The Atlantic Monthly, Volume V, Number 29, March, 1860
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Various >> The Atlantic Monthly, Volume V, Number 29, March, 1860
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While the vessels ordered by the Admiralty were on the stocks, it was
suggested by Mr. Lloyd that the model of their after-bodies was not that
most favorable to speed,--that they were too "full," and that a "finer
run" would be preferable. To settle this question, the Dwarf, a vessel
of fine run, was taken into dock, and her after-body filled out by three
separate layers of planking, so as to give it the form and proportions
of the vessels then building. These layers of planking could be removed
in succession, and the effects of a fuller or finer run upon the speed
of the vessel easily ascertained. A trial was then made, and the result
proved the correctness of Mr. Lloyd's opinion; the removal of the
different layers of planking increasing the speed from 3.75 to 5.75,
to 9, and finally to 11 knots. A trial between the Rifleman and the
Sharpshooter, vessels of four hundred and eighty tons and two hundred
horse-power, and the Minx and Teaser, of three hundred tons and one
hundred horse-power, gave similar results,--the speed in each trial
being twenty-four per cent. in favor of the finer run.
Although great efficiency and economy had now been attained, there was
still an important defect to be remedied, namely, the impediment to
speed and to evolution under sail presented by the dragging propeller;
which was accomplished by the invention of the "trunk" or "well," into
which the propeller can be raised at pleasure; and there is no longer
anything to prevent the construction of a screw-frigate which shall be
fit to accompany, under canvas only, a fleet of fast sailers, with the
assurance that she may arrive at the point of destination in company
with her consorts, having in reserve all her steam-power.
The mechanism by which the emersion of the screw is effected is as
follows:--There are two stern-posts; between these, and connecting them
with each other and with the keel, is a massive metallic frame, in which
rests another frame, or _chassis_, in which the screw is suspended; near
the water-line, the deck and wales are extended to the after stern-post,
and through an opening or trunk in this overhanging stern the frame
suspending the screw is raised by worms, working in a rack secured to
the frame, and operated from the deck, as shown in the accompanying
drawing,--or by a tackle, as is now most common. In the British ship
Agamemnon, of ninety guns, the propeller is raised by a hydrostatic
pump,--a neat arrangement, but liable to get out of order. When it is
desirable to raise the propeller, the blades are first placed in a
vertical position, and the operation of lifting is performed in a few
minutes.
The relative advantages of the propeller fitted to lift, and that which
is permanently fixed, have long been the subject of much discussion.
For merchant steamers, having an established route to perform, on which
the aid of steam is in constant demand, it is generally conceded that
the position of the screw should be permanent. The construction of the
ship is then less costly, while greater strength is preserved; and as
these vessels are out of port but for short intervals, should repairs be
needed, they have access to the docks. But for men-of-war the case is
widely different. Having frequently to keep the sea for long periods,
much under canvas, and often far distant from a dock-yard, they should
be provided with the means of lifting the screw to repair or to clear
it, or to be relieved from the impediment it offers to sailing and to
evolution, and also from the injurious "shake" occasioned by a dragging
propeller.
[Illustration: MODE OF LIFTING SCREW.]
On the other hand, the construction of a trunk or well impairs the
solidity of the stern, renders it much more vulnerable, and weakens its
defences, while it opposes to speed the very considerable resistance of
the after stern-post.[*] Nevertheless, no modern ship of the British
navy is without the means of raising her propeller, and the best opinion
of commanders and engineers of that service, of longest experience in
screw-ships, goes to establish the conviction, that, for men-of-war, the
advantages of being able to lift the propeller far more than outweigh
the objections urged against lifting. In this connection we mention the
fact, that all screw-ships "by the wind" have a strong tendency to
gripe. Would not this be obviated by having a gate or slide to fill out
the dead-wood when the screw is lifted?
[Footnote *: Might not a metallic stern-post, combining strength,
lightness, and little resistance, be introduced?]
The best illustration of the effects of a dragging propeller was
afforded on the departure of a Russian squadron from Cronstadt, bound to
the Amoor, in 1857-'58, consisting of three sloops of war bark-rigged,
and three three-masted schooners, under the flag of Commodore
Kouznetsoff. The vessels of each class were built from the same
moulds, and at the time of the experiment were of the same draft and
displacement. On clearing the land, signal was made to lift screws and
make sail. Soon after, all the squadron reported the execution of the
order, except the Voyerada sloop, which had the misfortune to break a
key in the couplings, and therefore could not lift her screw. Every
effort was tried to get out the key, and meanwhile a very instructive
example was presented to the squadron of the effect of a dragging
propeller on the speed of the vessel. The circumstances were as
follows:--The wind, a gentle breeze, right aft; the Voyerada carrying
all sail but the main course; the other two sloops holding way with
her with their topsails on the cap, and the schooners with their peaks
dropped. Under these conditions, the Voyerada, having her screw-blades
fixed horizontally, could scarcely keep her position, running two and a
half and three knots. The Voyerada next succeeded in getting her screw
vertical, when, without any change in the wind, the speed increased to
four and a half knots. The other sloops then mastheaded their topsails,
and the schooners peaked their gaffs. At length the Voyerada succeeded
in lifting her screw, when immediately all the sloops under the same
canvas continued their course, making six to six and a half knots. A
better example of the obstruction offered by a dragging propeller could
not have been afforded.[1]
The "shake," to which reference has been made, is the tremulous or
vibratory motion communicated to the after-body of the ship, and
particularly to the stern, by the revolution of the propeller, often
opening the seams, and in old ships sometimes starting the butts and
causing dangerous leaks. This movement arises from two causes,--one
inherent in the screw, the other due to its position in the deadwood.
The first cause is the difference in the propelling efficiency of the
upper and lower blades when in any other position than horizontal. The
centre of pressure of the lower blade, being at a greater depth below
the surface than the centre of pressure of the upper blade, acts upon a
medium of greater resistance to displacement, and the differential of
the pressures of the two blades produces inevitably a vibratory motion
in the stern of the vessel. This effect is greatly increased when the
clearance given to the screw in the dead-wood is too small; for the
reduction of the hydrostatic pressure at the stern-post, and the
increase of it at the rudder-post, on each passage of the blades, must
be followed by concussion. Therefore, if the "well," or distance between
the posts, be made sufficiently long in proportion to the screw, the
"shake" due to the latter cause can be almost entirely obviated.
In 1851, the British Admiralty selected three auxiliary screw-ships, of
different classes and qualities, for an experimental cruise, namely:--
[Footnote 1: _Russian Nautical Magazine_, No. XLI., December, 1857.]
---------------------------------------------------------------------
| Guns. | Horse | Screw. | Speed. | Day's | Sail
| | Power. | | | Fuel. | Equipment
---------------------------------------------------------------------
The | | | 2 | 9 | 8 |
Arrogant | 46 | 360 | blades | knots | days | Ship full rig
---------------------------------------------------------------------
The | | | 2 | 11 | 11 |
Dauntless | 24 | 580 | blades | knots | days | Ship light rig
---------------------------------------------------------------------
The | | | 2 | 10-1/2 | 6 |
Encounter | 14 | 360 | blades | knots | days | Barque
---------------------------------------------------------------------
They were ordered to pass round the Azores, each ship holding
her course, and using sail or steam, or both, as was deemed most
advantageous. An officer was sent on board each ship to keep a record of
her performance, and to note the time when and the position where, the
coal being entirely consumed, the contest ended. In this trial, the
Arrogant was found superior to the Dauntless, and both of them far
excelled the Encounter; indeed, no very different result was expected,
the object of the trial being to ascertain their relative as well as
positive value. These ships afterwards formed a part of the experimental
squadron stationed at Lisbon in the same year, which was composed of the
finest ships in the British navy.
It was believed by many officers, that a fast-sailing frigate, in a
reefed-topsail breeze, would be able to get away from any screw-ship;
but in a trial that took place between the Arethusa and the Encounter,
and the Phaeton and Arrogant, under circumstances the most favorable to
the sail-ships, it was found that the screw-ships, using both steam and
sail, had decidedly the superiority,--and that in fresh gales, with one,
two, or three reefs in the topsails, either "by the wind," or "going
free," the Phaeton and the Arethusa, the fastest sail-frigates in
the navy, were always beaten by the Arrogant. This result operated
powerfully in removing the repugnance to steam existing among all
classes of seamen; and the vast superiority of well-organized
screw-ships for the purposes of war is now so apparent, as to render
them the most important and indispensable part of every navy.
While the English were engaged in the trials here related, their rivals
on the opposite coast were not indifferent spectators. The French
were nearly as soon in the field of modern screw experiment as their
neighbors; and did the limits of this paper permit, it would be
instructive, as well as interesting, to trace the ingenious and
persevering steps by which they also approached the solution of that
difficult problem, the construction of a screw-man-of-war.
The first result of their efforts, La Pomone, screw-frigate, was shown
to the world in 1844, and after careful inspection, (in 1853,) it is
affirmed, such was the perfection of her general organization, that she
has hardly been excelled by any of her younger sisters.
The most complete course of experiments ever made, perhaps, with the
new motor, was that carried out by MM. Bourgois and Moll, of the French
navy, in 1847 and '48, which they verified by a second series in 1849.
These experiments were instituted to ascertain the relative efficiency
of all varieties of the screw-propeller, upon vessels of different
models and dimensions, and under all the varying conditions of wind and
sea, in order to determine the propeller best adapted to each particular
description of ship.[*]
Necessarily brief as is the notice of Gallic ingenuity and skill, the
acknowledgment must be made, that, for the invention of the trunk or
well, with its attendant advantages, navigation is indebted to Commander
Labrousse, of the French navy; and for a novel arrangement of the screw-
propeller, which has not attracted all the notice it deserves,
obligations are due to M. Allix, a distinguished engineer of that
service; and the propeller more recently introduced by M. Mangin, of the
same corps, if it performs all that is claimed for it, namely, that it
does away with the "shake," will be of great value.
[Footnote *: For a most interesting and instructive memoir upon these
experiments, the reader is referred to that admirable work, by Captain
E. Paris, of the French navy, _L'Helice Propulsive_.]
In concluding this recognition of the contributions by France to
screw-propulsion, it is desired to submit a few general observations on
the French navy; for, although upon every sea the tri-color waves
over ships proudly comparing with those under any other flag, it is
nevertheless too commonly believed that the docks of France are crowded
and her navy-list swollen with hulks which are but the mouldering
mementos of the vast armaments hastily created during the Consulate and
the Empire; an illusion most hazardous to our interests abroad and our
security at home.
At the period of _the coup d'etat_ of 1851, a Committee of Inquiry,
composed of the most experienced and intelligent officers and
distinguished legislators, had visited all departments of the navy, and
made the most careful investigations into every branch of the service.
Upon the evidence thus obtained, a report was submitted, providing for
the improvement of the condition of the officers and seamen, and the
increase, renewal, and remodelling of the _materiel_,--in fine, for the
correction of every abuse, the remedy of every evil, and the development
of all good existing in the navy. This report, stamped on every page
with patriotism and intelligence, commanded, even in the midst of
revolution, the support of all parties, the adhesion of every faction;
and has since, through all changes in the Ministry of the Marine, formed
the basis of the action of that department.
Under these auspices, France has in the last seven years organized the
means of promptly putting to sea a numerous fleet, composed of the most
modern and most powerful steamers, manned by efficient crews, commanded
by skilful officers; and now worthily maintains a position as a naval
power second only to that of Great Britain. At this moment, whilst
the British fleet includes but thirty-six screw line-of-battle ships,
mounting 3,400 guns, and propelled by 19,759 horse-power, that of France
may boast of forty such ships, mounting 3,700 guns, propelled by 27,500
horse-power; and while England has but thirty-eight screw-frigates,
France has forty-two.
In thus briefly summing up the forces of our ocean rivals, we cannot
avoid making some reflections suggested by the unpreparedness of this
country to meet any sudden burst of hostility. This not only involves
the risk of national humiliation, but paralyzes our diplomacy; since it
deprives us of that influence among the nations, which otherwise--from
the breadth of our territory, the value of our products, the activity
of our industry, the importance of our commerce, and the extent of our
maritime resources--we of right should hold.
No country is more interested than the United States in the maintenance
of peace; yet, even on the principle of economy, we may argue in favor
of a degree of preparation for war; for that calamity may best be
averted by taking from foreign powers the temptation to interfere with
us: all history showing that the justice and friendship of military
states are but slender guaranties for the peace of a nation unprepared
for attack.
It is vain to talk of husbanding financial resources for war, without
other preparation. When once embarked in hostilities, and in a position
to maintain our ground, large finances, judiciously used, will
ultimately command success; but no accumulation of funds can provide a
timely remedy for that weakness which cannot resist the first blow.
The national safety should no longer be left to chance, but be
established on a basis of certainty. A navy cannot be manufactured nor a
fortress built to meet an emergency, but should be kept ready-made.
In considering the auxiliary screw-frigate under the views already
offered, and in determining the canvas with which she should be
supplied, it will be well to refer, as the best guide, to the fastest
sail-ships,--the class which presents the greatest similarity in form to
that demanded in screw-ships. In these ships the great length of deck
offers every facility for the most advantageous spread of canvas;
consequently the centre of effort may he kept low, and the requisite
power and stability combined.
Intimately connected with her sailing-power is another branch of the
equipment of a screw-ship, which requires the most earnest, patient, and
intelligent consideration. Prepared to endure all the wear and tear of a
sail-ship, she should at the same time be ready for transmutation into
a steam-ship; namely, when, for any urgent service, her best powers of
steaming are required, she should be able to divest herself speedily of
yards and top-masts, and, the special service completed, resume all her
perfection as a sail-ship.
It would be out of place here to enter into details of equipment. In
naval affairs nothing is improvised, and a satisfactory conclusion upon
these points can be arrived at only through long experiment, and perhaps
frequent disappointment. Yet it is not doubted that the same ship may
exhibit a handy and efficient rig, develop a high velocity canvas, and,
without great power, a sufficient speed under steam.
In our navy, away from our own coast, sail must of necessity be the
rule, and steam the reserve or special power; and without abandonment of
our anti-colonial policy--with the depots of our rivals upon every sea,
yet not a ton of coal upon which we can rely--we should not dare to send
abroad a single ship which, whenever she gets up her anchor, must needs
also get up her steam.
Fortunately, in the creation of a steam-fleet, the United States will
not have to encounter tedious and costly experiments, nor to incur the
risk of failure.[1] The best form of hull, model of propeller, and plan
of engine are already so well established, that it is not easy to fall
into error; that which is most to be guarded against is the popular
demand, the prevailing mania for high speed,--for which single advantage
there is such a proneness to sacrifice every other warlike quality. That
measure of speed or power which will enable a ship to stem the currents
of rivers, to enter or leave a port in the face of a moderate gale, or
to meet the dangers of a lee-shore, should, it is conceived by many, be
sufficient; and for these exigencies a ship, which, with four months
supplies on board, can in calm weather and smooth water make nine to ten
knots under steam, has ample power. This moderate rate is far below the
popular mark; but, in considering this important question, it should not
be forgotten, that, unlike the paddle, the screw will always cooeperate
with sail,--and that, if a ship would go far under steam, she must be
content to go gently. The natural law regulating the speed of a ship
is, that the power requisite to propel her varies as the cube of the
velocity.
[Footnote 1: The constructors and engineers of the navy are unsurpassed
in professional art or science, and when conjoined with naval
officers--who should always determine the war-like essentials of
ships--they are capable of producing a steam-fleet that would meet the
requirements of all reasonable conditions. We venture to say, that
the failures with which they have been charged would be found,
on investigation, to be solely attributable to undue extraneous
influences.]
Let it be distinctly understood what power is here meant. As the power
applied to the propulsion of a vessel is only that which acts upon her
in the direction of the keel,--and as, of the gross indicated power
developed by her engine, one portion is absorbed in working the organs
of its mechanism, another in overcoming the friction of the load, while
still other proportions are expended in the slip of the propeller and
in the friction of its surfaces on the water,--only that portion of
the gross power which remains is applied to propulsion; and it is this
remainder which varies in the ratio of the cube of the speed.
Hence a steamer, that with five hundred horse-power can make eight knots
per hour, will require rather more than one thousand horse-power to
drive her at the speed of ten knots,--the law being thus modified by the
increased resistance consequent upon the greater weight of the large
engines; and thus a limit to speed is imposed, depending upon the weight
of machinery which, relative to her dimensions, a ship can carry. A
ship, that at the rate of ten knots under steam may run twelve hundred
miles, can, at the speed of eight knots, and with the expenditure of
rather less fuel, run the distance of eighteen hundred miles; and
therefore it is, many contend, that a man-of-war for distant service
should not be laden with large engines, whose full power can rarely be
wanted, and which monopolize so great a space and displacement as to
render it impossible to carry fuel for their proper development.
It is true, that, with large power of engine, the vessel may command,
so long as her coals last, the advantage of high speed, and her large
cylinders will enable her, by working the steam very expansively, to use
her fuel with great economy; but there still remains the disadvantage of
the increased first cost of the machinery, and its greater weight and
bulk, to be permanently carried, whether used or not, and which, by
increasing the displacement of the vessel, proportionally diminishes her
speed.
The last great improvement in connection with the screw remains to
be noticed, namely, lining the "bushings" and "bearings" with
lignum-vitae,--the invention of Mr. Penn, of Greenwich, near London.
The lignum-vitae is introduced in the manner shown in the drawing. In
connection therewith, it must be said, that the length and diameter of
bearings has been increased far beyond the proportions of former years.
The "brasses" are bored out about three-sixteenths of an inch larger
than the shaft; then the recesses are slotted out for the reception of
the wooden strips. If care be taken with this part of the operation, any
number of strips can be supplied ready fitted, and to put in a set of
spare strips becomes a short and simple operation.
[Illustration]
Strange as it appears, these wooden bearings are far more durable than
those of metal, and in some ships they have endured for years without
any perceptible wear in those parts which, previously to this invention,
had occasioned so much trouble and expense. But for this important
discovery, it is thought by some of the most competent engineers that
they would have been compelled to abandon the use of the screw in heavy
ships.
The Napoleon, the type of the new steam-ships of the line in the French
navy, is a good illustration of a first-class, full-powered steamer.
Her dimensions are as follows:--
FT. IN.
Length extreme. 262 6.40
Length at load-line. 234 0.94
Beam. 53 8.38
Height between decks. 6 8.72
Height of lower port sill. 7 2.63
Depth of hold. 26 9.34
Deep-load draft. 25 3
Immersed cross section, sq. ft. 1063.48
Displacement. tons. 6050
Diameter of cylinders. 8 2.45
Length of stroke. 5 3.06
Diameter of propeller. (4 bladed) 19 0.70
Pitch " " mean) 37 11
She has eight boilers, each having five furnaces, consuming, at full
speed, (12.14 knots,) 143 tons of coal per day, for which she stows five
days' supply. The boilers and engines occupy eighty-two feet in the
length of the ship.
The trial of this ship has established the practicability of adapting a
propeller to a ship of the largest class, so as to insure great speed,
and constitute a most effective man-of-war for certain purposes and
in certain situations; but when the great weight of the engines is
considered, and the large space they occupy in the vessel,--thereby
diminishing the stowage of supplies,--and further, that, after the coal
is exhausted, the ninety-gun ship has but the sail of a sixty-gun ship
to rely upon, it is not easy to avoid the conclusion, that, however
useful such a vessel may be for short passages,[1] and in those seas in
which her supplies of coal and provisions may be constantly replenished,
her sphere of action must be very limited, and she could not be relied
upon for the long cruises and various services on which an ordinary
line-of-battle ship is employed.
[Footnote 1: For debarking a regiment or two of Zouaves on the shores of
the Adriatic or upon the coast of Ireland.]
A ship constructed on the plan of the Napoleon, for the sake of gaining
a speed of twelve knots per hour for the distance of about two thousand
two hundred miles, is compelled to sacrifice a great part of her
efficiency in several most important particulars.
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