PROPULSION OF VESSELS 231 



PROPULSION OF VESSELS 



SLIP 



True Slip. In considering the speed of a stream projected 

 by a propelling instrument from a ship or other vessel in 

 motion, it must be borne in mind that while the stream is 

 propelled astern the vessel is advancing. Since the stream 

 must move astern faster than the vessel advances, the rear- 

 ward speed of the stream in relation to a fixed point of the 

 water some distance astern of the ship, as a floating piece of 

 wood, will be the difference between the speed of the vessel 

 in relation to the piece of wood and the rearward speed of 

 the stream in relation to the vessel. When the propeller 

 works in a wake, which has a forward motion, the speed with 

 which water is fed to the propelling instrument is reduced 

 thereby and it becomes equal to the difference between the 

 forward speed of the vessel in relation to a fixed point of the 

 water clear of the wake and the wake velocity. Thus, if the 

 speed of the ship is 15 mi. per hr. and a wake that has a forward 

 velocity of 3 mi. per hr. collects at the stern, the speed with 

 which the water is fed to a screw propeller is 15 3 = 12 mi. 

 per hr. The difference between the speed with which water 

 is fed to the propelling instrument and the speed with which 

 it is projected astern, both speeds being measured in relation 

 to the vessel, is called the true slip, and also the real slip, of 

 the stream. 



Apparent Slip. In practice, it is very inconvenient and 

 exceedingly difficult to measure either the wake velocity or 

 the speed of the ship in relation to the wake, but it is a very 

 simple matter to measure the speed of the vessel in relation 

 to a fixed point in the water clear of the wake by means of 

 an instrument called a log. The difference between the speed 

 of the stream projected by the propelling instrument and 

 the speed of the ship thus found is taken as the slip. When 

 calculated in this manner it obviously is not the same as the 

 true slip; it is called the apparent slip. 



