132 THE PROPULSIVE EFFICIENCY 



Summing up the results of both series of experiments, it appears safe to con- 

 clude that, for a well-designed hull with a propeller running at about 90 revolutions 

 per minute, a hull efficiency of 1.09 and a propulsive efficiency of 0.65 may be ex- 

 pected in a single-screw cargo ship. 



DISCUSSION. 



The President: — ^Commander McEntee's paper on "Propulsive Efficiency of Single- 

 Screw Cargo Ships" is now before you for discussion, gentlemen. 



Mr. a. J. C. Robertson, Member: — I wish that Mr. McEntee were present here so 

 that we could express to him our great gratitude for this paper and the paper of last year, 

 the two really constituting one paper. Mr. McEntee has worked out a system of self-pro- 

 pulsion tests which have given us very valuable results. They are presented in last year's 

 paper and this paper. In last year's paper Mr. McEntee took a single prismatic coefficient 

 for his model of about 80 per cent. He took a fixed parallel middle body and moved it by 

 steps from aft of midship toward the fore body of the ship and gave us the resistance 

 curves and the shaft horse-power that resulted, and if anyone takes the trouble to plot the 

 curves of last year, he will find that a very full bow with a very fine stem for speeds around 

 6, 6.5, 7 and up to 8 knots gives distinctly the best form. When the speeds go above 8 

 knots the formation of waves and the resistance due to the wave-making at the bow evi- 

 dently counter-balance the gain of a fine stern in the wake and eddy-making aft, so that 

 the proportion which is best for driving gradually moves the middle body aft. However, the 

 fine stern Mr. McEntee secured by putting the parallel body very far forward gave very 

 high propeller efficiency, and for that reason he selected a model with a parallel body placed 

 considerably further forward than it should be for the best power resistance results. 



In this paper Mr. McEntee refers to the change between last year's model and this 

 year's model, showing an increase in resistance for this year's model of 3 per cent. He 

 does not state that that 3 per cent is at 11 knots, which is the designer's speed of the vessel. 

 At 13 knots the increase of this model over the best model of last year is more than 11 per 

 cent, which shows that the fullness forward which he advocates in order to secure a good 

 propeller efficiency would have a serious effect if the vessel were going to meet a head sea, 

 which would augment its resistance over the resistance at 11 knots. 



There is one point further in regard to Mr. McEntee's figures I wish to refer to. The 

 65 per cent propulsive efficiency which he states is obtained with the prismatic coefficient of 

 the after body of the ship of only 70 per cent, and to increase the after body prismatic to 

 74 per cent, cuts down the coefficient of propulsion by 4 per cent, bringing it down to 61 

 per cent. Now a vessel around about 80 per cent prismatic coefficient would have its center 

 of buoyancy extremely far forward if it had to have an afterbody prismatic of only 70 per 

 cent, so that is a qualification of the use of this very high efficiency which Mr. McEntee 

 foretells. 



There is another qualification I want to make in regard to that figure. Suppose we 



