90 COMPARATIVE TESTS OF BILGE KEELS AND A GYRO-STABILIZER 
effective horse-power of 10 per cent and 20 per cent respectively. It is to be noted 
that this does not include the loss of power due to decrease in propeller efficiency 
for a twin-screw ship, when the propellers alternately approach the surface, if not 
actually coming out of the water. Also, as the method of towing did not permit 
free yawing of the model, the losses which occur on a ship, because of the increase 
in rudder resistance as it is moved from side to side in an effort to steer a straight 
course, were not present. 
These results confirm experience at sea that loss of speed is found to occur 
when ships are rolling heavily. Under these circumstances it appears that the 
power and weight devoted to the means for stabilizing a ship are more than amply 
repaid by the saving effected in the power required to drive her. 
DISCUSSION. 
TuE CHAIRMAN:—Gentlemen, you have heard a very excellent paper by Com- 
mander McEntee, and I desire to express the appreciation of the Society, not only for 
this paper but for the moving pictures which have accompanied it, and also for the 
pictures which accompanied the paper yesterday of Commander Saunders. These 
moving pictures form a distinct addition to the method of presenting information to 
the Society. Is there any discussion of this paper? If so, it is in order. 
Mr. W. W. Situ, Member:—I would like to bring up the point of the effect of 
rolling on the propulsive efficiency. If anyone could throw light on that subject, it 
would be of interest, and especially so in connection with twin-screw and single-screw 
types of vessels. 
Captain Henry M. Greason, Member:—I would like to bring out one point for 
the purpose of discussion. In the experiments carried out the increase in power required 
to draw the model through the water apparently is very carefully worked out, and 
reference was made to the figures in the propulsive coefficient. Will someone shed 
light on the loss in the power which is undoubtedly due to the yawing of the vessel 
that is part of the rolling operation. It might be interesting if anyone had any data 
relative to that loss, so that the three losses might be estimated and thus we would get 
an idea of the combined loss of the whole. 
ProFessor H. C. Sapier, Member of Council:—I had occasion to compare the 
average work of some of our Atlantic liners, over a year’s work, with the results secured 
on trial. The two types of vessels investigated were the high-speed type and the inter- 
mediate-speed type. In both the results worked out approximately as follows: The 
decrease in speed over a year’s work, with the same horse-power, as compared with the 
tank and trial results, was in the neighborhood of 9 to 10 per cent, and roughly, the 
other way, it took about 30 per cent increase in horse-power to maintain the same 
average sea speed, as obtained on the trial results. 
