JOINT DISCUSSION ON TWO PRECEDING PAPERS. 109 
test is used as the basis for determining the nominal slip ratio. For a single-screw vessel such 
as this, I believe the Cg value from the “behind” test is usually from 3 to 6 per cent higher 
than Cq value corresponding to the nominal slip determined as above. 
The rotative efficiencies for the tests which were described by Commander McEntee in 
the paper which he read before this Society in 1919 were from 94 to 96 per cent. 
From Plate 43 it appears that the axial position of the propeller has practically no effect 
on the hull efficiency. Mr. James Semple described a similar test in a paper read before 
the Institution of Naval Architects in 1919. The curves which he gave in Plate 51 of his 
article shows that the hull efficiency increases somewhat as the propeller is moved away from 
the stern post. It is my understanding, also, that the propulsive efficiency falls off if the pro- 
peller is moved towards the stern post, and especially if it is a large one. 
The large increase in hull efficiency due to raising the propeller is extraordinary. The 
gain in efficiency varies from 8 to 32 per cent, the average being 16 per cent. 
On pages 104 and 105 the author indicates that a high location of propeller is desirable 
because of the improved efficiency. This is contrary to the general idea that the lowest posi- 
tion is the most efficient. Admiral Dyson gives an example in his book where lowering the 
propeller improved the efficiency greatly. Prof. Bragg also calls attention to this feature 
on page 100. 
To investigate this matter fully, estimates were prepared for a small vessel having a 
form of after body exactly similar to that given in Plate 37. The following cases were used: 
Case 1. Given the speed of vessel and R. P.M. V = 8.75, Ha = 376, R = 135, 
1D) == SO) 
Case Givenpthe. oy Ey > vandken baie p18 On e135 1D) —— lO) 
Case 3. Given the wheel and R. P. M. D= 10, P= 8.29, R = 135. 
A comparative summary of these estimates is given in Table 1, page 110, from which it is 
seen that the higher propulsive efficiency is obtained in the lower position for cases 1 and 2. 
This result is due to the higher slip ratio which occurs when the wheel is in the higher position. 
The higher slip ratio of course gives a lower propeller efficiency. Also the pitch for the higher 
position must be lower, which augments this effect. 
Case 3 is not a very practical one, but is given as a matter of interest. 
The vessel referred to above is a small freighter driven by a Diesel engine. 
I hope that papers describing tests and experiments at the Washington tank will be read 
at each meeting of the Society. I might suggest that the effect of trim on self-propulsion be 
investigated. It is often stated that trim by the stern greatly improves the driving qualities 
and increases the speed for a given vessel. 
It would also be valuable to investigate the effect of rake of the shafting. 
Admiral Taylor has added another to the many valuable papers which he has contributed 
on this subject. The Society, and especially the shipbuilders, are much indebted to him for 
these valuable contributions. 
ProFEssor Brace :—First, I would like to make a few remarks about Admiral Taylor’s 
paper. I am wondering whether he has a record of the shaft horse-powers which it actually 
took to run these models under the different conditions, at one or more speeds. In 1911 Prof. 
Peabody reported to this Society some results which he had obtained from tests of the 
Froude. He gave both the hull efficiencies and the shaft horse-powers which it took to drive 
the boat at certain speeds. The Froude had a thick stern post and the propeller was placed 
anywhere from 1 to 30 inches back of it. 
