early engineers as a means of propulsion. Long after the experiment of
H. B. M. S. Rattler had demonstrated the contrary, the public faith
in the visible wheel was greater in reality and more sincere than
that in the invisible screw; and it is probable that it was more the
question of cost than anything else that gained the victory for the
screw for ocean and general service. The paddle engine is in itself
heavier and occupies more room than the screw engine; it is as a rule
more expensive per I. H.-P.; and in wear and tear—especially of the
propeller itself—it far exceeds the screw. It occupies the best part of
the ship, and its position is not a matter of choice, as with the screw
engine, but is, of necessity, at or near the middle of the ship.[14]
It is evident that a paddle steamer must require more room, and that
in moving among ships or other obstructions the liability to damage
the propeller is greater than with the screw steamer, and in the case
of a long voyage the paddle generally worked at a disadvantage, as
at the commencement it was too deeply immersed, and at the end not
immersed enough for efficient working. If the sails were set so as to
steady the vessel, or if set in sufficient quantity to be of any use in
quickening the speed, she was inclined until the lee wheel was “buried”
and the “weather” wheel doing very little work; besides there was a
general tendency on the part of the ship to turn round, which had to be
counterbalanced by the rudder. The race of water from the wheels past
the ship being at a high velocity, and raised above the normal level,
causes a resistance to the ship beyond that due to her passage through
the water, as in the case of a screw ship. On the other hand, the
paddle boat is more readily got into motion and her speed more rapidly
arrested than is the case with the screw steamer; and it is claimed for
the paddle-wheel—although the foundation for such a claim is rather
nebulous—that when the engines are working at full speed the ship is
prevented from the excessive rolling observable with a screw vessel.
But against this it must not be forgotten that the paddle engine is
far more trying to the structure of the ship, on account of the great
weight of the wheels being taken on the sides of the hull, as well as
from the effort of the wheels in propelling being applied at the same
place. Then there is the additional danger, and that not a remote one,
that in case of the shaft breaking and a wheel falling clear of the
ship, she would upset. An accident of this kind has occurred more than
once, but there is no record of the actual result being so calamitous
as just stated, owing to other fortuitous circumstances. That which
retains the paddle-wheel in favor to-day, and renders it a necessity
in spite of argument or prejudice, is the fact that the screw requires
that the draft of the ship shall not be less than its own diameter,
whereas in the largest paddle boats a dip of wheel of six feet is
generally sufficient. Hence it is that nearly all fast steamers plying
on rivers or shallow estuaries, and channel steamers running to ports
where there is little water when the tide is low, are of necessity
paddle-wheel. By employing two screws (one on each side instead of one
amidships) the draft of water can be reduced by at least thirty per
cent. Likewise, by increasing the number of revolutions smaller
screws will do, and the draft of water may be still less, so that
some thirty years ago, on the introduction of twin-screws, there were
soon many ships built for services that had hitherto been monopolized
by paddle boats;[15] and to-day, when there is a demand for higher
speed and more power, and where paddle-wheels are not admissible,
three screws are being employed. Ships have also been employed with
four screws, viz., two at the bow and two at the stern, and, for the
purpose for which they were required, answered very well indeed; but
the worst possible place for a propeller is obviously at the bow, and
therefore in these ships the bow screws were not very efficient, but
they undoubtedly added somewhat to the power of the ship. In the same
way some tug-boats have been fitted with a screw at each end.
All attempts at propulsion with internal propellers—that is, by turbine
wheels, pulsometers, ejectors, or by pumps—have failed in consequence
of the great friction set up by the water in its rapid passage through
the pipes from and to the sea; the motion must be rapid owing to the
size of the pipes being necessarily restricted. The best experiment
with this kind of propeller was made on a costly scale by the British
Admiralty in 1866, when they fitted the iron-clad gun-boat Waterwitch,
of 1,200 tons displacement, with a Ruthven’s hydraulic propeller,
consisting of a horizontal turbine wheel drawing its water through
the bottom of the ship and discharging it fore-and-aft-ways at each
side, and driven by an engine of 160 nominal horse-power; and although
this vessel was only 162 feet long, 32 feet broad, and drew 11 feet
4 inches of water, her speed was only a little over 9 knots, with an
indicated horse-power of 801. The speed co-efficients whereby her
performances could be compared with that of other ships were most
disappointing.
[Illustration: The Twin Screws of the City of New York]
But the achievements of screw steamers are not always satisfactory at
first, and time has shown some curious instances where what appeared
at first sight a little thing prevented great results. To-day we know
somewhat of the screw propeller, but it is very difficult, if not
impossible, for the cleverest and most experienced engineer to define
his knowledge or to classify his facts so as to deduce any rules from
them that shall enable him to lay down fixed laws for the practical
guidance of others. In past years more was professed, but still less
was actually known, and that which was to be a panacea for the ills
of every screw ship proved useless in many instances, and aggravated
the evil in others. The patents for propellers are numerous, and some
of the specifications interesting and amusing, but of them all there
are less than can be counted on the fingers of one hand that have any
practical value, or that have influenced the commerce of the world;
and we find to-day that the propeller which gives the best results
is very simple in form and its working surface a true helix. What is
better understood, however, are the proportions, and in them lies the
success of the instrument. It is quite true that the blades may be of
such a shape and so arranged as to give bad results, but it is very
difficult to alter the propeller blade now most generally used and get
much improvement thereby.
[Illustration: The Propeller of the North German Lloyd Steamer Havel.
(From a photograph of the steamship in Handren & Robins’s Erie Basin
Dry Docks, Brooklyn.)]
In 1865 H. B. M. S. Amazon was found to fall short of her designed
speed by nearly a knot, although the indicated horse-power was in
excess of the requirements. With a four-bladed Mangin propeller, 12
feet 6 inches pitch, it took 1,940 I. H.-P. to drive the vessel 12
knots. A two-bladed Griffith’s screw of 13 feet 9 inches pitch was
substituted, when 12.4 knots were obtained with only 1,664 I. H.-P.
But the most remarkable case was that of H. B. M. S. Iris, which had
been designed for a speed of 17-1/2 knots, but on her first trial trip,
although the 7,000 I. H.-P. was exceeded, the speed was only 16.58
knots. A series of trials was then entered upon to find out the cause
of this deficiency, with the result that the screws were discovered to
be too large; others of 2 feet 3 inches less diameter were substituted,
when a speed of 18.57 knots was attained with the same I. H.-P. Similar
instances could be adduced, if necessary, to show how comparatively
slight changes in the propeller can produce marked improvements in
speed.
It has already been shown that the frictional resistance of the skin
of the ship is very great, and generally speaking, in fast steamers,
is by far the largest portion of the whole resistance. It necessarily
follows, therefore, that for high speed it is essential that the
submerged portion shall be as smooth as possible; and to that end ships
are coated with enamel paints which, when dry, are perfectly smooth and
glassy, or remain in a smooth, slimy condition. They do not, however,
remain long in this state, as the action of sea-water destroys them,
and even the best of these compositions admits, at times, of marine
plant growth, and sometimes barnacles. The effect of a coating of weed
is very serious indeed; the resistance induced thereby being greater
than if the vessel were rough, from the fact that each filament of
weed has to be towed through the water, and the total surface thereby
exposed may be two or three times that of the ship herself. It is
a sound economy in any vessel to keep the bottom perfectly clean
and smooth, but in the case of high-speed steamers it is absolutely
essential, inasmuch as a very moderate amount of foulness will reduce
their speed by 2 or 3 knots.
The introduction of Siemens-Martin steel, about the year 1875, and
its continued and extended use since, have however been really the
means of rendering possible the construction of steamships of all
sizes with high rates of speed now so common, and are undoubtedly the
means whereby those ships can be so economically built and worked as
to pay as commercial ventures. The construction of their hulls with a
material fifty per cent. stronger than iron has rendered it possible to
make such appreciable decrease in weight as to admit of fining their
lines suitably for high speed without sacrificing carrying capacity.
With this same steel, boilers can be constructed for a pressure of
150 pounds per square inch without weighing very much more than iron
ones for 75 pounds. By using steel for castings, forgings, etc., the
weight of the machinery has been reduced from 5 hundredweight to 2
hundredweight per I. H.-P., and when forced draught is employed it is
as low as 1.6 hundredweight per I. H.-P. for large powers, and less
still for such engines as are used in torpedo boats and catchers.
[Illustration: Recent Naval Engine.
(Made by Earle’s Shipbuilding Company, Hull, England, for twin-screw
fast cruiser for the British Navy, of 9,000 I. H.-P.)]
It has already been remarked that the consumption of coal, which
enters as a most important factor into the question of high speed,
both from the weight and cost, had been reduced, by the introduction
of the compound engine, from 4 pounds to 2-1/2 pounds per I. H.-P.,
and latterly, as that engine was improved and higher pressures used,
the consumption was further reduced to 2 pounds, and in some cases as
low as 1-3/4 pound per I. H.-P. The triple expansion engine, developed
within the past eight years, and later the quadruple expansion, have
effected a still further saving, until with them and such other means
as are now employed, the consumption is under 1-1/2 pound of coal per
I. H.-P.
The success of the locomotive was very questionable until the exhaust
steam was turned into the chimney so as to create a rapid draught,
and the steam-blast to-day enables the locomotive to travel at its
great speed by causing the comparatively small boiler to generate
such a large amount of steam. When this form of boiler was tried
on board ship its power would have been very much crippled had not
some other means been adopted for forcing the draught, as the steam
could not in this case be allowed to escape through the funnel, but
must be condensed into water for the use of the boiler. By closing
the stoke-hole and forcing into it by mechanical means a plentiful
supply of air, this boiler was made to be as efficient for a torpedo
boat as for a locomotive. This forced draught has now been adopted on
large ships, and to-day the very high speed of naval vessels, and of
many mercantile steamers, is due to it. Consequently, with the same
weight of machinery, higher powers are developed with a corresponding
increase in speed, and the cruiser Piemonte, constructed by Sir William
Armstrong & Co., of which an illustration is shown on p. 91, had her
speed increased by means of forced draught from 20 knots to 22.3 knots,
at which speed she was going when the picture was taken.
Mr. James Howden patented a forced draught process by which the
incoming air is warmed by the heat (which would otherwise be wasted) in
the uptakes and funnels, and then conducted direct to the furnaces; and
he claims by this to be able to do with still smaller boilers, besides
avoiding the danger to the tubes now sometimes experienced in war ships
with closed stoke-holes.
[Illustration: Italian Cruiser Piemonte at Full Speed—22.3 knots=25-3/4
miles per hour.]
But there still remains the problem of how to feed the furnaces by
mechanical methods, so as to save the very large staff now required
in the boiler-room of our large steamships. So far all means hitherto
adopted with success on shore have proved failures at sea, and at
present there is no reason to suppose that any one of them can be so
adapted as to prove generally efficient for service. It is necessary
for such a purpose that the gear can go continuously for many days,
and the coal be small and tolerably uniform, and the supply regular.
Such coal is not convenient for passenger ships, and if the demand
for the present supply of small coal were increased the price would
preclude its use. Some success, however, has been achieved in saving
labor in the stoke-hole, and the most noticeable invention to this end
is that of Mr. Thomas Henderson, whose now well-known self-cleaning
fire-bars do away with the necessity for the firemen raking the fires
out to remove the clinkers which adhere to the grates and obstruct the
air-passages. By means of this apparatus, the alternate bars having a
very slight movement, the coal gradually travels to the back end of the
grate together with the clinker, which latter is eventually deposited
behind the bridges. Thus not only is considerable labor saved, but the
fires are always in such good condition that the full pressure of steam
is maintained, and so a better speed kept up by the vessel herself.
On shore the tendency is to substitute gas for solid fuel, or to use
the coke resulting from gas manufacture. That something of the same
kind might be done on shipboard is possible, although not at present
probable. The higher efficiency of the coal when treated in this way
would enable still more power to be obtained from a pound of it, and
there would be savings in other ways of a beneficial nature.
Then, again, if petroleum, or other liquid of a similar nature, could
be obtained at a fairly low price, it might be used on shipboard; and
as it has a heating power twenty-five per cent. higher than the best
coal, and fifty per cent. higher than some of the commonest kinds
weight for weight, the substitution of it would be a means of obtaining
better speed. But it is always a question of _cui bono_, and when it
is taken into consideration that the voyage between Sandy Hook and
Queenstown is now done in 140 hours, and to do the distance in 5 days
would require a speed of nearly 23-1/2 knots, with an increase in power
of sixty-two per cent., and in fuel consumption of thirty-eight per
cent., the cry must be regarded as a very far one at present. At the
same time it is not desirable to believe that there is now finality in
the speed of steamships, although by analogy with railway trains that
conclusion might be arrived at.
Footnotes:
[10] This, however, is not an absolute test of the fineness of the
_water-lines_ of a vessel, and it can only be used as such on the
assumption that the midship sections of ships are of similar form.
The best test of the fineness of water-lines is made by taking the
displacement as a percentage of the prism whose length is that of
the ship and whose section is the same as the midship section of a
ship, assuming, however, that the midship section of all ships is
approximately that found in general practice to-day; in speaking of
coefficients it will mean the percentage of the rectangular block above
named.
[11] More than thirty years ago this matter had been observed by the
officers of the British navy, and experiments were ordered to be tried
with H. B. M. S. Flying Fish, a 1,100-ton cruiser, her length being 200
feet, breadth 30 feet 4 inches, and her draft of water 10 feet 6 inches
forward and 13 feet aft. With 1,290 I. H.-P. her speed was only 11.64
knots, whereas with 577 I. H.-P. it was 9.923 knots, and a speed of
11.201 was obtained with but 878 I. H.-P. A false bow 18 feet long was
then fitted, so as to give finer lines forward, or, as sailors describe
it, “a better entrance,” when it was found that with 1,285 I. H.-P. a
speed of 12-1/2 knots was attained, and with 1,345 very nearly 12-3/4
knots. There is also every reason to suppose that could the stern have
been altered in a similar way, the speed would have been still higher,
in spite of the ship being larger and with a consequent increase of
immersed surface to cause resistance. It has, besides, been observed on
many occasions that when steamers have been cut in two and lengthened
there has been no diminution of the speed, but, on the contrary,
in some cases there has actually been a gain; so that in these two
instances there is an apparent anomaly, viz., that with the same power
the larger ship is propelled at a quicker speed.
The late Dr. Froude investigated this matter some years ago, and
showed that such results were quite possible, independently of any
fining of the lines, owing to the effect on the ship of the waves set
up when in motion. One very curious illustration of how such waves
may seriously affect a vessel is in that of a yacht built many years
ago by an eminent firm on the Clyde, which failed to come anywhere
near the performances guaranteed owing to the fact that as the speed
increased the hollow following the wave formed at the bow increased
and approached nearer and nearer to the paddle-wheels, until the water
dropped below the floats and allowed the wheels to spin in the air;
the propelling effect was thus entirely lost until the vessel slowed
down sufficiently for the water to rise again to the level of the
paddle-wheels. Such a thing could scarcely happen with a screw steamer;
but the very bad steering qualities of certain naval ships is due to
the fact that the inrush of water at the stern causes currents to flow
_with_ the ship, and therefore to produce quite different results with
the rudder from those which generally obtain.
[12] A nautical mile is 6,080 feet, the land mile being 5,280 feet. The
knot is a measure of _rate_ of speed per hour. A vessel makes 20 knots
when she is travelling at the rate of 20 nautical miles per hour.
[13] The dimensions, speed, etc., of the steamers here referred to, as
well as other representative steamers from 1836 to 1890, are shown in
the table on page 78.
[14] In the case of river steamers of moderate size there is not the
same restriction on the position of the wheel, and as a matter of fact,
as in the case of stern-wheelers, it is altogether at one end.
[15] It is now claimed for the twin-screw ship that she is not only
capable of entering shallower harbors, but that she is in every way
much safer, and it is most unfortunate that, owing to an act of
carelessness, this was not conclusively shown in the recent accident
to the City of Paris. But there is safety in the twin-screw beyond
that which is rendered possible, as in the cases of the City of Paris
and Majestic, by the division of the engine-rooms, viz., the fact that
if one engine breaks down it is improbable that the other would do
so at the same time, and that the vessel, although somewhat crippled
in speed, would still be able to pursue her voyage; also, that in
the event of accident to the steering apparatus the passage could be
continued and the direction of the ship guided by regulating with one
or both of the engines. Each of these features is pronounced, and the
advantages have been proved on many occasions.
THE BUILDING OF AN “OCEAN GREYHOUND.”
BY WILLIAM H. RIDEING.
THE COST OF AN OCEAN RACER—INTRICATE “FINANCING” OF SUCH AN
UNDERTAKING—THE CONTRACT WITH THE SHIP-BUILDERS—THE UNCERTAIN ELEMENT
IN DESIGNING—GREAT SHIP YARDS ALONG THE CLYDE—THE PLANS OF A STEAMER
ON PAPER—ENLARGEMENT OF PLANS IN THE “MOULD LOFT”—WHAT IS MEANT BY
“FAIRING THE SHIP”—THE “SCRIVE BOARD”—LAYING DOWN THE KEEL—MAKING
THE HUGE RIBS—WHEN A SHIP IS “IN FRAME”—SHAPING AND TRIMMING THE
PLATES—RIVETING AND CAULKING—READY FOR LAUNCHING—THE GREAT “PLANT”
WHICH IS NECESSARY FOR THE BUILDING OF A SHIP—DESCRIPTION OF A
TYPICAL YARD—WORKS COVERING SEVENTY-FOUR ACRES—WHERE THE SHAFT IS
FORGED—THE LATHES AT WORK—THE ADJUSTMENT OF PARTS—SEVEN THOUSAND
WORKMEN.
I.
As often as the “record is broken,” and the Atlantic voyage is reduced
by some unprecedentedly fast passage, we may be sure that there is a
flutter in the offices of the rival lines which have thus been left
behind. Between the Cunard, the Guion, the Inman, and the White Star
lines there has been a constant race for supremacy, now one, and then
the other, taking the first place. No ship has been allowed to keep
the lead for more than a year or two. When sixteen knots have been
developed by one line, seventeen knots have been aimed at by another,
and the ship of that speed is no longer a wonder. So when we read in
the newspapers of the “fastest passage” we may take it for granted that
it is no sooner heard of in Liverpool than the managers of the lines
momentarily surpassed are preparing to beat it. If the triumph belongs
to the Cunard line, at the very next meeting of the directors of the
White Star and Inman lines it will be discussed, and though an order
for another ship may not be given there and then, it is sure to follow.
An order for a new ship of the class required to compete in the
modern passenger service of the Atlantic is not by any means a matter
to be determined on without grave consideration. Speed is costly,
and as you increase it it is generally necessary to also increase
the tonnage. Thus if the problem before you is to beat the record
of a seven-thousand-ton ship, which has developed eighteen knots
with engines of twelve thousand five hundred horse-power, you must
(principally for economic reasons) have a larger hull as well as more
powerful engines for your competing vessel. This forces upon your
consideration tides, channels, harbor-bars, and dock accommodations,
all of which impose limitations upon you. And then the cost of the
ship herself is not a matter which even the wealthiest of corporations
can provide for at a moment’s notice: it is not one hundred thousand
dollars, or five hundred thousand dollars that the work calls for, but
about five times the latter sum, for it is safe to say that a vessel
superior to the City of New York or the Etruria could not be built for
less than two million and a half of dollars.
The “financing” of such an undertaking requires time: there are long
consultations between the directors, bankers, and ship-builders. If
we could follow the steps of the gentleman to whom these negotiations
are intrusted, we might see him flying off from Liverpool for Euston:
closeted in a private office down in Lombard Street or Cornhill with
some capitalists who are expected to contribute to the necessary funds;
again, after dinner, engaged in argument with these same capitalists in
a West End mansion to which they have adjourned, and then racing off in
the precarious hansom cab to catch the night train from King’s Cross
for Glasgow.