3o6 RUDDER TRIALS, U. S. S. STERETT. 



ordinate of the force curve the turn is considered finished after a swing of 

 90°. As soon as the helm is put back amidships, the tension in the tiller- 

 chain falls rapidly to its initial value. 



In Appendix VI is given a detailed description of the method employed 

 in constructing a new speed-revolution curve for the displacement (12.7 

 per cent greater than on standardization) at which turning trials were run. 

 From the record of port and starboard revolutions, together with the time 

 record, the average revolutions per minute on approaching the turn were 

 computed and the corresponding speed in knots obtained from the revised 

 speed-revolution curve (Fig. 18, Plate 128). The rate of putting helm over 

 was similarly obtained from the helm record line. 



The tension in the tiller chain in pounds at any point during a turn is 

 obtained from the corresponding ordinate of the force curve referred to the 

 calibration plot for the particular spring used. Both the maximum tension 

 and the tension on steady turning were measured and tabulated in Tables 

 I, II, III, IV, appended. 



As explained in Appendix V, the maximum forces recorded by the 

 dynamometer were diminished by a certain amount to allow for the dynamic 

 friction overcome in moving the rudder and gear. The forces recorded on 

 steady turning, however, were not corrected for friction because it is believed 

 that the static friction of rudder and gear was neutralized for this condition 

 by vibration. 



The forces recorded were reduced for average helm-angles of 16°, 26°, 

 31°, and 35°, these being the average values of the measured helm-angles 

 which varied not more than 1° or 2° on either side of the averages. 



CURVES OF TWISTING MOMENT. 



From the data given in Tables I, II, III, and IV, curves were plotted 

 with maximum twisting moments (Mm), and twisting moments on steady 

 turning (Ms), as ordinates, on speed of approach in knots, as abscissae. 

 These curves were for the four average helm-angles referred to above, as 

 shown on Figs. 10 and 11, Plate 120 and 121. On the curves of Fig. 10, 

 Plate 120, it will be observed that there is a pronounced hump, beginning 



at about 22 knots speedT— -7y= = 1.3V The average deviation of points 



from the curves is 1.5 per cent of the maximum value for Mm and i.o per 

 cent of the maximum for Ms. Cross-curves of Mm and Ms on helm-angles 

 as abscissae were then plotted at constant speeds of approach, as shown on 

 Figs. 12 and 13, Plates 122 and 123. From the following table, it appears 

 that the ratio of Mm to Ms varies from i .3 at 28 knots to 1.04 at 16 knots. 



