330 RUDDER TRIALS, U. S. S. STERETT. 



APPENDIX VI. 



SPEED-REVOLUTION CURVE. 



The mean displacement of the Sterett on the turning trials as determined 

 by change in draught from beginning to end of tests was 835 tons, i. e., 93 

 tons or 12.7 per cent greater than the normal displacement (742 tons) on 

 standardization, when the speed-revolution curve was obtained. Conse- 

 quently, it was necessary to make a correction to this curve for the over- 

 draught condition. Curves were obtained from the U. S. Model Tank at 

 Washington, showing effective horse-power on speed for the Sterett model, 

 at three displacements, viz., normal, 10 per cent below, and 10 per cent 

 above normal. From these curves cross-curves at constant speeds were 

 plotted as shown on Fig. 18, Plate 128, from which the effective horse-power 

 was obtained for these speeds and at displacement 12.7 per cent greater 

 than normal. 



Curves of effective horse-power on speed in knots, for both normal 

 and 12.7 per cent over-displacement, were then laid down as shown. The 

 speed-revolution curve for normal displacement was also plotted on the 

 same scale of speed. Then for a given speed, say 22 knots, the power on 

 normal displacement (from curve I) is 2,450 effective horse-power and the 

 corresponding revolutions per minute 414. For the same power, at 12.7 

 per cent greater displacement, a speed of only 21.7 knots would be obtained. 

 Furthermore, a curve of per cent slip on speed in knots shows a very gradual 

 increase of slip with speed. Consequently, for the small change of speed 

 (22 — 21.7 knots) in going from normal to 12.7 per cent over-displacement, 

 the slip may be assumed constant. Then for the same power and revolu- 

 tions (414 revolutions per minute) the speed is reduced from 22 knots to 

 21.7 knots. 



To obtain a new speed-revolution curve for the conditions of the trials, 

 from the curves of effective horse-power and speed revolutions at normal 

 displacement, a horizontal line ah of constant effective horse-power was 

 drawn at an arbitrary point. At a the line ac was laid off vertically up- 

 ward, equal in length to the intercept bd. This gave one point c of the 

 new speed-revolution curve. The new curve of revolution was completed 

 by a fair curve through a number of such points. 



The record of revolutions shows a notch for each 10 revolutions of port 



