PKOPULSION OF SHIPS 403 



solid nut, and if V P is the corresponding axial velocity so that V P = N P, 

 then V P V s is usually termed the slip, or more correctly the apparent 

 slip of the screw, and, if this worked in still water, would give the stern- 

 ward velocity in the propeller race relative to the surrounding water. 

 The real slip is the change actually produced by the screw on the velocity 

 of the water in the propeller race, and since the propeller works in an 

 accompanying current of watsr which has an initial absolute velocity V w 

 in the direction of the ship, this is given by (V P V s -f V w ). 



Evidently the apparent is always less than the true slip, and may, 

 if V w is sufficiently great, become negative. In general, however, it is 

 positive and, expressed as a percentage of V Pl has a value of about 20 per 

 cent. 



The real slip is always positive if the screw is to have any propelling 

 effect, as will be evident if it be remembered that it measures the change 

 in the sternward momentum of the water. 



Owing to the centrifugal action of the water, the pressure over the front 

 face of the propeller is negative, so that part of the total change of 

 velocity occurs before the water actually reaches the propeller. There is 

 thus a tendency to draw water from the stern of the vessel and to reduce 

 the forward pressure over this portion of its surface, with the result that 

 the resistance, as compared with the towing resistance, is considerably 

 augmented. 



The velocity with which water can be supplied to the screws is entirely 

 dependent on the atmospheric pressure augmented by the head of water 

 over the screw. Should this be insufficient to give the necessary velocity 

 of inflow the column of water in front of the screw is broken and 

 "cavitation " is caused, just as in a pump which is being driven at too 

 high a speed. This may be prevented either by increasing the depth of 

 immersion or by reducing the speed of the propeller. 



The maximum efficiency of the propeller would appear to be about 

 75 per cent., the magnitude of the various losses then probably 

 approximating to the following values : 



Friction losses ...... 9 per cent. 



Eddy losses 10 



Losses by kinetic energy rejected in wake . 6 ,, 

 Assuming an efficiency for the engine and shafting of 80 per cent, this 

 gives a combined efficiency of 60 per cent. 



In spite of much experimental work and theoretical investigation, very 

 little is actually known as to the best design of screw for any given con- 

 ditions. Mr. Froude, after an extended series of experiments, concluded 



i> D 2 



