202 MACHINERY AND TRIALS OF THE PASSENGER SHIPS 
a very important question, and it appears that a short stay out of the dock, without any 
apparent fouling, may have an appreciable effect on resistance. That matter is being in- 
vestigated, and I hope we will get at the bottom of it in time. Two weeks is not a very 
long time, but that is another factor working in the same direction. 
A thing in this paper which excited my curiosity more than anything else was Plate 56. 
Take the right-hand diagram in Plate 56—the upper curves of propulsive coefficient—you will 
see the upper curve c, which applies to the 16,000-ton condition, is decidedly above the others, 
reaching a maximum a little under seven-tenths. The next curve falls off materially until 
deep top load curve is quite a bit below 65. That seemed to be rather a large difference, 
and I inquired at the Model Basin and obtained some information which I hope that Mr. 
Warriner will get and add to his paper. 
The wake factors in the three conditions appear to be practically the same. There is a 
slight difference in the thrust deduction coefficients, the thrust deduction coefficient increas- 
ing as the draught increases, but not enough to account for the difference; and if you will 
examine curves of slip corrected for wake, you will find that the upper curve c corresponds 
to an actual slip of the propeller of about 20 per cent, and the lower curve a@ corresponds 
to an actual slip of about 30 per cent; in other words, the propeller is working well on the 
down grade, from the point of maximum efficiency. 
I think in this case, as well as in the majority of cases of actual ships, the revolutions 
of the vessel are too high to get the highest possible propulsive efficiency. As we all know, 
it is very hard with modern machinery to get it to turn slow enough to get a really efficient 
propeller, that is, to get the maximum efficiency of which the propeller is capable. Change 
in the revolutions of the President Pierce, from 110 to 125, which was necessitated by gear 
considerations, is also a cause for decreased efficiency, and I am rather inclined to suspect, 
if the President Pierce propeller were actually tested in the model, its efficiency would be 
less than estimated by the author. 
The previous factors which I mentioned are all on the side of increased resistance, such 
as fouling of the hull, shallowness of water, etc. An increase in resistance of the ship will 
necessarily add to the slip, and the efficiency will slide on down, the effect being more or 
less cumulative. I believe that this discrepancy of 15 per cent is accounted for by a number 
of small causes which would have been discovered if the model had been tested in the Model 
Basin under actual conditions of trial. There is no difficulty in testing a model in the Model 
Basin in shallow water. I was a little surprised, in view of the conditions, to hear the 
statement as to the wonderful speed made by one of this class at sea, because if we have 
to increase the power at sea to cover an average increase of say 20 per cent over the trial 
trip conditions, the propeller would have undoubtedly worked under exceedingly inefficient 
conditions. That is a case where, if you make a larger propeller, there is no question you 
would get somewhat better results on trial, and perhaps much better results at sea. 
Probably a four-bladed propeller would show even greater improvement. If you make 
a larger propeller, you necessarily slightly reduce the pitch ratio, and the result would be, 
in my mind, to slightly reduce the upper comparative curve of propulsive efficiency in Plate 
56 and materially raise the lower curve; in other words, to bring the curves of prelnalenve effi- 
ciency close together, as they should be. 
Mr. Bartey:—TI think that Mr. Warriner’s paper, together with the discussions, form 
a valuable contribution. I would like to inquire if Mr. Robertson could not give to the 
