3 14 SECTION-MECHANICS. 



relation between their entire resistances would follow the law just 

 enunciated as governing the relation between their wave-making 

 resistances; but as has been explained the mean resistance'pcr square 

 foot of a long area is less than that of a short area, and thus the ship's 

 surface friction is relatively a little less than the models, and her sur- 

 face friction must be calculated separately ; but when similar ships are 

 compared the differences in length cf area will be unimportant, and the 

 law may be treated as exactly true, name!y, that the entire resistances of 

 similar ships at corresponding speeds are as the cubes of their respective 

 dimensions. The principles on which this law depends render it equally 

 true, however abnormally the wave-making resistance may grow, in 

 terms of the speed. 



I aoi afraid it would take me too long if I were to attempt to tell 

 you in detail all the results ot this investigation, but speaking 

 generally, we must for high speeds have long ships, because short ships 

 at high speeds make very big waves ; but at moderate speeds short ones 

 do best, because there is less surface friction, and at such speed, that is 

 the chief element of resistance. Practically, what is meant by moderate 

 speed is a speed which, in knots, is quite short of the square root of 

 the ship's length in feet. If you make a well-shaped ship loofeet long 

 to go at a speed of ten knots she will begin to make a rapidly growing 

 resistance, and at such a speed with a ship of ordinary proportions 

 the total resistance will be about one two-hundredth part of the ship's 

 entire weight. At a speed in knots about 1*35 times the square root 

 of the length in feet, she would be making a very large resistance 

 indeed, probably there increasing nearly as the sixth power of the speed. 

 When that is the case, then a ship of larger dimensions and with the 

 same lines would make absolutely less resistance at that speed. I may 

 say the " Shah " in her recent measured mile trial, going at seventeen 

 knots wns propelled bya nett force which was equal to one two-hundrcth 

 part of her whole weight, while Mr. Thorneycroft's swift launch on the 

 contrary, which goes at such an extraordinary speed, requires, at a 

 speed of a little over eighteen knots, a propulsive force of one-fifteenth 

 part of her weight. So that you see at what an enormous rate the 

 resistance grows even with the finest forms when driven at extravagant 

 speeds. I think, therefore, the speed at which yen may rationally 

 aim is in knots about equal to the square root of the ship's length 



