revolving upon a shaft driven by the engine, and carrying upon its
circumference a number of flat boards, called paddle-boards, which
are secured by nuts and braces in a fixed position; and that
position is such that the planes [Pg473] of the paddle-boards
diverge nearly from the centre of the shaft on which the wheel
turns. The consequence of this arrangement is that each
paddle-board can only act in that direction which is most
advantageous for the propulsion of the vessel when it arrives near
the lowest point of the wheel. In _fig._ 132. let O be the shaft
on which the common paddle-wheel revolves; the position of the
paddle-boards are represented at A, B, C, &c.; X, Y represents the
water line, the course of the vessel being supposed to be from X
to Y; the arrows represent the direction in which the paddle-wheel
revolves. The wheel is immersed to the depth of the lowest
paddle-board, since a less degree of immersion would render a
portion of the surface of each paddle-board mechanically useless.
In the position A the whole force of the paddle-board is efficient
for propelling the vessel; but as the paddle enters the water in
the position H, its action upon the water, not being horizontal,
is only partially effective for propulsion: a part of the force
which drives the paddle is expended in depressing the water, and
the remainder in driving it contrary to the course of the vessel,
and, therefore, by its re-action producing a certain propelling
effect. The tendency, however, of the paddle entering the water at
H, is to form a hollow or trough, which the water, by its ordinary
property, has a continual tendency to fill up. After passing the
lowest point A, as the paddle approaches the position B, where it
[Pg474] emerges from the water, its action again becomes oblique,
a part only having a propelling effect, and the remainder having a
tendency to raise the water, and throw up a wave and spray behind
the paddle-wheel. It is evident that the more deeply the
paddle-wheel becomes immersed, the greater will be the proportion
of the propelling power thus wasted in elevating and depressing
the water; and if the wheel were immersed to its axis, the whole
force of the paddle-boards, on entering and leaving the water,
would be lost, no part of it having a tendency to propel. If a
still deeper immersion take place, the paddle-boards above the
axis would have a tendency to retard the course of the vessel.
When the vessel is, therefore, in proper trim, the immersion
should not exceed nor fall short of the depth of the lowest
paddle; but for various reasons it is impossible in practice to
maintain this fixed immersion: the agitation of the surface of the
sea, causing the vessel to roll, will necessarily produce a great
variation in the immersion of the paddle-wheels, one becoming
frequently immersed to its axle, while the other is raised
altogether out of the water. Also the draught of water of the
vessel is liable to change, by the variation in her cargo; this
will necessarily happen in steamers which take long voyages. At
starting they are heavily laden with fuel, which as they proceed
is gradually consumed, whereby the vessel is lightened.
(223.) To remove this defect, and economise as much as possible
the propelling effect of the paddle-boards, it would be necessary
so to construct them that they may enter and leave the water
edgeways, or as nearly so as possible; such an arrangement would
be, in effect, equivalent to the process called feathering, as
applied to oars. Any mechanism which would perfectly accomplish
this would cause the paddles to work in almost perfect silence,
and would very nearly remove the inconvenient and injurious
vibration which is produced by the action of the common paddles.
But the construction of feathering paddles is attended with great
difficulty, under the peculiar circumstances in which such wheels
work. Any mechanism so complex that it could not be easily
repaired when deranged, with such engineering implements and
skill [Pg475] as can be obtained at sea, would be attended with
great objections; and the efficiency of its propelling action
would not compensate for the dangers which must attend upon the
helpless state of a steamer, deprived of her propelling agents.
Feathering paddle-boards must necessarily have a motion
independently of the motion of the wheel, since any fixed position
which could be given to them, though it might be most favourable to
their action in one position would not be so in their whole course
through the water. Thus the paddle-board when at the lowest point
should be in a vertical position, or so placed that its plane, if
continued upwards, would pass through the axis of the wheel. In
other positions, however, as it passes through the water, it should
present its upper edge, not towards the axle of the wheel, but
towards a point above the highest point of the wheel. The precise
point to which the edge of the paddle-board should be directed is
capable of mathematical determination. But it will vary according to
circumstances, which depend on the motion of the vessel. The
progressive motion of the vessel, independently of the wind or
current, must obviously be slower than the motion of the
paddle-boards round the axle of the wheel; since it is by the
difference of these velocities that the re-action of the water is
produced by which the vessel is propelled. The proportion, however,
between the progressive speed of the vessel and the rotative speed
of the paddle-boards is not fixed: it will vary with the shape and
structure of the vessel, and with its depth of immersion;
nevertheless it is upon this proportion that the manner in which the
paddle-boards should shift their position must be determined. If the
progressive speed of the vessel were nearly equal to the rotative
speed of the paddle-boards, the latter should so shift their
position that their upper edges should be presented to a point very
little above the highest point of the wheel. This is a state of
things which could only take place in the case of a steamer of a
small draught of water, shallop-shaped, and so constructed as to
suffer little resistance from the fluid. On the other hand, the
greater the depth of immersion, and the less fine the lines of the
[Pg476] vessel, the greater will be the resistance in passing
through the water, and the greater will be the proportion which the
rotative speed of the paddle-boards will bear to the progressive
speed of the vessel. In this latter case the independent motion of
the paddle-boards should be such that their edges, while in the
water, shall be presented towards a point considerably above the
highest point of the paddle-wheel.
A vast number of ingenious mechanical contrivances have been
invented and patented for accomplishing the object just explained.
Some of these have failed from the circumstance of their inventors
not clearly understanding what precise motion it was necessary to
impart to the paddle-board: others have failed from the complexity
of the mechanism by which the desired effect was produced.
(224.) In the year 1829 a patent was granted to Elijah Galloway
for a paddle-wheel with movable paddles, which patent was
purchased by Mr. William Morgan, who made various alterations in
the mechanism, not very materially departing from the principle of
the invention.
[Illustration: _Fig._ 133.]
This paddle-wheel is represented in _fig._ 133. The contrivance
may be shortly stated to consist in causing the wheel which bears
the paddles to revolve on one centre, and the radial arms which
move the paddles to revolve on another centre. Let A B C D E F G H
I K L be the polygonal circumference of the paddle-wheel, formed
of straight bars, securely connected together at the extremities
of the spokes or radii of the wheel which turns on the shaft which
is worked by the engine; the centre of this wheel being at O. So
far this wheel is similar to the common paddle-wheel; but the
paddle-boards are not, as in the common wheel, fixed at A B C,
&c., so as to be always directed to the centre O, but are so
placed that they are capable of turning on axles which are always
horizontal, so that they can take any angle with respect to the
water which may be given to them. From the centres, or the line
joining the pivots on which these paddle-boards turn, there
proceed short arms K, firmly fixed to the paddle-boards at an
angle of about 120°. On a motion given to this arm K, it will
therefore give a corresponding angular motion to the paddle-board,
so as to make it turn on its pivots. At [Pg477] the extremities
of the several arms marked K is a pin or pivot, to which the
extremities of the radial arms L are severally attached, so that
the angle between each radial arm L and the short paddle-arm K is
capable of being changed by any motion imparted to L; the radial
arms are connected at the other end with a centre, round which
they are capable of revolving. Now, since the points A B C, &c.,
which are the pivots on which the paddle-boards turn, are moved in
the circumference of a circle, of which the centre is O, they are
always at the same distance from that point; consequently they
will continually vary their distance from the other centre P.
Thus, when a paddle-board arrives at that point of its revolution
at which the centre round which it revolves lies precisely between
it and the centre O, its distance from the former centre is less
than in any other position. As it departs from that point, its
distance from that centre gradually increases until it arrives at
the opposite point of its revolution, where the centre O is
exactly between it and the former centre; then the distance of the
paddle-board from the former centre is greatest. [Pg478] This
constant change of distance between each paddle-board and the
centre P is accommodated by the variation of the angle between the
radial arm L and the short paddle-board arm K; as the paddle-board
approaches the centre P this gradually diminishes; and as the
distance of the paddle-board increases, the angle is likewise
augmented. This change in the magnitude of the angle, which thus
accommodates the varying position of the paddle-board with respect
to the centre P, will be observed in the figure. The paddle-board
D is nearest to P; and it will be observed that the angle
contained between L and K is there very acute; at E the angle
between L and K increases, but is still acute; at G it increases
to a right angle; at H it becomes obtuse; and at K, where it is
most distant from the centre P, it becomes most obtuse. It again
diminishes at K, and becomes a right angle between A and B. Now
this continual shifting of the direction of the short arm K is
necessarily accompanied by an equivalent change of position in the
paddle-board to which it is attached; and the position of the
second centre P is, or may be, so adjusted that this paddle-board,
as it enters the water and emerges from it, shall be such as shall
be most advantageous for propelling the vessel, and therefore
attended with less of that vibration which arises chiefly from the
alternate depression and elevation of the water, owing to the
oblique action of the paddle-boards.
(225.) In the year 1833, Mr. Field, of the firm of Maudslay and
Field, constructed a paddle-wheel with fixed paddle-boards, but
each board being divided into several narrow slips arranged one a
little behind the other, as represented in _fig._ 134. These
divided boards he proposed to arrange in such cycloidal curves
that they must all enter the water at the same place in immediate
succession, avoiding the shock produced by the entrance of the
common board. These split paddle-boards are as efficient in
propelling when at the lowest point as the common paddle-boards,
and when they emerge the water escapes simultaneously from each
narrow board, and is not thrown up, as is the case with common
paddle-boards.[39]
[Illustration: _Fig._ 134.]
[Pg479] The theoretical effect of this wheel is the same as that
of the common wheel, and experience alone, the result of which has
not yet been obtained, can prove its efficiency. The number of
bars, or separate parts into which each paddle-board is divided,
has been very various. When first introduced by Mr. Galloway each
board was divided into six or seven parts: this was subsequently
reduced, and in the more recent wheels of this form constructed
for the government vessels the paddle-boards consist only of two
parts, coming as near to the common wheel as is possible, without
altogether abandoning the principle of the split paddle.
(226.) To obtain an approximate estimate of the extent to which
steam-power is applicable to long sea-voyages, it would be
necessary to investigate the mutual relation which, in the
existing state of this application of steam-power, exists between
the capacity or tonnage of the vessel, the magnitude, weight, and
power, of the machinery, the available stowage for fuel, and the
average speed attainable in all [Pg480] weathers, as well as the
general purposes to which the vessel is to be appropriated,
whether for the transport of goods or merchandise, or merely for
despatches and passengers, or for both of these combined. That
portion of the capacity of the vessel which is appropriated to the
moving power consists of the space occupied by the machinery and
the fuel. The distribution of it between these must mainly depend
on the length of the voyage which the vessel must make without
receiving a fresh supply of coals. If the trips be short, and
frequent relays of fuel can be obtained, then the space allotted
to the machinery may bear a greater proportion to that assigned to
the fuel; but in proportion as each uninterrupted stage of the
voyage is increased, a greater stock of coals will be necessary,
and a proportionally less space left for the machinery. Other
things being the same, therefore, steam-vessels intended for long
sea-voyages must be less powerful in proportion to their tonnage.
It will be apparent that every improvement which takes place in
the application of the steam-engine to navigation will modify all
these data on which such an investigation must depend. Every
increased efficiency of fuel, from whatever cause it may be
derived, will either increase the useful tonnage of the vessel, or
increase the length of the voyage of which it is capable. Various
improvements have been and are still in progress, by which this
efficiency has undergone continual augmentation, and voyages may
now be accomplished with moderate economy and profit, to which a
few years since marine engines could not be applied with permanent
advantage. The average speed of steam-vessels has also undergone a
gradual increase by such improvements. During the four years
ending June, 1834, it was found that the average rate of steaming
obtained from fifty-one voyages made by the Admiralty steamers
between Falmouth and Corfu, exclusive of stoppages, was seven
miles and a quarter an hour direct distance between port and port.
The vessels which performed this voyage varied from 350 to 700
tons measured burden, and were provided with engines varying from
100 to 200 horse-power, with stowage for coals varying from 80 to
240 tons. The proportion of the power to the [Pg481] tonnage
varied from one horse to three tons to one horse to four tons.
Thus the MESSENGER had a power of 200 horses and measured 730
tons; the FLAMER had a power of 120 horses, and measured 500 tons;
the COLUMBIA had a power of 120 horses, and measured 360 tons. In
general it may be assumed that for the shortest class of trips,
such as those of the Channel steamers, the proportion of the power
to the tonnage should be about one horse for every two tons; but
for the longer class of voyages, the proportion of power to
tonnage should be about one horse-power to from three to four tons
measured tonnage. These data, however, must be received as very
rough approximations, subject to considerable modifications in
their application to particular vessels. We have already stated
that the nominal horse-power is itself extremely indefinite; and
if, as is now customary in the longer class of voyages, the steam
be worked expansively, then the nominal power almost ceases to
have any definite relation to the actual performance of the
vessel. It is usual to calculate the horse-power by assuming a
uniform pressure of steam upon the piston, and, consequently, by
excluding the consideration of the effect of expansion. The most
certain test of the amount of mechanical power exerted by the
machinery would be obtained from the quantity of water actually
transmitted in the form of steam from the boiler to the cylinder.
But the effect of this would also be influenced by the extent to
which the expansive principle has been brought into operation.
From the reported performances of the larger class of steam-ships
within the last few years, it would appear that the average speed
has been increased since the estimate above mentioned, which was
obtained in 1834; and on comparing the consumption of fuel with
the actual performance, it would appear that the efficiency of
fuel has also been considerably augmented. No extensive course of
accurate experiments or observations have, however, been obtained
from which correct inferences may be drawn of the probable limits
to which steam-navigation, in its present state, is capable of
being extended. The jealousy of rival companies has obstructed the
inquiries of those who, solicitous more [Pg482] for the general
advancement of the art than for the success of individual
enterprises, have directed their attention to this question; and
it is hardly to be expected that sufficiently correct and
extensive data can be obtained for this purpose.
(227.) Increased facility in the extension and application of
steam-navigation is expected to arise from the substitution of iron
for wood, in the construction of vessels. Hitherto iron steamers
have been chiefly confined to river-navigation; but there appears no
sufficient reason why their use should be thus limited. For
sea-voyages they offer many advantages; they are not half the weight
of vessels of equal tonnage constructed of wood; and, consequently,
with the same tonnage they will have less draught of water, and
therefore less resistance to the propelling power; or, with the same
draught of water and the same resistance, they will carry a
proportionally heavier cargo. The nature of their material renders
them more stiff and unyielding than timber; and they do not suffer
that effect which is called _hogging_, which arises from a slight
alteration which takes place in the figure of a timber vessel in
rolling, accompanied by an alternate opening and closing of the
seams. Iron vessels have the further advantage of being more proof
against fracture upon rocks. If a timber vessel strike, a plank is
broken, and a chasm opened in her many times greater than the point
of rock which produces the concussion. If an iron vessel strike, she
will either merely receive a dinge, or be pierced by a hole equal in
size to the point of rock which she encounters. Some examples of the
strength of iron vessels were given by Mr. Macgregor Laird, in his
evidence before the Committee of the Commons on Steam Navigation,
among which the following may be mentioned:—An iron vessel, called
the ALBURKAH, in one of their experimental trials got aground, and
lay upon her anchor: in a wooden vessel the anchor would probably
have pierced her bottom; in this case, however, the bottom was only
dinged. An iron vessel, built for the Irish Inland Navigation
Company, was being towed across Lough Derg in a gale of wind, when
the towing rope broke, and she was driven upon rocks, on which she
bumped for a considerable time [Pg483] without any injury. A wooden
vessel would in this case have gone to pieces. A further advantage
of iron vessels (which in warm climates is deserving of
consideration) is their greater coolness and perfect freedom from
vermin.
Iron steam-vessels on a very large scale are now in preparation in
the ports of Liverpool and Bristol, intended for long sea-voyages.
The largest vessel of this description which has yet been
projected is stated to be in preparation for the voyage between
Bristol and New York, by the company who have established the
steam-ship called the Great Western, plying between these places.
Several projects for the extension of steam-navigation to voyages
of considerable length have lately been entertained both by the
public and by the legislature, and have imparted to every attempt
to improve steam-navigation increased interest. A committee of the
House of Commons collected evidence and made a report in the last
session in favour of an experiment to establish a line of
steam-communication between Great Britain and India. Two routes
have been suggested by the committee, each being a continuation of
the line of Admiralty steam-packets already established to Malta
and the Ionian Isles. One of the routes proposed is through Egypt,
the Red Sea, and across the Indian Ocean to Bombay, or some of the
other presidencies; the other across the north part of Syria to
the banks of the Euphrates, by that river to the Persian Gulf, and
from thence to Bombay. Each of these routes will be attended with
peculiar difficulties, and in both a long sea-voyage will be
encountered.
In the route by the Red Sea it is proposed to establish steamers
between Malta and Alexandria (eight hundred and sixty miles). A
steamer of four hundred tons' burden and one hundred horse-power
would perform this voyage, upon an average of all weathers
incident to the situation, in from five to six days, consuming ten
tons of coal per day. But it is probable that it might be found
more advantageous to establish a higher ratio between the power
and the tonnage. From Alexandria the transit might be effected by
land across the isthmus to Suez—a journey of from four to five
days—by caravan and camels; or the transit might be made either
[Pg484] by land or water from Alexandria to Cairo, a distance of
one hundred and seventy-three miles; and from Cairo to Suez,
ninety-three miles, across the desert, in about five days. At Suez
would be a station for steamers, and the Red Sea would be
traversed in three runs or more. If necessary, stations for coals
might be established at Cosseir, Judda, Mocha, and finally at Aden
or at Socatra—an island immediately beyond the mouth of the Red
Sea, in the Indian Ocean; the run from Suez to Cosseir would be
three hundred miles—somewhat more than twice the distance from
Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty
miles; from Judda to Mocha, five hundred and seventeen miles; and
from Mocha to Socatra, six hundred and thirty-two miles. It is
evident that all this would, without difficulty, in the most
unfavourable weather, fall within the present powers of
steam-navigation. If the terminus of the passage be Bombay, the
run from Socatra to Bombay will be twelve hundred miles, which
would be from six to eight days' steaming. The whole passage from
Alexandria to Bombay, allowing three days for delay between Suez
and Bombay, would be twenty-six days: the time from Bombay to
Malta would therefore be about thirty-three days; and adding
fourteen days to this for the transit from Malta to England, we
should have a total of forty-seven days from London to Bombay, or
about seven weeks.
If the terminus proposed were Calcutta, the course from Socatra
would be one thousand two hundred and fifty miles south-east to
the Maldives, where a station for coals would be established. This
distance would be equal to that from Socatra to Bombay. From the
Maldives, a run of four hundred miles would reach the southern
point of Ceylon, called the Point de Galle, which is the best
harbour (Bombay excepted) in British India: from the Point de
Galle, a run of six hundred miles will reach Madras, and from
Madras to Calcutta would be a run of about six hundred miles. The
voyage from London to Calcutta would be performed in about sixty
days.
At a certain season of the year there exists a powerful physical
opponent to the transit from India to Suez: from [Pg485] the
middle of June until the end of September, the south-west monsoon
blows with unabated force across the Indian Ocean, and more
particularly between Socatra and Bombay. This wind is so violent
as to leave it barely possible for the most powerful steam-packet
to make head against it, and the voyage could not be accomplished
without serious wear and tear upon the vessels during these
months.
The attention of parliament has therefore been directed to another
line of communication, not liable to this difficulty: it is
proposed to establish a line of steamers from Bombay through the
Persian Gulf to the Euphrates.