By D. P.
The achievement of one triumph after another in the matter of high
speed steamships, and especially the confidence with which pledges of
certain results are given and accepted long before actual trials are
made, form one of the most convincing proofs of the important part
which scientific methods play in modern shipbuilding. This is evident
in the case of ships embodying novel or hitherto untried features, and
more especially so in cases where shipbuilders, having no personal
practical experience or data, achieve such results. This was notably
illustrated in the case of the Fairfield Co. undertaking some five
years ago to build and engine a huge craft of most phenomenal form and
proportions, and to propel the vessel at a given speed under
conditions which appeared highly impracticable to many engaged in the
same profession. The contract was proceeded with, however, and the
Czar of Russia's wonderful yacht Livadia was the result, which
(however much she may have justified the professional strictures as to
form and proportions) entirely answered the designer's anticipations
as to speed. Equally remarkable and far more interesting instances are
the Inman liners City of Paris and City of New York, in whose design
there was sufficient novelty to warrant the degree of misgiving which
undoubtedly existed regarding the Messrs. Thomson's ability to attain
the speed required. In the case at least of the City of Paris, Messrs.
Thomson's intrepidity has been triumphantly justified. An instance
still more opposite to our present subject is found in the now
renowned Channel steamers Princess Henrietta and Princess Josephine,
built by Messrs. Denny, of Dumbarton, for the Belgian government. The
speed stipulated for in this case was 20½ knots, and although in one
or two previous Channel steamers, built by the Fairfield Co., a like
speed had been achieved, still the guaranteeing of this speed by
Messrs. Denny was remarkable, in so far as the firm had never
produced, or had to do with, any craft faster than 15 or 16 knots. The
attainment not only of the speed guaranteed, but of the better part of
a knot in excess of that speed, was triumphant testimony to the skill
and care brought to bear upon the undertaking. In this case, at least,
the result was not one due to a previous course of "trial and error"
with actual ships, but was distinctly due to superior practical skill,
backed and enhanced by knowledge and use of specialized branches in
the science of marine architecture. Messrs. Denny are the only firm of
private shipbuilders possessing an experimental tank for recording the
speed and resistance of ships by means of miniature reproductions of
the actual vessels, and to this fact may safely be ascribed their
confidence in guaranteeing, and their success in obtaining, a speed so
remarkable in itself and so much in excess of anything they had
previously had to do with. Confirmatory evidence of their success with
the Belgian steamers is afforded by the fact that they have recently
been instructed to build for service between Stranraer and Larne a
paddle steamer guaranteed to steam 19 knots, and have had inquiries as
to other high speed vessels.
In estimating the power required for vessels of unusual types or of
abnormal speed, where empirical formulæ do not apply, and where data
for previous ships are not available, the system of experimenting with
models is the only trustworthy expedient. In the case of the Czar's
extraordinary yacht, the Livadia, already referred to, it may be
remembered that previous to the work of construction being proceeded
with, experiments were made with a small model of the vessel by the
late Dr. Tideman, at the government tank at Amsterdam. On the strength
of the data so obtained, coupled with the results of trials made with
a miniature of the actual vessel on Loch Lomond, those responsible for
her stipulated speed were satisfied that it could be attained. The
actual results amply justified the reliance placed upon such
experiments.
The design of many of her Majesty's ships has been altered after
trials with their models. This was notably the case in connection with
the design of the Medway class of river gunboats. The Admiralty
constructors at first determined to make them 110 ft. long, by only 26
ft. in breadth. A doubt arising in their minds, the matter was
referred to the late Mr. Froude, who had models made of various
breadths, with which he experimented. The results satisfied the
Admiralty officers that a substantial gain, rather than a loss, would
follow from giving them much greater beam than had been proposed, and
this was amply verified in the actual ships.
So long ago as the last decade of last century, an extended series of
experiments with variously shaped bodies, ships as well as other
shapes, were conducted by Colonel Beaufoy, in Greenland dock, London,
under the auspices of a society instituted to improve naval
architecture at that time. Robert Fulton, of America, David Napier, of
Glasgow, and other pioneers of the steamship, are related to have
carried out systematic model experiments, although of a rude kind in
modern eyes, before entering on some of their ventures. About 1840 Mr.
John Scott Russell carried on, on behalf of the British Association,
of which he was at that time one of its most distinguished members, an
elaborate series of investigations into the form of least resistance
in vessels. For this purpose he leased the Virginia House and grounds,
a former residence of Rodger Stewart, a famous Greenock shipowner of
the early part of the century, the house being used as offices, while
in the grounds an experimental tank was erected. In it tests were made
of the speed and resistance of the various forms which Mr. Russell's
ingenuity evolved--notably those based on the well-known stream line
theory--as possible types of the steam fleets of the future. All the
data derived from experiment was tabulated, or shown graphically in
the form of diagrams, which, doubtless, proved of great interest to
the _savants_ of the British Association of that day. Mr. Russell
returned to London in 1844, and the investigations were discontinued.
It will thus be seen that model experiments had been made by
investigators long before the time of the late Dr. William Froude, of
Torquay. It was not, however, until this gentleman took the subject of
resistance of vessels in hand that designers were enabled to render
the results from model trials accurately applicable to vessels of full
size. This was principally due to his enunciation and verification by
experiment of what is now known as the "law of comparison," or the law
by which one is enabled to refer accurately the resistance of a model
to one of larger size, or to that of a full sized vessel. In effect,
the law is this--for vessels of the same proportional dimensions, or,
as designers say, of the same lines, there are speeds appropriate to
these vessels, which vary as the square roots of the ratio of their
dimensions, and at these appropriate speeds the resistances will vary
as the cubes of these dimensions. The fundament upon which the law is
based has recently been shown to have found expression in the works of
F. Reech, a distinguished French scientist who wrote early in the
century. There are no valid grounds for supposing that the discovery
of Reech was familiar to Froude; but even were this so, it is
abundantly evident that, although never claimed by himself, there are
the best of grounds for claiming the law of comparison, as now
established, to be an independent discovery of Froude's.
Dr. Froude began his investigations with ships' models at the
experimental tank at Torquay about 1872, carrying it on
uninterruptedly until his death in 1879. Since his decease, the work
of investigation has been carried on by his son, Mr. R. E. Froude, who
ably assisted his father, and originated much of the existing
apparatus. At the beginning of 1886, the whole experimental appliances
and effects were removed from Torquay to Haslar, near Portsmouth,
where a large tank and more commodious offices have been constructed,
with a view to entering more extensively upon the work of experimental
investigation. The dimensions of the old tank were 280 ft. in length,
36 ft. in width, and 10 ft. in depth. The new one is about 400 ft.
long, 20 ft. wide, and 9 ft. deep. The new establishment is more
commodious and better equipped than the old, and although the
experiments are taken over a greater length, the operators are enabled
to turn out results with as great dispatch as in the Torquay tank. The
adjacency of the new tank to the dockyard at Portsmouth enables the
Admiralty authorities to make fuller and more frequent use of it than
formerly. Since the value of the work carried on for the British
government has become appreciated, several experimental establishments
of a similar character have been instituted in other countries. The
Dutch government in 1874 formed one at Amsterdam which, up till his
death in 1883, was under the superintendence of Dr. Tideman, whose
labors in this direction were second only to those of the late Dr.
Froude. In 1877 the French naval authorities established an
experimental tank in the dockyard at Brest, and the Italian government
have just completed one on an elaborate scale in the naval dockyard at
Spezia. The Spezia tank, which is 500 ft. in length by about 22 ft. in
breadth, is fully equipped with all the special and highly ingenious
instruments and appliances which the scientific skill of the late Dr.
Froude brought into existence, and have been since his day improved
upon by his son, Mr. R. E. Froude, and other experts.
Through the courtesy of our own Admiralty and of Messrs. Denny, of
Dumbarton, the Italians have been permitted to avail themselves of the
latest improvements which experience has suggested, and the
construction of the special machinery and apparatus required has been
executed by firms in this country having previous experience in this
connection--Messrs. Kelso & Co., of Commerce Street, Glasgow; and Mr.
Robert W. Munro, of London.
Having briefly traced the origin and development of the system of
model experiment, it may now be of interest to describe the _modus
operandi_ of such experiments, and explain the way in which they are
made applicable to actual ships. The models with which experiments are
made in those establishments conducted on the lines instituted by Mr.
Froude are made of paraffin wax, a material well adapted for the
purpose, being easily worked, impervious to water, and yielding a fine
smooth surface. Moreover, when done with, the models may be remelted
for further use and all parings utilized. They are produced in the
following manner: A mould is formed in clay by means of cross sections
made somewhat larger than is actually required, this allowance being
made to admit of the cutting and paring afterward required to bring
the model to the correct point. Into this mould a core is placed,
consisting of a light wooden framework covered with calico and coated
with a thick solution of clay to make it impervious to the melted
paraffin. This latter substance is run into the space between the core
and the mould and allowed to cool. This space, forming the thickness
of the model, is usually from ¾ in. for a model of 10 ft. long to 1¼
in. and 1½ in. for one of 16 ft. and 18 ft. long. When cold, the model
is floated out of the mould by water pressure and placed bottom upward
on the bed of a shaping machine, an ingenious piece of mechanism
devised by the late Dr. Froude, to aid in reducing the rough casting
to the accurate form. The bed of this machine, which travels
automatically while the machine is in operation, can be raised or
lowered to any desired level by adjusting screws. A plan of water
lines of the vessel to be modeled is placed on a tablet geared to the
machine, the travel of which is a function of the travel of the bed
containing the model. With a pointer, which is connected by a system
of levers to the cutting tools, the operator traces out the water
lines upon the plan as the machine and its bed are in motion, with the
result that corresponding lines are cut upon the model. The cutting
tools are swiftly revolving knives which work on vertical spindles
moved in a lateral direction (brought near or removed from each
other), according to the varying breadth of the water lines throughout
the length of the model, as traced out by the operator's pointer. In
this way a series of longitudinal incisions are made on the model at
different levels corresponding to the water lines of the vessel. The
model is now taken from the bed of the machine and the superfluous
material or projection between the incisions is removed by means of a
spokeshave or other sharp hand tool, and the whole surface brought to
the correct form, and made fair and smooth.
To test accuracy of form, the weight of model is carefully taken, and
the displacement at the intended trial draught accurately determined
from the plan of lines. The difference between the weight of model and
the displacement at the draught intended is then put into the bottom
of the model in the form of small bags of shot, and by unique and very
delicately constructed instruments for ascertaining the correct
draught, the smallest error can at once be detected and allowed for.
The models vary in size from about one-tenth to one-thirtieth of the
size of the actual ship. A model of the largest size can be produced
and its resistance determined at a number of speeds in about two days
or so. The mode of procedure in arranging the model for the resistance
experiment, after the model is afloat in the tank at the correct
draught and trim, consists in attaching to it a skillfully devised
dynamometric apparatus secured to a lightly constructed carriage. This
carriage traverses a railway which extends the whole length of the
tank about 15 in. or 18 in. above the water. The floating model is
carefully guided in its passage through the water by a delicate
device, keeping it from deviating either to the right or left, but at
the same time allowing a free vertical and horizontal motion. The
carriage with the model attached is propelled by means of an endless
steel wire rope, passing at each end of the tank around a drum, driven
by a small stationary engine, fitted with a very sensitive governor,
capable of being so adjusted that any required speed may be given to
the carriage and model. The resistance which the model encounters in
its passage through the water is communicated to a spiral spring, and
the extension this spring undergoes is a measure of the model's
resistance. The amount of the extension is recorded on a revolving
cylinder to a much enlarged scale through the medium of levers or bell
cranks supported by steel knife edges resting on rocking pieces. On
the same cylinder are registered "time" and "distance" diagrams, by
means of which a correct measure of the speed is obtained. The time
diagram is recorded by means of a clock attached to an electric
circuit, making contact every half second, and actuating a pen which
forms an indent in what would otherwise be a straight line on the
paper. The distance pen, by a similar arrangement, traces another line
on the cylinder in which are indents corresponding to fixed distances
of travel along the tank, the indents being caused by small
projections which strike a trigger at the bottom of the carriage as it
passes, and make electric contact. From these time and distance
diagrams accurate account can be taken of the speed at which the model
and its supporting carriage have been driven. Thus on the same
cylinder is recorded graphically the speed and resistance of the
model. The carriage may be driven at any assigned speed by adjusting
the governor of the driving engine already alluded to, but the record
of the speed by means of the time and distance diagrams is more
definite. When the resistances of the model have been obtained at
several speeds, varying in some cases from 50 to 1,000 feet per
minute, the speeds are set off in suitable units along a base line,
and for every speed at which resistance is measured, the resistance is
set off to scale as an ordinate value at those speeds. A line passing
through these spots forms the "curve of resistance," from which the
resistance experienced by the model at the given trial speeds or any
intermediate speed can be ascertained. The resistance being known, the
power required to overcome resistance and drive the actual ship at any
given speed is easily deduced by applying the rule before described as
the law of comparison.--_The Steamship._
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