328 RUDDER TRIALS, U. S. S. STERETT. 



obtained on the subsequent turning trials, it was decided to use the data 

 from friction measurements afloat in making corrections for friction (see 

 curves I, II, and III, Fig. 17, Plate 127). The average deviation of points 

 from the curve I is about 60 pounds, or 5 per cent of the stress at 25° helm. 

 This means an error of less than i per cent at 28 knots speed and 35° helm. 

 As all turns were made with starboard helm, the two cases of greatest 

 interest are : — 



I. Ship under way, helm moving to starboard. 



II. Ship under way, helm steady a-starboard. 



Case I represents at its limit the condition at which maximum force 

 was obtained. Then the friction of the rudder and the friction of overhauling 

 B both tend to increase the tension in A , and consequently the dynamometer 

 reading must be diminished by the amount of these forces to obtain the 

 force to overcome moment of water-pressure on rudder, i. e.:—r 



Or, subtracting from the dynamometer reading {B+^R) (curve I) for 

 proper helm-angle, we obtain the half value of force to overcome moment 

 of water-pressure against the rudder. 



\T={D-hR-B) (4) 



Case II represents the condition which existed when the so-called 

 "steady" value of stress (considerably less than the maximum) was obtained 

 after the ship had turned through approximately 90°. With the helm 

 stationary at a definite angle with the center-line of the ship, the frictional 

 forces become static. The tension in lead A still holds the rudder against 

 the turning force of the water. The rudder friction {R) opposes any motion 

 of the rudder in either direction ; and since it helps the tension in A to hold 

 the rudder, the dynamometer reading should be increased by the amount 

 of the force. The tension in B set up in overhauling, due to friction of 

 sheaves, etc., opposes the pull in A and would therefore be subtracted from 

 the dynamometer reading, i. e.: 



D = IT+B-\R (5) 



However, any tendency of the rudder to move in one direction or the other 

 would change these various frictional forces. At best, the eftect of friction 

 in the condition of "steady starboard" is somewhat indefinite, but is 

 certainly diminished by vibration, as considered below. 



When steaming on a steady course with helm amidships, the vibration 

 at the stern of the ship was appreciable, but not all violent. On starting 



