170 VARIATIONS OF SHAFT HORSE-POWER, ETC., 



the impression that the "straight frame" construction with a sharp knuckle line amidships 

 would be advantageous, especially in ships with a parallel middle body; that is, that the in- 

 crease in resistance to propulsion would be so small that it could be disregarded in compari- 

 son with the gain due to simplified construction. In 1909 I suggested this form of hull in Jane's 

 "Fighting Ships" for battleships with external bottom armor where fitting of the heavy plates 

 on the round turn of bilges would present special difficulties, but I did not contemplate con- 

 tinuing the knuckle line much beyond about one-half of the length of the ship. I was, how- 

 ever, prepared to find that the increase in resistance due to this feature might be prohibi- 

 tive in ships of such high speed-length ratios and such form as found in modem battleships. 



The experiments of Professor Sadler and Mr. Yamamato throw much light on this 

 problem, and seem to show that in cargo vessels with speed-length ratios smaller than 0.9 

 the increase in resistance by this construction is on the whole insignificant, and that the 

 adoption of the "straight frame" type seems well worth serious consideration. Perhaps it 

 will be found most advantageous to adopt the square corner and to stop the knuckle a little 

 beyond the parallel middle body. 



It would be of interest to carry the investigation a step further and to determine the 

 resistance of a ship form approaching even more closely to a pure flatiron shape. The form 

 I have in mind is one by which the knuckle line lies in a horizontal plane and where the 

 floors have a uniform but small ris'e from end to end, making the corner angle a little larger 

 than a right angle. The bottom would then be almost quite flat, formed of two plane sur- 

 faces, slightly inclined upwards from the kee on either side. Some flare should, of course, 

 be given to the sections in the bow and in the stern, and locally, some deviation from the 

 simple form would probably be necessary in way of the propellers and the rudder, but prac- 

 tically all the plates in the bottom would be plane or developable with a slight twist, and the 

 frames would be straight with only one knuckle. If experiments on such a ship form have 

 not already been made, it would seem worth while to make them and to investigate whether 

 the gain in the first cost of construction would be outweighed or not by the increased 

 resistance to driving. 



Rear Admiral D. W. Taylor, U. S. N., Vice-President (Communicated) : — The paper 

 by Commander McEntee breaks new ground and afifords another illustration of the value of 

 the apparatus for investigating wake, thrust deduction and propulsive efficiency which has 

 been installed at the Model Basin by Commander McEntee. It seems to me that this very 

 interesting paper gives another illustration of what is, I believe, the fact, namely, that there 

 are many cases where a careful model basin investigation of a low-speed ship discloses that 

 there is a greater field for improvement than in the case of high-speed ships, although we all 

 know that, as regards the latter, the last word is far from said. 



The propeller tried is seen in Fig. 5, Plate 100, to have a maximum efficiency somewihat 

 greater than 70 per cent. Although the author does not give us the hull efficiencies, he 

 states that they are greater than unity, as might be expected and as is evident from inspec- 

 tion of Fig. 8, Plate 103. With the maximum propeller efficiency of over 70 per cent, and 

 the hull efficiency greater than unity, making the maximum possible propulsive efficiency well 

 over 70 per cent, we find in Fig. 7, Plate 102, the maximum propulsive efficiency is only 

 about 65 per cent. The reason is very simple. We find in Fig. 8 that, although the apparent 

 slip at 10 knots averages, roughly, a.bout 15 per cent, the true slip averages over 40 per cent, 

 and at 40 per cent slip the efficiency of the propeller, instead of approaching its maximum 



