in the water resembles in this respect that of the wheel of a
carriage running along the ground: that point in the radius of the paddle
of which the rotative speed is just equal to the velocity of the vessel
will describe a cycloid; points nearer the centre, prolate cycloids, and
points further from the centre, curtate cycloids. The circle described by
the point whose velocity equals the velocity of the ship, is called the
_rolling circle_, and the resistance due to the difference of velocity of
the rolling circle and centre of pressure is that which operates in the
propulsion of the vessel. The resistance upon any part of the float,
therefore, will vary as the square of its distance from the rolling circle,
supposing the float to be totally immerged; but, taking into account the
greater length of time during which the extremity of the paddle acts,
whereby the resistance will be made greater, we shall not err far in
estimating the resistance upon any point at the third power of its distance
from the rolling circle in the case of light immersions, and the 2.5 power
in the case of deep immersions.
554. _Q._--How is the position of the centre of pressure to be determined?
_A._--With the foregoing assumption, which accords sufficiently with
experiment to justify its acceptation, the position of the centre of
pressure may be found by the following rule:--from the radius of the wheel
substract the radius of the rolling circle; to the remainder add the depth
of the paddle board, and divide the fourth power of the sum by four times
the depth; from the cube root of the quotient subtract the difference
between the radii of the wheel and rolling circle, and the remainder will
be the distance of the centre of pressure from the upper edge of the
paddle.
555. _Q._--How do you find the diameter of the rolling circle?
_A._--The diameter of the rolling circle is very easily found, for we have
only to divide 5,280 times the number of miles per hour, by 60 times the
number of strokes per minute, to get an expression for the circumference of
the rolling circle, or the following rule may be adopted:--divide 88 times
the speed of the vessel in statute miles per hour, by 3.1416 times the
number of strokes per minute; the quotient will be the diameter in feet of
the rolling circle. The diameter of the circle in which the centre of
pressure moves or the effective diameter of the wheel being known, and also
the diameter of the rolling circle, we at once find the excess of the
velocity of the wheel over the vessel.
556. _Q._--Will you illustrate these rules by an example?
_A._--A steam vessel of moderately good shape, and with engines of 200
horses power, realises, with 22 strokes per minute, a speed of 10.62 miles
per hour. To find the diameter of the rolling circle, we have 88 times
10.62, equal to 934.66, and 22 times 3.1416, equal to 69.1152; then 934.66
divided by 69.1152 is equal to 13.52 feet, which is the diameter of the
rolling circle. The diameter of the wheel is 19 ft. 4 in., so that the
diameter of the rolling circle is about 2/3ds of the diameter of the wheel,
and this is a frequent proportion. The depth of the paddle board is 2 feet,
and the difference between the diameters of the wheel and rolling circle
will be 5.8133, which will make the difference of their radii 2.9067; and
adding to this the depth of the paddle board, we have 4.9067, the fourth
power of which is 579.64, which, divided by four times the depth of the
paddle board, gives us 72.455, the cube root of which is 4.1689, which,
diminished by the difference of the radii of the wheel and rolling circle,
leaves 1.2622 feet for the distance of the centre of pressure from the
upper edge of the paddle board in the case of light immersions. The radius
of the wheel being 9.6667, the distance from the centre of the wheel to the
upper edge of the float is 7.6667, and adding to this 1.2622, we get 8.9299
feet as the radius, or 17.8598 feet as the diameter of the circle in which
the centre of pressure revolves. With 22 strokes per minute, the velocity
of the centre of pressure will be 20.573 feet per second, and with 10.62
miles per hour for the speed of the vessel, the velocity of the rolling
circle will be 15.576 feet per second. The effective velocity will be the
difference between these quantities, or 4.997 feet per second. Now the
height from which a body must fall by gravity, to acquire a velocity of
4.997 feet per second, is about .62 feet; and twice this height, or 1.24
feet, multiplied by 62-1/2, which is the number of Lbs. weight in a cubic
foot of water, gives 77-1/2 Lbs. as the pressure on each square foot of the
vertical paddle boards. As each board is of 20 square feet of area, and
there is a vertical board on each side of the ship, the total pressure on
the vertical paddle boards will be 2900 Lbs.
557. _Q._--What pressure is this equivalent to on each square inch of the
pistons?
_A._--A vessel of 200 horses power will have two cylinders, each 50 inches
diameter, and 5 feet stroke, or thereabout. The area of a piston of 50
inches diameter is 1963.5 square inches, so that the area of the two
pistons is 3927 square inches, and the piston will move through 10 feet
every revolution; and with 22 strokes per minute this will be 220 feet per
minute, or 3.66 feet per second. Now, if the effective velocity of the
centre of pressure and the velocity of the pistons had been the same, then
a pressure of 2900 Lbs. upon the vertical paddles would have been balanced
by an equal pressure on the pistons, which would have been in this case
about .75 Lbs. per square inch; but as the effective velocity of the centre
of pressure is 4.997 feet per second, while that of the pistons is only
3.66 feet per second, the pressure must be increased in the proportion of
4.997 to 3.66 to establish an equilibrium of pressure, or, in other words,
it must be 1.02 Lbs. per square inch. It follows from this investigation,
that, in radial wheels, the greater part of the engine power is distributed
among the oblique floats.
558. _Q._--How comes this to be the case?
_A._--To understand how it happens that more power is expended upon the
oblique than upon the vertical floats, it is necessary to remember that the
only resistance upon the vertical paddle is that due to the difference of
velocity of the wheel and the ship; but if the wheel be supposed to be
immersed to its axle, so that the entering float strikes the water
horizontally, it is clear that the resistance on such float is that due to
the whole velocity of rotation; and that the resistance to the entering
float will be the same whether the vessel is in motion or not. The
resistance opposed to the rotation of any float increases from the position
of the vertical float-where the resistance is that due to the difference of
velocity of the wheel and vessel--until it reaches the plane of the axis,
supposing the wheel to be immersed so far, where the resistance is that due
to the whole velocity of rotation; and although in any oblique float the
total resistance cannot be considered operative in a horizontal direction,
yet the total resistance increases so rapidly on each side of the vertical
float, that the portion of it which is operative in the horizontal
direction, is in all ordinary cases of immersion very considerable. In the
feathering wheel, where there is little of this oblique action, the
resistance will be in the proportion of the square of the horizontal
velocities of the several floats, which may be represented by the
horizontal distances between them; and in the feathering wheel, the
vertical float having the greatest horizontal velocity will have the
greatest propelling effect.
559. _Q._--Should the floats in feathering wheels enter and leave the water
vertically?
_A._--The floats should be so governed by the central crank or eccentric,
that the entering and emerging floats have a direction intermediate between
a radius and a vertical line.
560. _Q._--Can you give any practical rules for proportioning paddle
wheels?
_A._--A common rule for the pitch of the floats is to allow one float for
every foot of diameter of the wheel; but in the case of fast vessels a
pitch of 2-1/2 feet, or even less, appears preferable, as a close pitch
occasions less vibration. If the floats be put too close, however, the
water will not escape freely from between them, and if set too far apart
the stroke of the entering paddle will occasion an inconvenient amount of
vibratory motion, and there will also be some loss of power. To find the
proper area of a single float:--divide the number of actual horses power of
both engines by the diameter of the wheel in feet; the quotient is the area
of one paddle board in square feet proper for sea going vessels, and the
area multiplied by 0.6 will give the length of the float in feet. In very
sharp vessels, which offer less resistance in passing through the water,
the area of paddle board is usually one-fourth less than the above
proportion, and the proper length of the float may in such case be found by
multiplying the area by 0.7. In sea going vessels about four floats are
usually immersed, and in river steamers only one or two floats. There is
more slip in the latter case, but there is also more engine power exerted
in the propulsion of the ship, from the greater speed of engine thus
rendered possible.
561. _Q._--Then is it beneficial to use small floats?
_A._--Quite the contrary. If to permit a greater speed of the engine the
floats be diminished in area instead of being raised out of the water, no
appreciable accession to the speed of the vessel will be obtained; whereas
there will be an increased speed of vessel if the accelerated speed of the
engine be caused by diminishing the diameter of the wheels. In vessels
intended to be fast, therefore, it is expedient to make the wheels small,
so as to enable the engine to work with a high velocity; and it is
expedient to make such wheels of the feathering kind, to obviate loss of
power from oblique action. In no wheel must the rolling circle fall below
the water line, else the entering and emerging floats will carry masses of
water before them. The slip is usually equal to about one-fourth of the
velocity of the centre of pressure in well proportioned wheels; but it is
desirable to have the slip as small as is possible consistently with the
observance of other necessary conditions. The speed of the engine and also
the speed of the vessel being fixed, the diameter of the rolling circle
becomes at once ascertainable, and adding to this the slip, we have the
diameter of the wheel.
CONFIGURATION AND ACTION OF THE SCREW.
562. _Q._--Will you describe more in detail than you have yet done, the
configuration and mode of action of the screw propeller?
_A._--The ordinary form of screw propeller is represented in figs. 46 and
47; fig. 46 being a perspective view, and fig. 47 an end view, or view such
as is seen when looking upon the end of the shaft. The screw here
represented is one with two arms or blades. Some screws have three arms,
some four and some six; but the screw with two arms is the most usual, and
screws with more than three arms are not now much employed in this country.
The screw on being put into revolution by the engine, preserves a spiral
path in the water, in which it draws itself forward in the same way as a
screw nail does when turned round in a piece of wood, whereas the paddle
wheel more resembles the action of a cog wheel working in a rack.
[Illustration: Fig. 46. Fig. 47. ORDINARY FORM OF SCREW PROPELLER.]
563. _Q._--But the screw of a steam vessel has no resemblance to a screw
nail?
_A._--It has in fact a very close resemblance if you suppose only a very
short piece of the screw nail to be employed, and if you suppose, moreover,
the thread of the screw to be cut nearly into the centre to prevent the
wood from stripping. The original screw propellers were made with several
convolutions of screw, but it was found advantageous to shorten them, until
they are now only made one-sixth of a convolution in length.
564. _Q._--And the pitch you have already explained to be the distance in
the line of the shaft from one convolution to the next, supposing the screw
to consist of two or more convolutions?
_A._--Yes, that is what is meant by the pitch. If a thread be wound upon a
cylinder with an equal distance between the convolutions, it will trace a
screw of a uniform pitch; and if the thread be wound upon the cylinder with
an increasing distance between each convolution, it will trace a screw of
an increasing pitch. But two or more threads may be wound upon the cylinder
at the same time, instead of a single thread. If two threads be wound upon
it they will trace a double-threaded screw; if three threads be wound upon
it they will trace a treble-threaded screw; and so of any other number. Now
if the thread be supposed to be raised up into a very deep and thin spiral
feather, and the cylinder be supposed to become very small, like the newel
of a spiral stair, then a screw will be obtained of the kind proper for
propelling vessels, except that only a very short piece of such screw must
be employed. Whatever be the number of threads wound upon a cylinder, if
the cylinder be cut across all the threads will be cut. A slice cut out of
the cylinder will therefore contain a piece of each thread. But the
threads, in the case of a screw propeller, answer to the arms, so that in
every screw propeller the number of threads entering into the composition
of the screw will be the same as the number of arms. An ordinary screw with
two blades is a short piece of a screw of two threads.
565. _Q._--In what part of the ship is the screw usually placed?
[Illustration: Fig. 48]
_A._--In that part of the run of the ship called the dead wood, which is a
thin and unused part of the vessel just in advance of the rudder. The usual
arrangement is shown in fig. 48, which represents the application to a
vessel of a species of screw which has the arms bent backwards, to
counteract the centrifugal motion given to the water when there is a
considerable amount of slip.
566. _Q._--How is the slip in a screw vessel determined?
_A._--By comparing the actual speed of the vessel with the speed due to the
pitch and number of revolutions of the screw, or, what is the same thing,
the speed which the vessel would attain if the screw worked in a solid nut.
The difference between the actual speed and this hypothetical speed, is the
slip.
567. _Q._--In well formed screw propellers what is the amount of slip found
to be?
_A._--If the screw be properly proportioned to the resistance that the
vessel has to overcome, the slip will not be more than 10 per cent., but in
some cases it amounts to 30 per cent., or even more than this. In other
cases, however, the slip is nothing at all, and even less than nothing; or,
in other words the vessel passes through the water with a greater velocity
than if the screw were working in a solid nut.
568. _Q._--Then it must be by the aid of the wind or some other extraneous
force?
_A._--No; by the action of the screw alone.
569. _Q._--But how is such a result possible?
_A._--It appears to be mainly owing to the centrifugal action of the screw,
which interposes a film or wedge of water between the screw itself and the
water on which the screw reacts. This negative slip, as it is called,
chiefly occurs when the pitch of the screw is less than its diameter, and
when, consequently, the velocity of rotation is greater than if a coarser
pitch had been employed. There is, moreover, in all vessels passing through
the water with any considerable velocity, a current of water following the
vessel, in which current, in the case of a screw vessel, the screw will
revolve; and in certain cases the phenomenon of negative slip may be
imputable in part to the existence of this current.
570. _Q._--Is the screw propeller as effectual an instrument of propulsion
as the radial or feathering paddle?
_A._--In all cases of deep immersion it appears to be quite as effectual as
the radial paddle, indeed, more so; but it is scarcely as effectual as the
feathering paddle, with any amount of immersion, and scarcely as effectual
as the common paddle in the case of light immersions.
COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS.
571. _Q._--Whether do you consider paddle or screw vessels to be on the
whole the most advantageous?
_A._--That is a large question, and can only receive a qualified answer. In
some cases the use of paddles is indispensable, as, for example, in the
case of river vessels of a limited draught of water, where it would not be
possible to get sufficient depth below the water surface to enable a screw
of a proper diameter to be got in.
572. _Q._--But how does the matter stand in the case of ocean vessels?
_A._--In the case of ocean vessels, it is found that paddle vessels fitted
with the ordinary radial wheels, and screw vessels fitted with the ordinary
screw, are about equally efficient in calms and in fair or beam winds with
light and medium immersions. If the vessels are loaded deeply, however, as
vessels starting on a long voyage and carrying much coal must almost
necessarily be, then the screw has an advantage, since the screw acts in
its best manner when deeply immersed, and the paddles in their worst. When
a screw and paddle vessel, however, of the same model and power are set to
encounter head winds, the paddle vessel it is found has in all cases an
advantage, not in speed, but in economy of fuel. For whereas in a paddle
vessel, when her progress is resisted, the speed of the engine diminishes
nearly in the proportion of the diminished speed of ship, it happens that
in a screw vessel this is not so,--at least to an equal extent,--but the
engines work with nearly the same rate of speed as if no increase of
resistance had been encountered by the ship. It follows from this
circumstance, that whereas in paddle vessels the consumption of steam, and
therefore of fuel, per hour is materially diminished when head winds occur,
in screw vessels a similar diminution in the consumption of steam and fuel
does not take place.
573. _Q._--But perhaps under such circumstances the speed of the screw
vessel will be the greater of the two?
_A._--No; the speed of the two vessels will be the same, unless the
strength of the head wind be so great as to bring the vessels nearly to a
state of rest, and on that supposition the screw vessel will have the
advantage. Such cases occur very rarely in practice; and in the case of the
ordinary resistances imposed by head winds, the speed of the screw and
paddle vessel will be the same, but the screw vessel will consume most
coals.
574. _Q._--What is the cause of this peculiarity?
_A._--The cause is, that when the screw is so proportioned in its length as
to be most suitable for propelling vessels in calms, it is too short to be
suitable for propelling vessels which encounter a very heavy resistance. It
follows, therefore, that if it is prevented from pursuing its spiral course
in the water, it will displace the water to a certain extent laterally, in
the manner it does if the engine be set on when the vessel is at anchor;
and a part of the engine power is thus wasted in producing a useless
disturbance of the water, which in paddle vessels is not expended at all.
575. _Q._--If a screw and paddle vessel of the same mould and power be tied
stern to stern, will not the screw vessel preponderate and tow the paddle
vessel astern against the whole force of her engines?
_A._--Yes, that will be so.
576. _Q._--And seeing that the vessels are of the same mould and power, so
that neither can derive an advantage from a variation in that condition,
does not the preponderance of the screw vessel show that the screw must be
the most powerful propeller?
_A._---No, it does not.
577. _Q._--Seeing that the vessels are the same in all respects except as
regards the propellers, and that one of them exhibits a superiority, does
not this circumstance show that one propeller must be more powerful than
the other?
_A._--That does not follow necessarily, nor is it the fact in this
particular case. All steam vessels when set into motion, will force
themselves forward with an amount of thrust which, setting aside the loss
from friction and from other causes, will just balance the pressure on the
pistons. In a paddle vessel, as has already been explained, it is easy to
tell the tractive force exerted at the centre of pressure of the paddle
wheels, when the pressure urging the pistons, the dimensions of the wheels
and the speed of the vessel are known; and that force, whatever be its
amount, must always continue the same with any constant pressure on the
pistons. In a screw vessel the same law applies, so that with any given
pressure on the pistons and discarding the consideration of friction, it
will follow that whatever be the thrust exerted by a paddle or a screw
vessel, it must remain uniform whether the vessel is in motion or at rest,
and whether moving at a high or a low velocity through the water. Now to
achieve an equal speed during calms in two vessels of the same model, there
must be the same amount of propelling thrust in each; and this thrust,
whatever be its amount, cannot afterward vary if a uniform pressure of
steam be maintained. The thrusts, therefore, caused by their respective
propelling instruments, when a screw and paddle vessel are tied stern to
stern, must be the same as at other times; and as at other times those
thrusts are equal, so must they be when the vessels are set in the
antagonism supposed.
578. _Q._--How comes it then that the screw vessel preponderates?
_A._--Not by virtue of a larger thrust exerted by the screw in pressing
forward the shaft and with it the vessel, but by the gravitation against
the stern of the wave of water which the screw raises by its rapid
rotation. This wave will only be raised very high when the progress of the
vessel through the water is nearly arrested, at which time the centrifugal
action of the screw is very great; and the vessel under such circumstances
is forced forward partly by the thrust of the screw, and partly by the
hydrostatic pressure of the protuberance of water which the centrifugal
action of the screw raises up at the stern.
579. _Q._--Can you state any facts in corroboration of this view?
_A._--The screw vessel will not preponderate if a screw and paddle vessel
be tied bow to bow and the engines of each be then reversed. In, some screw
vessels the amount of thrust actually exerted by the screw under all its
varying circumstances, has been ascertained by the application of a
dynamometer to the end of the shaft. By this instrument--which is formed by
a combination of levers like a weighing machine for carts--a thrust or
pressure of several tons can be measured by the application of a small
weight; and it has been found, by repeated experiment with the dynamometer,
that the thrust of the screw in a screw vessel when towing a paddle vessel
against the whole force of her engines, is just the same as it is when the
two vessels are maintaining an equal speed in calms. The preponderance of
the screw vessel must, therefore, be imputable to some other agency than to
a superior thrust of the screw, which is found by experiment not to exist.
580. _Q._--Has the dynamometer been applied to paddle vessels?
_A._--It has not been applied to the vessels themselves, as in the case of
screw vessels, but it has been employed on shore to ascertain the amount of
tractive force that a paddle vessel can exert on a rope.
581. _Q._--Have any experiments been made to determine the comparative
performances of screw and paddle vessels at sea?
_A._--Yes, numerous experiments; of which the best known are probably those
made on the screw steamer Rattler and the paddle steamer Alecto, each
vessel of the same model, size, and power,--each vessel being of about 800
tons burden and 200 horses power. Subsequently another set of experiments
with the same object was made with the Niger screw steamer and the Basilisk
paddle steamer, both vessels being of about 1000 tons burden and 400 horses
power. The general results which were obtained in the course of these
experiments are those which have been already recited.
582. _Q._--Will you recapitulate some of the main incidents of these
trials?
_A._--I may first state some of the chief dimensions of the vessels. The
Rattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tons
burden, 200 horses power, and has an area of immersed midship section of
380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alecto
is of the same dimensions in every respect, except that she is only of 800
tons burden, the difference in this particular being wholly owing to the
Rattler having been drawn out about 15 feet at the stern, to leave abundant
room for the application of the screw. The Rattler was fitted with a
dynamometer, which enabled the actual propelling thrust of the screw shaft
to be measured; and the amount of this thrust, multiplied by the distance
through which the vessel passed in a given time, would determine the amount
of power actually utilized in propelling the ship. Both vessels were fitted
with indicators applied to the cylinders, so as to determine the amount of
power exerted by the engines.
583. _Q._--How many trials of the vessels were made on this occasion?
_A._--Twelve trials in all; but I need not refer to those in which similar
or identical results were only repeated. The first trial was made under
steam only, the weather was calm and the water smooth. At 54 minutes past 4
in the morning both vessels left the Nore, and at 30-1/2 minutes past 2 the
Rattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutes
afterward she was joined by the Alecto. The mean speed achieved by the
Rattler during this trial was 9.2 knots per hour; the mean speed of the
Alecto was 8.8 knots per hour. The slip of the screw was 10.2 per cent. The
actual power exerted by the engines, as shown by the indicator, was in the
case of the Rattler 334.6 horses, and in the case of the Alecto 281.2
horses; being a difference of 53.4 horses in favor of the Rattler. The
forward thrust upon the screw shaft was 3 tons, 17 cwt., 3 qrs., and 14
lbs. The horse power of the shaft--or power actually utilized--ascertained
by multiplying the thrust in pounds by the space passed through by the
vessel in feet per minute, and dividing by 33,000, was 247.8 horses power.
This makes the ratio of the shaft to the engine power as 1 to 1.3, or, in
other words, it shows that the amount of engine power utilized in
propulsion was 77 per cent. In a subsequent trial made with the vessels
running before the wind, but with no sails set and the masts struck, the
speed realized by the Rattler was 10 knots per hour. The slip of the screw
was 11.2 per cent. The actual power exerted by the engines of the Rattler
was 368.8 horses. The actual power exerted by the engines of the Alecto was
291.7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4
cwt., 1 qr., 1 lb. The horse power of the shaft was 290.2 horses, and the
ratio of the shaft to the engine power was 1 to 1.2. Here, therefore, the
amount of the engine power utilized was 84 per cent.
584. _Q._--If in any screw vessel the power of the engine be diminished by
shutting off the steam or otherwise, you will then have a larger screw
relatively with the power of the engine than before?
_A._--Yes.
585. _Q._--Was any experiment made to ascertain the effect of this
modification?
_A._--There was; but the result was not found to be better than before. The
experiment was made by shutting off the steam from the engines of the
Rattler until the number of strokes was reduced to 17 in the minute. The
actual power was then 126.7 horses; thrust upon the shaft 2 tons, 2 cwt., 3
qrs., 14 lbs; horse power of shaft 88.4 horses; ratio of shaft to engine
power 1 to 1.4; slip of the screw 18.7 per cent. In this experiment the
power utilized was 71 per cent.
586. _Q._--Was any experiment made to determine the relative performances
in head winds?
_A._--The trial in which this relation was best determined lasted for seven
hours, and was made against a strong head wind and heavy head sea. The
speed of the Rattler by patent log was 4.2 knots; and at the conclusion of
the trial the Alecto had the advantage by about half a mile. Owing to an
accidental injury to the indicator, the power exerted by the engines of the
Rattler in this trial could not be ascertained; but judging from the power
exerted in other experiments with the same number of revolutions, it
appears probable that the power actually exerted by the Rattler was about
300 horses. The number of strokes per minute made by the engines of the
Rattler was 22, whereas in the Alecto the number of strokes per minute was
only 12; so that while the engines of the Alecto were reduced, by the
resistance occasioned by a strong head wind, to nearly half their usual
speed, the engines of the Rattler were only lessened about one twelfth of
their usual speed. The mean thrust upon the screw shaft during this
experiment, was 4 tons, 7 cwt., 0 qr., 16 lbs. The horse power of the shaft
was 125.9 horses, and the slip of the screw was 56 per cent. Taking the
power actually exerted by the Rattler at 300 horses, the power utilized in
this experiment is only 42 per cent.
587. _Q._--What are the dimensions of the screw in the Rattler?
_A._--Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet. The
foregoing experiments show that with a larger screw a better average
performance would be obtained. The best result arrived at, was when the
vessel was somewhat assisted by the wind, which is equivalent to a
reduction of the resistance of the hull, or to a smaller hull, which is
only another expression for a larger proportionate screw.
588. _Q._--When you speak of a larger screw, what increase of dimension do
you mean to express?
_A._--An increase of the diameter. The amount of reacting power of the
screw upon the water is hot measured by the number of square feet of
surface of the arms, but by the area of the disc or circle in which the
screw revolves. The diameter of the screw of the Rattler being 10 feet, the
area of its disc is 78.5 square feet; and with the amount of thrust already
mentioned as existing in the first experiment, viz. 8722 lbs., the reacting
pressure on each square foot of the screw's disc will be 108-1/2 lbs. The
immersed midship section being 380 square feet, this is equivalent to 23
lbs. per square foot of immersed midship section at a speed of 9.2 knots
per hour.
589. _Q._--In smaller vessels of similar form, will the resistance per
square foot of midship section be more than this?
_A._--It will be considerably more. In the Pelican, a vessel of 109-3/4
square feet of midship section, I estimate the resistance per square foot
of midship section at 30 lbs., when the speed of the vessel is 9.7 knots
per hour. In the Minx with an immersed midship section of 82 square feet,
the resistance per square foot of immersed midship section was found by the
dynamometer to be 41 lbs. at a speed of 8-1/2 knots; and in the Dwarf, a
vessel with 60 square feet of midship section, I estimate the resistance
per square foot of midship section at 46 lbs. at a speed of 9 knots per
hour, which is just double the resistance per square foot of the Rattler.
The diameter of the screw of the Minx is 4-1/2 feet, so that the area of
its disc is 15.9 square feet, and the area of immersed midship section is
about 5 times greater than that of the screw's disc. The diameter of the
screw of the Dwarf is 5 feet 8 inches, so that the area of its disc is
25.22 square feet, and the area of immersed midship section is 2.4 times
greater than that of the screw's disc. The pressure per square foot of the
screw's disc is 214 lbs. in the case of the Minx, and 109-1/2 lbs. in the
case of the Dwarf.
590. _Q._--From the greater proportionate resistance of small vessels, will
not they require larger proportionate screws than large vessels?
_A._--They will.
591. _Q._--Is there any ready means of predicting what the amount of thrust
of a screw will be?
_A._--When we know the amount of pressure on the pistons, and the velocity
of their motion relatively with the velocity of advance made by the screw,
supposing it to work in a solid nut, it is easy to tell what the thrust of
the screw would be if it were cleared of the effects of friction and other
irregular sources of disturbance. The thrust, in fact, would be at once
found by the principle of virtual velocities; and if we take this
theoretical thrust and diminish it by one fourth to compensate for friction
and lateral slip, we shall have a near approximation to the amount of
thrust that will be actually exerted.[1]
[1] See Treatise on the Screw Propeller, by J. Bourne, C. E.
COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS.
592. _Q._--What species of screw do you consider the best?
_A._--In cases in which a large diameter of screw can be employed, the
ordinary screw or helix with two blades seems to be as effective as any
other, and it is the most easily constructed. If, however, the screw is
restricted in diameter, or if the vessel is required to tow, or will have
to encounter habitually strong head winds, it will be preferable to employ
a screw with an increasing pitch, and also of such other configuration that
it will recover from the water some portion of the power that has been
expended in slip.
593. _Q._--How can this be done?
_A._--There are screws which are intended to accomplish, this object
already in actual use. When there is much slip a centrifugal velocity is
given to the water, and the screw, indeed, if the engine be set on when the
vessel is at rest, acts very much as a centrifugal fan would do if placed
in the same situation. The water projected outward by the centrifugal force
escapes in the line of least resistance, which is to the surface; and if
there be a high column of water over the screw, or, in other words, if the
screw is deeply immersed, then the centrifugal action is resisted to a
greater extent, and there will be less slip produced. The easiest
expedient, therefore, for obviating loss by slip is to sink the screw
deeply in the water; but as there are obvious limits to the application of
this remedy, the next best device is to recover and render available for
propulsion some part of the power which has been expended in giving motion
to the water. One device for doing this consists in placing the screw well
forward in the dead wood, so that it shall be overhung by the stern of the
ship. The water forced upward by the centrifugal action of the screw will,
by impinging on the overhanging stern, press the vessel forward in the
water, just in the same way as is done by the wind when acting on an
oblique sail. I believe, the two revolving vanes without any twist or
obliquity on them at all, would propel a vessel if set well forward in the
dead wood or beneath the bottom, merely by the ascent of the water up the
inclined plane of the vessel's run; and, at all events, a screw so placed
would, in my judgment, aid materially in propelling the vessel when her
progress was resisted by head winds.
594. _Q._--But you said there are some kinds of screws which profess to
accomplish this?
[Illustration: Fig. 49. THE EARL OF DUNDONALD'S PROPELLER.]
_A._--There are screws which profess to counteract the centrifugal velocity
given to the water by imparting to it an equal centripetal force, the
consequence of which will be, that the water projected backward by the
screw, instead of taking the form of the frustum of a cone, with its small
end next the screw, will take the form of a cylinder. One of these forms of
screw is that patented by the Earl of Dundonald in 1843, and which is
represented in fig. 49. Another is the form of screw already represented in
fig. 48, and which was patented by Mr. Hodgson in 1844. Mr. Hodgson bends
the arms of his propellers backward, not into the form of a triangle, but
into the form of a parabola, to the end that the impact of the screw on the
particles of the water may cause them to converge to a focus, as the rays
of light would do in a parabolic reflector. But this particular
configuration is not important, seeing that the same convergence which is
given to the particles of the water, with a screw of uniform pitch bent
back into the form of a parabola, will be given with a screw bent back into
the form of a triangle, if the pitch be suitably varied between the centre
and the circumference.
595. _Q._--Then the pitch may be varied in two ways?
_A._--Yes: a screw may have a pitch increasing in the direction of the
length, as would happen in the case of a spiral stair, if every successive
step in the ascent was thicker than the one below it; or it may increase
from the centre to the circumference, as would happen in the case of a
spiral stair, if every step were thinner at the centre of the lower than at
its outer wall. When the pitch of a screw increases in the direction of its
length, the leading edge of the screw enters the water without shock or
impact, as the advance of the leading edge per revolution will not be
greater than the advance of the vessel. When the pitch of a screw increases
in the direction of its diameter, the central part of the screw will
advance with only the same velocity as the water, so that it cannot
communicate any centrifugal velocity to the water; and the whole slip, as
well as the whole propelling pressure, will occur at the outer part of the
screw blades.
596. _Q._--Is there any advantage derived from these forms of screws?
_A._--There is a slight advantage, but it is so slight as hardly to balance
the increased trouble of manufacture, and, consequently, they are not
generally or widely adopted.
597. _Q._--What other kinds of screw are there proposing to themselves the
same or similar objects?
_A._--There is the corrugated screw, the arms of which are corrugated, so
as it were to gear with the water during its revolution, and thereby
prevent it from acquiring a centrifugal velocity. Then there is Griffith's
screw, which has a large ball at its centre, which, by the suction it
creates at its hinder part, in passing through the water, produces a
converging force, which partly counteracts the divergent action of the
arms. Finally, there is Holm's screw, which has now been applied to a good
number of vessels with success.
598. _Q._--Will you describe the configuration and action of Holm's screw?
_A._--First, then, the screw increases in the direction of its length, and
this increase is very rapid at the following edge, so that, in fact, the
following edge stands in the plane of the shaft, or in the vertical
longitudinal plane of the vessel. Then the ends of the arms are bent over
into a curved flange, the edge of which points astern, and the point where
this curved flange joins the following edge of the screw is formed, not
into an angle, but into a portion of a sphere, so that this corner
resembles the bowl of a spoon. When the screw is put into revolution, the
water is encountered by the leading edge of the screw without shock, as its
advance is only equal to the advance of the vessel, and before the screw
leaves the water it is projected directly astern. At the same time, the
curved flange at the rim of the screw prevents the dispersion of the water
in a radial direction, and it consequently assumes the form of a column or
cylinder of water, projected backward from the ship.
599. _Q._--What is the nature of Beattle's screw?
_A._--Beattie's screw is an arrangement of the screw propeller whereby it
is projected beyond the rudder, and the main object of the arrangement is
to take away the vibratory motion at the stern,--an intention which it
accomplishes in practice. There is an oval eye in the rudder, to permit the
screw shaft to pass through it.
600. _Q._--When the diameter of the cylinder of water projected backward by
a screw, and the force urging it into motion are known, may not the
velocity it will acquire be approximately determined?
_A._--That will not be very difficult; and I will take for illustration the
case of the Minx, already referred to, which will show how such a
computation is to be conducted. The speed of this vessel, in one of the
experiments made with her, was 8.445 knots; the number of revolutions of
the screw per minute, 231.32; and the pressure on each square foot of area
of the screw's disc, 214 lbs. If a knot be taken to be 6075.6 feet, then
the distance advanced by the vessel, when the speed is 8.445 knots, will be
3.7 feet per revolution, and this advance will be made in about .26 of a
second of time. Now the distance which a body will fall by gravity, in .26
of a second, is 1.087 feet; and a weight of 214 lbs. put into motion by
gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity
of 1.087 feet during the time one revolution of the screw is being
performed. The weight to be moved, however, is 3.7 cubic feet of water,
that being the new water seized by the screw each revolution for every
square foot of surface in the screw's disc; and 3.7 cubic feet of water
weigh 231.5 lbs., so that the urging force of 214 lbs. is somewhat less
than the force of gravity, and the velocity of motion communicated to the
water will be somewhat under 1.087 feet per revolution, or we may say it
will be in round numbers 1 foot per revolution. This, added to the progress
of the vessel, will make the distance advanced by the screw through the
water 4.7 feet per revolution, leaving the difference between this and the
pitch, namely 1.13 feet, to be accounted for on the supposition that the
screw blades had broken laterally through the water to that extent. It
would be proper to apply some correction to this computation, which would
represent the increased resistance due to the immersion of the screw in the
water; for a column of water cannot be moved in the direction of its axis
beneath the surface, without giving motion to the superincumbent water, and
the inertia of this superincumbent water must, therefore, be taken into the
account. In the experiment upon the Minx, the depth of this superincumbent
column was but small. The total amount of the slip was 36.53 per cent.; and
there will not be much error in setting down about one half of this as due
to the recession of the water in the direction of the vessel's track, and
the other half as due to the lateral penetration of the screw blades.
601. _Q._--Is it not important to make the stern of screw vessels very
fine, with the view of diminishing the slip, and increasing the speed?
_A._--It is most important. The Rifleman, a vessel of 486 tons, had
originally engines of 200 horses power, which propelled her at a speed of 8
knots an hour. The Teazer, a vessel of 296 tons, had originally engines of
100 horses power, which propelled her at a speed of 6-1/2 knots an hour.
The engines of the Teazer were subsequently transferred to the Rifleman,
and new engines of 40 horse power were put into the Teazer. Both vessels
were simultaneously sharpened at the stern, and the result was, that the
100 horse engines drove the Rifleman, when sharpened, as fast as she had
previously been driven by the 200 horse engines; and the 40 horse engines
drove the Teazer, when sharpened, a knot an hour faster than she had
previously been driven by the 100 horse engines. The immersion of both
vessels was kept unchanged in each case; and the 100 horse engines of the
Teazer, when transferred to the Rifleman, drove that vessel, after she had
been sharpened, 2 knots an hour faster than they had previously driven a
vessel not much more than half the size. These are important facts for
every one to be acquainted with who is interested in the success of screw
vessels, and who seeks to obtain the maximum of efficiency with the minimum
of expense.[1]
[1] See Treatise on the Screw Propeller, by John Bourne, C. E.
PROPORTIONS OF SCREWS.
602. _Q._--In fixing upon the proportions of a screw proper to propel any
given vessel, how would you proceed?
_A._--I would first compute the probable resistance of the vessel, and I
would be able to find the relative resistances of the screw and hull, and
in every case it is advisable to make the screw as large in diameter as
possible. The larger the screw is, the greater will be the efficiency of
the engine in propelling the vessel; the larger will be the ratio of the
pitch to the diameter, which produces a maximum effect; and the smaller
will be the length of the screw or the fraction of a convolution to produce
a maximum effect.
603. _Q._--Will you illustrate this doctrine by a practical example?
_A._--The French screw steamer Pelican was fitted successively with two
screws of four blades, but the diameter of the first screw was 98.42
inches, and the diameter of the second 54 inches. If the efficiency of the
first screw by represented by 1, that of the second screw will be
represented by .823, or, in other words, if the first screw would give a
speed of 10 knots, the second would give little more than 8. The most
advantageous ratio of pitch to diameter was found to be 2.2 in the case of
the large screw, and 1.384 in the case of the small. The fraction of a
convolution which was found to be most advantageous was .281 in the case of
the large screw, and .450 in the case of the small screw.
604. _Q_--Were screws of four blades found to be more efficient than screws
with two?
_A_--They were found to have less slip, but not to be more
efficient, the increased slip in those of two blades being balanced by the
increased friction in those of four. Screws of two blades, to secure a
maximum efficiency, must have a finer pitch than screws of four.
605. _Q._--Are the proportions found to be most suitable in the case of the
Pelican applicable to the screws of other vessels?
_A._--Only to those which have the same relative resistance of screw and
hull. Taking the relative resistance to be the area of immersed midship
section, divided by the square of the screw's diameter, it will in the case
of the Rattler be 380/100 or 3.8. From the experiments made by MM. Bourgois
and Moll on the screw steamer Pelican, they have deduced the proportions of
screws proper for all other classes of vessels, whether the screws are of
two, four, or six blades.
606. _Q._--Will you specify the nature of their deductions?
_A._--I will first enumerate those which bear upon screws with two blades.
When the relative resistance is 5.5 the ratio of pitch to diameter should
be 1.006, and the fraction of the pitch or proportion of one entire
convolution should be 0.454. When the relative resistance is 5, the ratio
of pitch to diameter should be 1.069, and fraction of pitch 0.428; relative
resistance 4.5, pitch 1.135, fraction 0.402; relative resistance 4, pitch
1.205, fraction 0.378; relative resistance 3.5, pitch 1.279, fraction
0.355; relative resistance 3, pitch 1.357, fraction 0.334; relative
resistance 2.5, pitch 1.450, fraction 0.313; relative resistance 2, pitch
1.560, fraction 0.294; relative resistance 1.5, pitch 1.682, fraction
0.275. The relative resistance of 4 is that which is usual in an auxiliary
line of battle ship, 3.5 in an auxiliary frigate, 3 in a high speed line of
battle ship, 2.5 in a high speed frigate, 2 in a high speed corvette, and
1.5 in a high speed despatch boat.
607. _Q._--What are the corresponding proportions of screws of four blades?
_A._--The ratios of the pitches to the diameter being for each of the
relative resistances enumerated above, 1.342, 1.425, 1.513, 1.607, 1.705,
1.810, 1.933, 2.080, and 2.243, the respective fractions of pitch or
fractions of a whole convolution will be 0.455, 0.428, 0.402, 0.378, 0.355,
0.334, 0.313, 0.294, and 0.275.
608. _Q._--And what are the corresponding proportions proper for screws of
six blades?
_A._--Beginning with the relative resistance of 5.5 as before, the proper
ratio of pitch to diameter for that and each of the successive resistances
in the case of screws with six blades, will be 1.677, 1.771, 1.891, 1.2009,
2.131, 2.262, 2.416, 2.600, 2.804; and the respective fractions of pitch
will be 0.794, 0.749, 0.703, 0.661, 0.621, 0.585, 0.548, 0.515, and 0.481.
These are the proportions which will give a maximum performance in every
case.[1]
[1] In my Treatise on the Screw Propeller I have gone into these various
questions more fully than would consort with the limits of this
publication.
SCREW VESSELS WITH FULL AND AUXILIARY POWER.
609. _Q._--Do you consider that the screw propeller is best adapted for
vessels of full power, or for vessels with auxiliary power?
_A._--It is, in my opinion, best adapted for vessels with auxiliary power,
and it is a worse propeller than paddle wheels for vessels which have
habitually to encounter strong head winds. Screw vessels are but ill
calculated--at least as constructed heretofore--to encounter head winds,
and the legitimate sphere of the screw is in propelling vessels with
auxiliary power.
610. _Q._--Does the screw act well in conjunction with sails?
_A._--I cannot say it acts better than paddles, except in so far as it is
less in the way and is less affected by the listing or heeling over of the
ship. A small steam power, however, acts very advantageously in aid of
sails, for not only does the operation of the sails in reducing the
resistance of the hull virtually increase the screw's diameter, but the
screw, by reducing the resistance which has to be overcome by the sails and
by increasing the speed of the vessel, enables the sails to act with
greater efficiency, as the wind will not rebound from them with as great a
velocity as it would otherwise do, and a larger proportion of the power of
the wind will also be used up. In the case of beam winds, moreover, the
action of the screw, by the larger advance it gives to the vessel will
enable the sails to intercept a larger column of wind in a given time. It
appears, therefore, that the sails add to the efficiency of the screw, and
that the screw also adds to the efficiency of the sails.
611. _Q._--What is the comparative cost of transporting merchandise in
paddle steamers of full power, in screw steamers of auxiliary power, and in
sailing ships?
_A._--That will depend very much upon the locality where the comparison is
made. In the case of vessels performing distant ocean voyages, in which
they may reckon upon the aid of uniform and constant winds, such as the
trade winds or the monsoon, sailing ships of large size will be able to
carry more cheaply than any other species of vessel. But where the winds
are irregular and there is not much sea room, or for such circumstances as
exist in the Channel or Mediterranean trades, screw vessels with auxiliary
power will constitute the cheapest instrument of conveyance.
612. _Q._--Are there any facts recorded illustrative of the accuracy of
this conclusion?
_A._--A full paddle vessel of 1000 tons burden and 350 horses power, will
carry about 400 tons of cargo, besides coal for a voyage of 500 miles, and
the expense of such a voyage, including wear and tear, depreciation, &c.,
will be about 190_l_. The duration of the voyage will be about 45-1/2
hours. A screw vessel of 400 tons burden and 100 horses power, will carry
the same amount of cargo, besides her coals, on the same voyage, and the
expense of the voyage, including wear and tear, depreciation, &c., will be
not much more than 60_l_. An auxiliary screw vessel, therefore, can carry
merchandise at one third of the cost of a full-powered paddle vessel. By
similar comparisons made between the expense of conveying merchandise in
auxiliary screw steamers and sailing ships on coasting voyages, it appears
that the cost in screw steamers is about one third less than in the sailing
ships; the greater expedition of the screw steamers much more than
compensating for the expense which the maintenance of the machinery
involves.
SCREW AND PADDLES COMBINED.
613. _Q._--Would not a screw combined with paddles act in a similarly
advantageous way as a screw or paddles when aided by the wind?
_A._--If in any given paddle vessel a supplementary screw be added to
increase her power and speed, the screw will act in a more beneficial
manner than if it had the whole vessel to propel itself, and for a like
reason the paddles will act in a more beneficial manner. There will be less
slip both upon the paddles and upon the screw than if either had been
employed alone; but the same object would be attained by giving the vessel
larger paddles or a larger screw.
614. _Q._--Have any vessels been constructed with combined screw and
paddles?
_A._--Not any that I know of, except the great vessel built under the
direction of Mr. Brunel. The Bee many years since was fitted with both
screw and paddles, but this was for the purpose of ascertaining the
relative efficiency of the two modes of propulsion, and not for the purpose
of using both together.
615. _Q._--What would be the best means of accelerating the speed of a
paddle vessel by the introduction of a supplementary screw?
_A._--If the vessel requires new boilers, the best course of procedure
would be to work a single engine giving motion to the screw with high
pressure steam, and to let the waste steam from the high pressure engine
work the paddle engines. In this way the power might be doubled without any
increased expenditure of fuel per hour, and there would be a diminished
expenditure per voyage in the proportion of the increased speed.
616. _Q._--What would the increased speed be by doubling the power?
_A._--The increase would be in the proportion of the cube root of 1 to the
cube root of 2, or it would be 1.25 times greater. If, therefore, the
existing speed were 10 miles, it would be increased to 12-1/2 miles by
doubling the power, and the vessel would ply with about a fourth less coals
by increasing the power in the manner suggested.
617. _Q._--Is not high pressure steam dangerous in steam vessels?
_A._--Not necessarily so, and it has now been introduced into a good number
of steam vessels with satisfactory results. In the case of locomotive
engines, where it is used so widely, very few accidents have occurred; and
in steam vessels the only additional source of danger is the salting of the
boiler. This may be prevented either by the use of fresh water in the
boiler, or by practising a larger amount of blowing off, to insure which it
should be impossible to diminish the amount of water sent into the boiler
by the feed pump, and the excess should be discharged overboard through a
valve near the water level of the boiler, which valve is governed by a
float that will rise or fall with the fluctuating level of the water. If
the float be a copper ball, a little water should be introduced into it
before it is soldered or brazed up, which will insure an equality of
pressure within and without the ball, and a leakage of water into it will
then be less likely to take place. A stone float, however, is cheaper, and
if properly balanced will be equally effective. All steam vessels should
have a large excess of boiling feed water constantly flowing into the
boiler, and a large quantity of water constantly blowing off through the
surface valves, which being governed by floats will open and let the
superfluous water escape whenever the water level rises too high. In this
way the boiler will be kept from salting, and priming will be much less
likely to occur. The great problem of steam navigation is the economy of
fuel, since the quantity of fuel consumed by a vessel will very much
determine whether she is profitable or otherwise. Notwithstanding the
momentous nature of this condition, however, the consumption of fuel in
steam vessels is a point to which very little attention has been paid, and
no efficient means have yet been adopted in steam vessels to insure that
measure of economy which is known to be attainable, and which has been
attained already in other departments of engineering in which the benefits
of such economy are of less weighty import. It needs nothing more than the
establishment of an efficient system of registration in steam vessels, to
insure a large and rapid economy in the consumption of fuel, as this
quality would then become the test of an engineer's proficiency, and would
determine the measure of his fame. In the case of the Cornish engines, a
saving of more than half the fuel was speedily effected by the introduction
of the simple expedient of registration. In agricultural engines a like
economy has speedily followed from a like arrangement; yet in both of these
cases the benefits of a large saving are less eminent than they would be in
the case of steam navigation; and it is to be hoped that this expedient of
improvement will now be speedily adopted.
CHAPTER X.
EXAMPLES OF ENGINES.
* * * * *
OSCILLATING PADDLE ENGINES.
618. _Q._--Will you describe the structure of an oscillating engine as made
by Messrs. Penn?
_A._--To do this it will be expedient to take an engine of a given power,
and then the sizes may be given as well as an account of the configuration
of the parts: we may take for an example a pair of engines of 21-1/2 inches
diameter of cylinder, and 22 inches stroke, rated by Messrs. Penn at 12
horses power each. The cylinders of this oscillating engine are placed
beneath the cranks, and, as in all Messrs. Penn's smaller engines, the
piston rod is connected to the crank pin by means of a brass cap, provided
with a socket, by means of which it is cuttered to the piston rod. There is
but one air pump, which is situated within the condenser between the
cylinders, and it is wrought by means of a crank in the intermediate
shaft--this crank being cut out of a solid piece of metal as in the
formation of the cranked axles of locomotive engines. The steam enters the
cylinder through the outer trunnions, or the trunnions adjacent to the
ship's sides, and enters the condenser through the two midship trunnions--a
short three ported valve being placed on the front of the cylinder to
regulate the flow of steam to and from the cylinder in the proper manner.
The weight of this valve on one side of the cylinder is balanced by a
weight hung upon the other side of the cylinder; but in the most recent
engines this weight is discarded, and two valves are used, which balance
one another. The framing consists of an upper and lower frame of cast iron,
bound together by eight malleable iron columns: upon the lower frame the
pillow blocks rest which carry the cylinder trunnions, and the condenser
and the bottom frame are cast in the same piece. The upper frame supports
the paddle shaft pillow blocks; and pieces are bolted on in continuation of
the upper frame to carry the paddle wheels, which are overhung from the
journal.
619. _Q._--What are the dimensions and arrangement of the framing?
_A._--The web, or base plate of the lower frame is 3/4 of an Inch thick,
and a cooming is earned all round the cylinder, leaving an opening of
sufficient size to permit the necessary oscillation. The cross section of
the upper frame is that of a hollow beam 6 inches deep, and about 3-1/2
inches wide, with holes at the sides to take out the core; and the
thickness of the metal is 13/16ths of an inch. Both the upper and the lower
frame is cast in a single piece, with the exception of the continuations of
the upper frame, which support the paddle wheels. An oval ring 3 inches
wide is formed in the upper frame, of sufficient size to permit the working
of the air pump crank; and from this ring feathers run to the ends of the
cross portions of the frame which supports the intermediate shaft journals.
The columns are 1-1/2 inches in diameter; they are provided with collars at
the lower ends, which rest upon bosses in the lower frame, and with collars
at the upper ends for supporting the upper frame; but the upper collars of
two of the corner columns are screwed on, so as to enable the columns to be
drawn up when it is required to get the cylinders out. The cross section of
the bottom frame is also of the form of a hollow beam, 7 inches deep,
except in the region of the condenser, where it is, of course, of a
different form. The depth of the boss for the reception of the columns is a
little more than 7 inches deep on the lower frame, and a little more than 6
inches deep on the upper frame; and the holes through them are so cored
out, that the columns only bear at the upper and lower edges of the hole,
instead of all through it--a formation by which the fitting of the columns
is facilitated.
620. _Q._--What are the dimensions of the condenser?
_A._--The condenser, which is cast upon the lower frame, consists of an
oval vessel 22-1/2 inches wide, by 2 feet 4-1/4 inches long, and 1 foot
10-1/2 inches deep; it stands 9 inches above the upper face of the bottom
frame, the rest projecting beneath it; and it is enlarged at the sides by
being carried beneath the trunnions.
621. _Q._--What are the dimensions of the air pump?
_A._--The air pump, which is set in the centre of the condenser, is 15-1/4
inches in diameter, and has a stroke of 11 inches. The foot valve is
situated in the bottom of the air pump, and its seat consists of a disc of
brass, in which there is a rectangular flap valve, opening upwards, but
rounded on one side to the circle of the pump, and so balanced as to enable
the valve to open with facility. The balance weight, which is formed of
brass cast in the same piece as the valve itself, operates as a stop, by
coming into contact with the disc which constitutes the bottom of the pump;
the disc being recessed opposite to the stop to enable the valve to open
sufficiently. This disc is bolted to the barrel of the pump by means of an
internal flange, and before it can be removed the pump must be lifted out
of its place. The air pump barrel is of brass to which is bolted a cast
iron mouth piece, with a port for carrying the water to the hot well;
within the hot well the delivery valve, which consists of a common flap
valve, is situated. The mouth piece and the air pump barrel are made tight
to the condenser, and to one another, by means of metallic joints carefully
scraped to a true surface, so that a little white or red lead interposed
makes an air tight joint. The air pump bucket is of brass, and the valve of
the bucket is of the common pot lid or spindle kind. The injection water
enters through a single cock in front of the condenser--the jet striking
against the barrel of the air pump. The air pump rod is maintained in its
vertical position by means of guides, the lower ends of which are bolted to
the mouth of the pump, and the upper to the oval in the top frame, within
which the air pump crank works; and the motion is communicated from this
crank to the pump rod by means of a short connected rod. The lower frame is
not set immediately below the top frame, but 2-1/2 inches behind it, and
the air pump and condenser are 2-1/2 inches nearer one edge of the lower
frame than the other.
622. _Q._--What are the dimensions of the cylinder?
_A._--The thickness of the metal of the cylinder is 9/16ths of an inch; the
depth of the belt of the cylinder is 9-1/2 inches, and its greatest
projection from the cylinder is 2-1/2 inches. The distance from the lower
edge of the belt to the bottom of the cylinder is 11-1/2 inches, and from
the upper edge of the belt to the top flange of the cylinder is 9 inches.
The trunnions are 7-1/4 inches diameter in the bearings, and 3-1/2 inches
in width; and the flanges to which the glands are attached for screwing in
the trunnion packings are 1-1/2 inch thick, and have 7/8ths of an inch of
projection. The width of the packing space round the trunnions is 5/8ths of
an inch, and the diameter of the pipe passing through the trunnion
4-5/8ths, which leaves 11/16ths for the thickness of the metal of the
bearing. Above and below each trunnion a feather runs from the edge of the
belt or bracket between 3 and 4 inches along the cylinder, for the sake of
additional support; and in large engines the feather is continued through
the interior of the belt, and cruciform feathers are added for the sake of
greater stiffness. The projection of the outer face of the trunnion flange
from the side of the cylinder is 6-1/2 inches; the thickness of the flange
round the mouth of the cylinder is 3/4 of an inch, and its projection 1-3/8
inch; the height of the cylinder stuffing box above the cylinder cover is
4-1/8 inches, and its external diameter 4-3/8 inches--the diameter of the
piston rod being 2-1/8 inches. The thickness of the stuffing box flange is
1-1/8 inch.
623. _Q._--Will you describe the nature of the communication between the
cylinder and condenser?
_A._--The pipe leading to the condenser from the cylinder is made somewhat
bell mouthed where it joins the condenser, and the gland for compressing
the packing is made of a larger internal diameter in every part except at
the point at which the pipe emerges from it, where it accurately fits the
pipe so as to enable the gland to squeeze the packing. By this construction
the gland may be drawn back without being jammed upon the enlarged part of
the pipe; and the enlargement of the pipe toward the condenser prevents the
air pump barrel from offering any impediment to the free egress of the
steam. The gland is made altogether in four pieces: the ring which presses
the packing is made distinct from the flange to which the bolts are
attached which force the gland against the packing, and both ring and
flange are made in two pieces, to enable them to be got over the pipe. The
ring is half checked in the direction of its depth, and is introduced
without any other support to keep the halves together, than what is
afforded by the interior of the stuffing box; and the flange is half
checked in the direction of its thickness, so that the bolts which press
down the ring by passing through this half-checked part, also keep the
segments of the flange together. The bottom of the trunnion packing space
is contracted to the diameter of the eduction pipe, so as to prevent the
packing from being squeezed into the jacket; but the eduction pipe does not
fit quite tight into this contracted part, but, while in close contact on
the lower side, has about 1/32nd of an inch of space between the top of the
pipe and the cylinder, so as to permit the trunnions to wear to that extent
without throwing a strain upon the pipe. The eduction pipe is attached to
the condenser by a flange joint, and the bolt holes are all made somewhat
oblong in the perpendicular direction, so as to permit the pipe to be
slightly lowered, should such an operation be rendered necessary by the
wear of the trunnion bearings; but in practice the wear of the trunnion
bearings is found to be so small as to be almost inappreciable.
624. _Q._--Will you describe the valve and valve casing?
_A._--The length of the valve casing is 16-1/2 inches, and its projection
from the cylinder is 3-1/2 inches at the top, 4-1/4 inches at the centre,
and 2-1/2 inches at the bottom, so that the back of the valve casing is not
made flat, but is formed in a curve. The width of the valve casing is 9
inches, but there is a portion of the depth of the belt 1-1/2 inch wider,
to permit the steam to enter from the belt into the casing. The valve
casing is attached to the cylinder by a metallic joint; the width of the
flange of this joint is 1-1/4 inch, the thickness of the flange on the
casing 1/2 inch, and the thickness of the flange on the cylinder 5/8ths of
an inch. The projection from the cylinder of the passage for carrying the
steam upwards, and downwards, from the valve to the top and bottom of the
cylinder, is 2-1/4 inches, and its width externally 8-5/8 inches. The valve
is of the ordinary three ported description, and both cylinder and valve
faces are of cast iron.
625. _Q._--What description of piston is used?
_A._--The piston is packed with hemp, but the junk ring is made of
malleable iron, as cast iron junk rings have been found liable to break:
there are four plugs screwed into the cylinder cover, which, when removed,
permit a box key to be introduced, to screw down the piston packing. The
screws in the junk ring are each provided with a small ratchet, cut in a
washer fixed upon the head, to prevent the screw from turning back; and the
number of clicks given by these ratchets, in tightening up the bolts,
enables the engineer to know when they have all been tightened equally. In
more recent engines, and especially in those of large size, Messrs. Penn
employ for the piston packing a single metallic ring with tongue piece and
indented plate behind the joint; and this ring is packed behind with hemp
squeezed by the junk ring as in ordinary hemp-packed pistons.
626. _Q._--Will you describe the construction of the cap for connecting the
piston rod with the crank pin?
_A._--The cap for attaching the piston rod to the crank pin, is formed
altogether of brass, which brass serves to form the bearing of the crank
pin. The external diameter of the socket by which this cap is attached to
the piston rod is 3-5/16 inches. The diameter of the crank pin is 3 inches,
and the length of the crank pin bearing 3-7/8 inches. The thickness of the
brass around the crank pin bearing is 1 inch, and the upper portion of the
brass is secured to the lower portion, by means of lugs, which are of such
a depth that the perpendicular section through the centre of the bearing
has a square outline measuring 7 inches in the horizontal direction, 3-7/8
inches from the centre of the pin to the level of the top of the lugs, and
2-1/2 inches from the centre of the pin to the level of the bottom of the
lugs. The width of the lugs is 2 inches, and the bolts passing through them
are 1-1/4 inch in diameter. The bolts are tapped into the lower portion of
the cap, and are fitted very accurately by scraping where they pass through
the upper portion, so as to act as steady pins in preventing the cover of
the crank pin bearing from being worked sideways by the alternate thrust on
each side. The distance between the centres of the bolts is 5 inches, and
in the centre of the cover, where the lugs, continued in the form of a web,
meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, and
provided with an internal pipe, is cast upon the cover, to contain oil for
the lubrication of the crank pin bearing. The depth of the cutter for
attaching the cap to the piston rod is 1-1/4 inch and its thickness is
3/8ths of an inch.
627. _Q._--Will you describe the means by which the air pump rod is
connected with the crank which works the air pump?
[Illustration: Fig. 50. AIR PUMP CONNECTING ROD AND CROSS HEAD. Messrs.
Penn.]
_A._--A similar cap to that of the piston rod attaches the air pump crank
to the connecting rod by which the air pump rod is moved, but in this
instance the diameter of the bearing is 5 inches, and the length of the
bearing is about 3 inches. The air pump connecting rod and cross head are
shown in perspective in fig. 50. The thickness of the brass encircling the
bearing of the shaft is three fourths of an inch upon the edge, and 1-1/8
inch in the centre, the back being slightly rounded; the width of the lugs
is 1-5/8 inch, and the depth of the lugs is 2 inches upon the upper brass,
and 2 inches upon the lower brass, making a total depth of 4 inches. The
diameter of the bolts passing through the lugs is 1 inch, and the bolts are
tapped into the lower brass, and accurately fitted into the upper one, so
as to act as steady pins, as in the previous instance. The lower eye of the
connecting rod is forked, so as to admit the eye of the air pump rod; and
the pin which connects the two together is prolonged into a cross head, as
shown in fig. 50. The ends of this cross head move in guides. The forked
end of the connecting rod is fixed upon the cross head by means of a
feather, so that the cross head partakes of the motion of the connecting
rod, and a cap, similar to that attached to the piston rod, is attached to
the air pump rod, for connecting it with the cross head. The diameter of
the air pump rod is 1-1/2 inch, the external diameter of the socket
encircling the rod is 2-1/8 inches, and the depth of the socket 4-1/2
inches from the centre of the cross head. The depth of the cutter for
attaching the socket to the rod is 1 inch, and its thickness 5/16 inch. The
breadth of the lugs is 1-3/8 inch, the depth 1-1/4 inch, making a total
depth of 2-1/2 inches; and the diameter of the bolts seven eighths of an
inch. The diameter of the cross head at the centre is 2 inches, the
thickness of each jaw around the bearing 1 inch, and the breadth of each
9/16 inch.
628. _Q._--What are the dimensions of the crank shaft and cranks?