RESISTANCE, PROPULSION AND SPEED OF SHIPS 



243 



^/L=0.2 

 i=5.3-3.e 



0.G5 



0.70 



0.75 



80 



V 



Fig. 6 Loss of ships' speed in North Atlantic as a function of 

 block coefficient with ratio (wave length) to (ship length) 

 as parameter. Three ranges of horsepower per ton of displace- 

 ment values are indicated (from Miickel, 1944) 



0.245-0.160 

 0.757-0.768 



0.365-0.305 

 0.721-0.745 



Fig. 7 Mean loss of ships' speed at different values of horse- 

 power per ton of displacement plotted versus (wave length)/ 

 (ship length) ratio (from Miickel, 1944) 



6 Constant torque for the ship. 



The results of this investigation are sumnuirizeil in 

 Fig. 5. The speed loss of a model, towed with a constant 

 force (cur\'e /?,„), is found to be identical with the speed 

 loss of a ship at constant RPM. This latter is the usual 

 operating condition of a ship, the engines of which are 

 governor controlled. 



2.4 Propeller Efficiency. The increase of the shaft 

 horsepower due to waves is larger than the increase of 

 the resistance shown in Fig. 3. The increased propeller 

 thrust and decreased ship speed lead to a rapid increase 

 of the projjeller thrust-loading coefficient. This in turn 

 leads to a pronounced reduction of the propeller effi- 

 ciency. This reduction can be evaluated readily from 

 the usual propeller-data curves. However, it has been 

 necessary to assume that thrust deduction and wake 

 fraction are not affected by waves. Kenipf (3-1935, 

 193fi) vividly demonstrated large losses of the propeller 



efficiency in waves and showed that these occur primarily 

 as a result of increased resistance and decreased ship 

 speed, ^'ertical velocity of a ship's stern in pitching and 

 heaving has only a small effect on the efficiency. 



The losses of propeller efficiency are also shown in the 

 work of Kent. They are, however, not so conspicuous 

 in this case because Kent used rather mild sea conditions 

 in his work. 



2.5 Estimate of Ship Resistance and Speed Loss in 

 Operating Conditions. The discu.ssion of the ship resist- 

 ance in the pre\'ious sections was limited to evaluating 

 the effect of the regular long-crested waves. The estima- 

 tion of the speed loss in operating conditions requires the 

 con.sideration of: 



(a) Sea irregularity. 



(b) Other cau.ses of speed loss. 



