584 THK I'KiXiHKSS TX STEAM NAVTrrATTON. 



It is impossible here to attempt aii}^ 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 working of 

 this method, in the form of a contrast between a destroyer of 300 tons, 

 212 feet long, capable of steaming HO 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 1.9:1. To 12 knots in the 

 small vessel would correspond 22.8 knots in the large vessel, and the 

 resistance experienced hy 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 expciiment this resistance for the small vessel 

 was found to be 1.8 ton. Hence, for the large vessel at 22.8 knots 

 the resistanci^ should be 84.6 tons. This would correspond to an '' ett'ect- 

 \yv hoi'sopower" of ov«n' 13.00(1. or to about 26,000 indicated horse- 

 powei'. riie frictional coi-rectioii would reduce this to al)()ut 25,000 

 horsepower, or about l.s horsepower })er ton. Now. turning to the 

 desti'oy»'r, it is found experimentally that at 22.8 knots she experiences 

 a resistance of a])out 11 tons, corresponding to an eft'cctix c hoi'sepower 

 of over 1.700 horsepower and an indicated horsepower of al)out 3,000 

 horsepowei" say. lo horsepower per ton, or nearly five and a iialf times 

 the power ])er ton rc(|uir<'d in the larger vessel. This ilhistratfts the 

 economy <»f pi'o))ulsion 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 resist- 

 anct' at that speed is experimentally determined to l)e 3.5 tons, and 

 the resistance of the large ship at 30 knots, neglecting frictional correc 

 tion, is about 165 tons. The effective horsepower of the large ship at 

 30 knots is, therefore, about 34,000 horsepower, corresponding to 

 68,000 horsepower indicated. Allowing for the frictional correction, 

 this would drop to about 62,000 horsepower, or 4.4 horsepower per 

 ton. For the destroyer at 30 knots the resistance is aliout 17^ tons; 

 the effective horsepower is 3,600 horsepower, and the indicated horse- 

 power about 6,000 horsepower, or 20 horsepower 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 i)oilers 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 alread}' been given. Her 

 propelling apparatus is about one-fourth as heavy in relation to its 

 maximum power, and her load is onl)^ about one-third as great in rela- 

 tion to the displacement, when compared with the corresponding 

 features in the cruiser. 



