OF SINGLE-SCREW CARGO SHIPS. 131 



natural increase in effective horse-power resulting from the larger displacement 

 and coefficient is magnified or augmented by an accompanying fall in the propulsive 

 coefficient from 0.665 to 0.587. 



In the tests the same propeller was used on all models, and it may be considered 

 that the propeller is too small for the ship represented by Model 2185, as the revo- 

 lutions per minute have increased from 84.5 to 93.2. This is to some extent true, but 

 separate estimates indicate that if a propeller of different diameter or pitch were 

 fitted to 2185 for the purpose of reducing the revolutions to 84.5, such change would 

 make but little change in the propulsive coefficient, not over one per cent, so for all 

 practical purposes the models may be compared on the basis of the shaft horse-power 

 curves shown in Plate 52 without taking the revolutions into consideration. 



As another comparison of these results, it is interesting to consider the effect 

 of increasing the longitudinal coefficient and displacement of the ship represented 

 by Model 2183. The curves show that at a speed of 11 knots an increase of longi- 

 tudinal coefficient from 0.78 to 0.80 with a corresponding increase in displacement 

 of 335 tons or 2.57 per cent requires an 8 per cent increase in effective horse-power 

 and 12.27 per cent increase in shaft horse-power. Of the increased displacement, 

 only a part would be available as cargo-carrying capacity, depending upon the neces- 

 sary increase in the weight of the hull, machinery, and fuel. On a given trade 

 route, with a knowledge as to the cost of fuel and other expenses proportional to the 

 power, it may readily be determined whether the additional cargo carried for the 

 additional displacement would be economically possible with the increased power 

 charges. 



In Plate 53 are shown the curves of wake fraction, thrust deduction coefficient, 

 apparent slip and true slip for the ships. In extending the results of the model ex- 

 periments to the full-sized ships it has been assumed that the wake fraction and 

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



As the American practice in defining the wake as a percentage of the ship's 

 speed varies from that followed in Great Britain, the following definitions of the 

 thrust deduction coefficient and the wake fraction are given :■ — 



. r—R V—V 



r ' -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. 

 With regard to the shaft horse-power required for the ships represented by the 

 different models, it should be noted that the curves show what may be expected 

 under trial conditions, that is, with a clean, freshly painted bottom and a smooth 

 bronze propeller running in smooth water. To maintain the same sea speed over 

 considerable periods, it is necessary to allow a margin of power to cover the in- 

 creased resistance due to average sea and weather conditions and a moderate 

 amount of fouling of bottom which occurs between each docking and painting. 



