TO PROPELLERS OF OCEAN-GOING MERCHANT VESSELS. 91 
power. As the angle of the fin is increased until it finally reaches the vertical, in this latter 
position the revolutions are those which would be obtained in cavitation while the K factor 
becomes squared. Where the fin is so located as to have no directive force on the water 
flowing to the screw or to interfere with the water closing in around the hull, its effect on 
basic slip and K is nil. 
In the problem of type 2 ship, loaded down very deeply, her revolutions are consider- 
ably below those actually obtained. In type 2 ships I have found it better to make no cor- 
rection as the ship is loaded down, but to use for H the height from the base line to the 
upper limit of the stream-line body. Any increase in block above this has no effect on the 
revolutions, but it does effect the value of K. 
Mr. STEVENS:—In answer to Admiral Dyson’s remarks the following should be men- 
tioned: The 20 to 30 per cent that should be added to the effective horse-power, as found 
in the model test, is to allow for the increased resistance of a ship at sea over that on the 
trial trip. This increase is not intended to allow for any difference in the draught, but that 
due to the foulness of the ship’s bottom and to adverse weather conditions. 
I might state here, on analysing the data on various voyages of vessels, that the in- 
creased power at sea seems to run anywhere from 15 to 40 per cent over that on the trial trip, 
depending on the weather and the condition of the ship’s bottom. A fair average would be 
about 25 to 30 per cent. The U.S. Navy has found that after a ship has been in the water a 
month, the increase due to the roughness of the ship’s bottom is about 10 per cent. 
Referring to ships fitted with spectacle frames, Admiral Dyson’s experience seems to 
confirm mine, when the fins follow the stream line and when they are horizontal or 
nearly so, in as far as they effect the slip. 
Referring to Problem 12, if the estimated effective horse-power (as calculated by Tay- 
lor’s method of hull resistance) is correct, the K value for this ship would be about the one- 
half power of that shown on the chart. From Admiral Dyson’s experience regarding the 
effect that the bosses have on the power augment factor K, it would seem advisable to place 
the fins horizontal or nearly so, if by so doing the resistance of the ship is not increased 
enough to counterbalance the reduction of the power augment factor. It appears that this 
question would be well worth studying, as the necessary power might be considerably re- 
duced by the proper placing of the fins, or by using spectacle frames in place of the struts. 
All that I can say about Mr. Smith’s remarks is, that his ideas and mine are too much 
alike to permit of any discussion. 
THE PRESIDENT :—We thank you, Mr. Stevens, for reading the paper. I am sure that 
all the members have been highly interested in its statements. It takes us back to old times 
to have Mr. Stevens present a paper to us, gentlemen; his father used to read valuable papers 
before this Society, and it is a fine thing now that the son has taken up the work in the foot- 
steps of his father, and that his father’s mantle has fallen on his shoulders. 
The next two papers on the program, Nos. 5 and 6, are along similar lines, and if they 
are presented and considered together, we shall have the best results in the discussion. We 
will therefore call for Paper No. 5, entitled “A Study of the Wake of Certain Models by 
means of a Current Meter,’ by Professor Edward M. Bragg, Member. 
