IN A SINGLE-SCREW CARGO SHIP. 167 



reliability of the method followed it may be stated that in similar experiments made 

 on a model of the twin-screw collier Jupiter, for which are available accurate trial 

 data, the results of the model tests, when extended to the ship, agreed identically as 

 regards the propeller revolutions and within one per cent as regards power and pro- 

 pulsive coefficient. 



In Fig. 8, Plate 103, are shown the curves of wake fraction, thrust deduction 

 coefficient, apparent slip, and true slip for the ship. In extending the results of the 

 model experiments to the full-size ships it has been assumed that the wake fraction 

 and thrust deduction coefficient for the ships are the same as for the models. The 

 apparent slip and the true slip for the ship are less than for the model, because of 

 less relative resistance, owing to the fact that the f rictional resistance increases less 

 rapidly than if it followed the law of comparison. 



In order to avoid confusion in terms the following definitions of thrust deduc- 

 tion coefficient, t, and wake fraction, w, are given : — 



, T-R V-V 



in which T is the thrust of the propeller, R the resistance of the ship, V the speed of 

 the ship, V' the speed of advance of the propeller in the water in which it works. 



As was to be expected, the wake fraction and thrust deduction coefficient both 

 increase with fullness of the stern. It is interesting to note that in all cases the wake 

 fraction is considerably greater than the thrust deduction coefficient, resulting in a 

 hull efficiency greater than unity. The wake fraction obtained for the different 

 models at a speed corresponding to eleven knots for the ship varies from 0.29 to 0.35. 



The formula for wake fraction given by Taylor is as follows : — vo = — 0.05 

 -\- 0.5&, in which b is the block coefficient. This would give for all of these models 

 a wake fraction of 0.336. The average of the results obtained in these experiments 

 is 0.31, which is but slightly less than that estimated by the above formula. 



DISCUSSION. 



The President: — These two papers, No. 9, entitled "Experiments upon Simplified 

 Forms of Ships," and No. 10, entitled, "Variation of Shaft Horse-Power, Propeller Revo- 

 lutions and Propulsive Coefficient with Longitudinal Position of the Parallel Middle Body 

 in a Single-Screw Cargo Ship," are now open for discussion. These papers are particularly 

 interesting and will be discussed together. We will be glad to hear from anyone in the room. 



Constructor-Commander S. V. Goodall, R. N., Member: — Mr. President and gen- 

 tlemen, it has been my privilege, during my stay in this country, to spend some little time at 

 the U. S. Model Basin in Washington, and to see these experiments, or experiments similar 

 to them, carried out by Commander McEntee. I was very much impressed with the sys- 



