134 THE PROPULSIVE EFFICIENCY 



propeller, but will affect it more in a seaway than in smooth water, and consequently, in 

 order to decide what sort of a propeller a ship should have, it would presumably be neces- 

 sary to investigate what the propeller is doing in rough water. 



Then there is another matter in that connection which struck me as interesting, namely, 

 that this effect would be greatest in a relatively low-speed propeller, with large area and 

 small slip, and these variations might be a reason for using smaller higher speed propellers 

 which might, in smooth water, be less efficient, but which might in rough water show a better 

 result, as they would not be so greatly affected bv slight variations of the relative speed of 

 the water. 



PROifESsoR H. C. Sadler, Member of Council: — I would confirm, what Mr. Robertson 

 said about propulsive coefficients. We have found with the ordinary tank tests and bare 

 hull that the ratio of effective to indicated horse-power in practice works out between 50 and 

 53 per cent — 53 per cent with a good machinery installation. If you have reason to sup- 

 pose that ordinary care has not been given to the installation of the machinery, it is a little 

 safer to take 50 per cent. I mention this because the figure of 65 per cent given in the 

 paper may be a little misleading, if memlbers feel that is the ratio of indicated horse-power 

 to effective. 



With regard to the question of trial trip speed and average sea speed, it has been my 

 experience that it is safe to say that with the same horse-power the reduction in speed over 

 a year's working in a vessel amounts to very nearly 10 per cent — somewhere between 8 and 

 10 per cent — so that if you want to figure on sustained sea speeds, and have the trial trip 

 result, I think you are fairly safe in saying that the speed will be reduced somewhere about 

 10 per cent. 



Mr. F. M. Hiatt, Member: — In speaking of the propulsive coefficient, are you re- 

 ferring to the ratio of the resistance, with appendages, or the bare hull resistance, plain 

 model resistance? 



Commander William McEntee, C. C, U. S. N. (Communicated) : — Referring to 

 Mr. Robertson's discussion as to the relative fineness of the bow and the stern, the distri- 

 bution of displacement was not intended to be that which would give the absolute minimum 

 resistance for the various prismatic coefficients. It was admitted in the paper that in the 

 fuller models the bow is too bluff at any except low speed. It is intended to give an idea as 

 to the variation of power and propeller efficiency with fullness of lines. The general trend 

 of these variations might differ a little if absolutely the best model were chosen for each dis- 

 placement, but the variation as to the prismatic coefficient to use under different conditions 

 would not be very much, and it is believed that the data given in the paper may be used with 

 considerable confidence in determining which is a proper prismatic coefficient to- use for a 

 single-cargo ship under a given set of conditions. 



With regard to propeller efficiency and to the statements by Mr. Robertson and Pro- 

 fessor Sadler that a propulsive coefficient of 65 per cent appears high, I am perfectly willing 

 to admit that I was rather surprised at the relatively high efficiency obtained when I first 

 examined the data resulting from the tests. It must be remembered, however, that propul- 

 sive coefficient in the present paper refers to the ratio between the effective horse-power and 

 the shaft horse-power and not to the ratio between the effective horse-power and indicated 



