232 PROPULSION OF VESSELS 



Formulas for Slip. It is customary to express slip in per 

 cent, of the velocity of the stream projected by the propelling 

 instrument. 



Let 5 t = true slip in per cent., expressed decimally; 



S a = apparent slip in reference to the ship's motion 

 through the water, expressed decimally and 

 in per cent.; 

 V velocity of stream projected by propelling instru- 



ment, in relation to vessel; 



Vi = velocity of water fed to propelling instrument; 

 that is, speed of vessel in relation to the sur- 

 rounding water diminished by the wake velocity 

 at the point where the propelling instrument 

 is located, for a vessel under way. 

 Vt = speed of vessel in relation to the water, as shown 



by the log. 

 Then, the true slip is given by the formula 



and the apparent slip by the formula 

 V-Vt 



S ^~T (2) 



It is customary among writers on marine propulsion to 

 refer to apparent slip simply as slip; when the true slip is meant, 

 it is usually called distinctly the true slip. 



SCREW PROPELLERS 



Definitions. If a point is caused to rotate at a uniform dis- 

 tance from and about an axis, and if the point at the same time 

 is caused to advance at a uniform rate in the direction of axis, 

 its path will be a helix. If the point, when moving away from 

 the observer, moves in the direction of the hands of a watch, 

 the helix will be right-handed; if in an opposite direction, left- 

 handed. The distance that the point advances in one complete 

 revolution is known as the pitch. If a line passing through 

 the axis is caused to rotate about the axis, and to pass along 



