130 THE PROPULSIVE EFFICIENCY 



propeller were repeated. The model was next connected with the resistance dyna- 

 mometer on the towing carriage and the usual model resistance data taken. This 

 insured that the conditions of test, both for self-propulsion and for the resistance of 

 the model, would be uniform as regards conditions of the model, temperature of 

 water, etc. 



The following are the dimensions of the propeller used in the experiments and 

 also the dimensions expanded to the ship scale : — 



Model. Ship. 



Diameter 10.125 inches 16 feet 7 inches 



Pitch . 9.0 inches 14 feet 9 inches 



Pitch ratio 0.889 



Mean width ratio 0.20 



Number of blades 3 



Ratio of projected to disc area 0.266 



Blade thickness fraction 0.04 



The propeller had three blades of Taylor's standard form. 



The propeller characteristics were obtained by separate tests of the propeller 

 model run in free water, that is, in a separate apparatus where the propeller shaft 

 projected well ahead, so that the propeller ran in water undisturbed by the action 

 of the testing apparatus. The same motor dynamometer was used for tests as 

 was used for the self-propulsion tests, the only difference being that the propeller 

 shaft was coupled to the forward end of the armature shaft instead of the after end. 



The characteristics of the propeller are given in Plate 50. The thrust constant, 

 Ct, and the torque constant, Cq, are plotted on nominal slip following the method 

 used by Schaffran.* These constants, which are in non-dimensional form, lend 

 themselves well to the analysis of self-propulsion experiments and to the extension 

 of the results to the full-sized ship. 



The results of the investigation are given in Plates 51, 52 and 53. An examina- 

 tion of the estimated horse-power curves and the shaft horse-power curves for the 

 various models shows, as was to be expected, wide variation in power require- 

 ments, especially at higher speeds. For example, at 1 1 knots passing from Model 

 2181 longitudinal coefificient 0.74 to Model 2185 longitudinal coefficient 0.82, with 

 a change in displacement of 1,337 tons, or 10.8 per cent, increases the effective 

 horse-power from 1,165 to 1,585 or 36 per cent, while the shaft horse-power is in- 

 creased by 950 or 54 per cent. In this range the percentage increase in effective 

 horse-power required is about three and one-half times as much as the increase in 

 displacement, while the increase in shaft horse-power is five times as great as that 

 in displacement. At a speed of 11 knots the increased displacement is obtained by 

 an apparently unwarranted increase in power. This results from the fact that the 



*"Systematische Propellerversuche" ; K. Schaffran, Schiffbau, September 22, 1915. 



