SEPTEMBER 28, 1899 | 
WA TURE 
533 
of the extraordinary speed is to be found in the extreme light- 
ness of propelling apparatus and small load. 
No doubt in the 7zréénia lightness has been pushed further 
than it would be in vessels of larger size and greater power. In 
such vessels a lower rate of revolution would probably be ac- 
cepted, additional motors would be fitted for manceuvring and 
going astern, boilers of relatively greater weight would be 
adopted and other changes made. But, after making ample 
allowance for all such increases in weight, it is unquestionable 
that considerable economies must be possible with rotary 
engines. Two other vessels of the destroyer type with turbo- 
motors (one for the Royal Navy) are now approaching com- 
pletion. Their trials will be of great interest, as they will 
furnish a direct comparison with vessels of similar size and 
form, fitted with similar boilers and driven by reciprocating 
engines. 
On the side of coal consumption, Mr. Parsons claims at least 
equality with the best triple expansion engines. Into the other 
advantages attending the use of rotary engines it is not necessary 
now to enter, 
Reference must be made, however, to one matter in which 
Mr. Parsons has done valuable and original work. In torpedo 
vessels of high speed the choice of the most efficient propellers 
has always been a matter of difficulty, and the solution of the 
problem has in many instances involved extensive experimental 
trials. By means of alterations in propellers alone, very large 
increases in speed have been effected ; and even now there are 
difficulties to be faced. When Mr. Parsons adopted the extra- 
ordinary speed of revolution just named for the Zwdznza, he 
went far beyond all experience and precedent and had to face 
unknown conditions. He has found the solution, after much 
patient and original investigation, in the use of multiple screws 
of small diameter. His results in this direction are of general 
interest to all who have to deal with screw propulsion. 
Such radical changes in propelling machinery as are involved 
in the adoption of turbo-motors must necessarily be subjected to 
thorough test before they will be widely adopted. The experi- 
ment which the Admiralty are making is not on a small scale as 
regards power. Although it is made in a destroyer, about 
10,000 horse-power will probably be developed and a corre- 
spondingly high speed attained. It may well happen that from 
this experiment very far-reaching effects may follow. Mr. 
Parsons himself has prepared many designs illustrating various 
applications of the system to sea-going, cross-Channel and 
special service vessels. Where shallowmess of draught is un- 
avoidable, the small diameter of the screws possible with the 
quick-running turbines is clearly an important matter. 
Comparisons between Large and Small Vessels. 
It has been shown that the attainment of very high speeds 
by vessels of small size involves many conditions not applicable 
to large sea-going steamships. But it is equally true that 
in many ways the trials of small swift vessels constitute model 
experiments from which interesting information may be ob- 
tained as to what would be involved in driving ships of large 
size at speeds much exceeding any of which we have experience. 
When the progressive steam-trials of such small vessels can be 
studied side by side with experiments made on models to deter- 
mine their resistance at various speeds, then the fullest inform- 
ation is obtained and the best guide to progress secured. This 
advantage, as has been said, we owe to William Froude. 
His contributions to the Reports of the British Association 
are classics in the literature of the resistance and propulsion of 
ships. In 1874 he practically exhausted the subject of frictional 
resistance so far as it is known ; and his Presidential Address 
to this Section in 1875 dealt fully and lucidly with the modern 
or stream-line theory of resistance. No doubt there would be 
advantage in extending Froude’s experiments on frictional 
résistance to greater lengths and to ship-shaped forms. It is 
probable also that dynamometric determinations of the resist- 
ance experienced by ships of modern forms and considerable 
size when towed at various speeds would be of value if they 
could be conducted. These extensions of what Froude accom- 
plished are not easily carried out; and in this country the 
pressure of work on shipbuilding for the Royal Navy has, for 
many years past, taxed to the utmost limits the capacity of the 
Admiralty experimental establishment so ably superintended by 
Mr. R. E. Froude, allowing little scope for purely scientific in- 
vestigations, and making it difficult to deal with the numerous 
experiments incidental to the designs of actual ships. Now that 
NO. 1561, VOL. 60] 
Holland, Russia, Italy and the United States have equipped 
experimental establishments, while Germany and France are 
taking steps in that direction, we may hope for extensions of 
purely scientific work and additions to our knowledge. In this 
direction, however, I am bound to say that much might be 
done if experimental establishments capable of dealing with 
questions of a general nature relating to resistance and pro- 
pulsion were added to the equipment of some of our universities 
and colleges. Engineering laboratories have been multiplied, 
but there is as yet no example of a model experimental tank 
devoted to instruction and research. 
It is impossible, and possibly is unnecessary, to attempt in 
this Address any account of Froude’s ‘‘ scale of comparison ” 
between ships and models at ‘‘corresponding speeds.’ But 
it may be of interest to give a few illustrations of the work- 
ing of this method, in the form of a contrast between a 
destroyer of 300 tons, 212 feet long, capable of steaming 30 
knots an hour, and a vessel of similar form enlarged to 765 feet 
in length and 14,100 tons. The ratio of dimensions is here 
about 3°61 : 1; the ratio of displacements is 47 : 1 ; and the 
ratio of corresponding speeds is 1°9 : I. 
To 12 knots in the small vessel would correspond 22°8 knots 
in the large vessel ; and the resistance experienced by the large 
vessel at 22°8 knots (neglecting a correction for friction) should 
be forty-seven times that of the small vessel at 12 knots. By ex- 
periment, this resistance for the small vessel was found to be 
18 tons. Hence, for the large vessel at 22°8 knots the resist- 
ance should be 84°6 tons. This would correspond to an 
“effective horse-power’’ of over 13,000, or to about 26,000 
indicated horse-power. The frictional correction would reduce 
this to about 25,000 horse-power, or about 1°8 horse-power per 
ton. Now turning to the destroyer, it is found experimentally 
that at 22°8 knots she experiences a resistance of about I1 tons, 
corresponding to an effective horse-power of over 1700, and an 
indicated horse-power of about 3000: say I0 horse-power per 
ton, or nearly five and a half times the power per ton required 
in the larger vessel. This illustrates the economy of propulsion 
arising from increased dimensions. 
Applying the same process to a speed of 30 knots in the large 
ship, the corresponding speed in the small ship is 15 8 knots. 
Her resistance at that speed is experimentally determined to be 
3'5 tons, and the resistance of the large ship at 30 knots (ne- 
glecting frictional correction) is about 165 tons. The effective 
horse-power of the large ship at 30 knots is, therefore, about 
34,000, corresponding to 68,000 horse-power indicated. Allow- 
ing for the frictional correction, this would drop to about 
62,000 horse-power, or 4°4 horse-power per ton. For the 
destroyer at 30 knots the resistance is about 174 tons; the 
effective horse-power is 3600, and the indicated horse-power 
about 6000, or 20 horse-power per ton, nearly five times as 
great as the corresponding power for the large ship. But while 
the destroyer under her trial conditions actually reaches 30 
knots, it is certain that in the large ship neither weight nor 
space could be found for machinery and boilers of the power 
required for 30 knots, and of the types usually adopted in large 
cruisers, in association with an adequate supply of fuel. The 
explanation of the methods by which the high speed is reached 
in the destroyer has already been given. Her propelling 
apparatus is about one-fourth as heavy in relation to its maxi- 
mum power, and her load is only about one-third as great in 
relation to the displacement, when compared with the corre- 
sponding features in a swift modern cruiser. 
It will, of course, be understood that in practice, under 
existing conditions, a cruiser of 14,000 tons would not be'made 
765 feet long, but probably about 500 feet. The hypothetical 
cruiser has been introduced simply for purposes of comparison 
with the destroyer. 
The earlier theories of resistance assumed that the resistance 
experienced by ships varied as the square of the speed. We 
now know that the frictional resistances of clean-painted sur- 
faces of considerable length vary as the 1°83 power of the speed. 
This seems a small difference, but it is sensible in its effects, 
causing a reduction of 32 per cent. at 10 knots, nearly 40 per cent. 
at 20 knots, and 42 per cent. at 25 knots. On the other hand, it 
isnow known that the laws of variation of the residual or wave- 
making resistance may depart very widely from the law of 
the square of the speed, and it may be interesting to trace for 
the typical destroyer how the resistance actually varies. 
Take first the otal resestance. Up to 11 knots it varies 
nearly as the square of the speed; at 16 knots it has reached 
