NOTES ON FUEI. ECONOMY. 237 



to drive them at the given speed of 10 knots. If we go into the question of the 

 skin effective horse-power and residuary effective horse-power, we find that in the 

 case of the 370-foot ship, the residuary resistance is more than 38 per cent of the 

 total resistance, while the percentage of residuary resistance in the case of the 

 400-foot ship is about 35 per cent. In a ship of this class, where we are interested 

 in the maintenance of this speed of 10 knots at sea, the question arises as to whether 

 this 370-foot ship with a larger residuary resistance in per cent of the total resistance 

 than the 400-foot ship, will maintain her sea speed as easily as the 400-foot ship. 

 I think the experience of everybody has been that in order to obtain a certain speed 

 at sea, we have to build the ship for more speed in the smooth-water trials, and that 

 as a general proposition if we take two lo-knot ships, one of which will drive at 

 10.5 or II knots in smooth water easier than the other will at this increased speed, 

 we will find that it will be easier to maintain 10 knots with that ship in sea service 

 than with the other ship whose power curve runs up more quickly above 10 knots 

 and requires at 10.5 or 11 knots more power than the first ship. Has Mr. Rigg 

 drawn his conclusions in Example No. 2 entirely from the results of model tank 

 tests? It will be interesting for us to have the results of these tests, also to know 

 the relative performances in service of these vessels. 



Referring to Example No. 6, it will be interesting to know what actual economy 

 is realized in sea service with a coastwise steamer with a bulbous form of bow. 



The results cited in Example No. 7 seem to conform to the experience that, 

 generally speaking, if we wish to design a ship for shallow water we must adopt 

 such a form as will in deep water, at a higher speed than the designed speed in 

 shallow water, have a comparatively moderate wave-making resistance. 



In the last example. No. 8, it seems to me the reason for the gain is very 

 apparent. We find there that the prismatic or. longitudinal coefficient in the case 

 of the boat in column A is 0.634, ^^'^ i^ the case of the boat in column B is 0.568; 

 in other words, the longitudinal or prismatic coefficient has been reduced by increas- 

 ing the area of the midship section and fining the ends of the vessel. 



From an inspection of the contours of residuary resistance given in Mr. Taylor's 

 work, we find that we can expect to obtain, in the case of the two vessels cited in 

 this last example, a reduction in residuary resistance by this reduction in the longi- 

 tudinal coefficient of as much as from 40 to 50 per cent. We can consequently 

 accept some increase in skin resistance and still materially gain by the changes 

 made in dimensions and proportions of the vessel at the speed in proportion to 

 the length at which it is designed to run. 



Mr. George Simpson, Visitor : — This is a very interesting paper, but I think 

 that Mr. Rigg is a little on the wrong track. I am of the opinion that all this talk 

 about the prismatic coefiicient is entirely wrong as applied to a freight carrier. 

 It is all right when we are dealing with high-speed ships, as Mr. Taylor has shown 

 and proved, because to put beam on with these vessels to fine the ship toward the 

 ends is an entirely different matter. 



