320 POWER AND SPEED TRIALS OF 



but is affected by appreciable tidal currents, which, however, usually run in a direction parallel 

 to the course. The water is brackish with a density slightly greater than fresh water. The 

 ranges are well defined, and, in addition, the terminals are marked by buoys. 



METHOD OF RUNNING TRIALS. 



The trials were run continuously and consecutively, beginning with the slow speeds and 

 working up, except for runs 2 and 4, which are transposed on account of a misunderstanding 

 of signals. The power was obtained with Crosby outside spring indicators, and at least three 

 diag'rams were worked up for each cylinder on each run. The reducing motion was of the 

 quadrant type. The time over the course was taken by two observers with stop watches. 

 Long turns were made at each end and conditions well stabilized on the straightaway before 

 crossing the starting lines. 



PRELIMINARY CURVES. 



From the observed data two curves were plotted (Plate 64), one for the northerly runs 

 and one for the southerly runs. These curves were then corrected in two steps, first for 

 the wind effect, and second for the tidal or current effect. 



The observations made were time over course, revolutions per minutes and indicator 

 cards. From the time over course, speed by shore can be derived; from the indicator cards, 

 the mean effective pressure for each cylinder was figured and the mean effective pressure re- 

 ferred to low-pressure cylinder calculated. (Denote revolutions by R. P. M., speeds by V and 

 mean effective pressures referred to low-pressure cylinder by M. E. P.) As F and M. E. P. 

 both vary with R. P. M., it is convenient to use R. P. M. as the primary variable. 



When plotted on R. P. M., speeds and M. E. Ps. fall into two groups — northbound and 

 southbound. Northboimd, the ship was rtmning against both wind and tide; southbound, 

 with both. Except for a few erratic points, fair curves can be drawn through these two 

 groups of Vs. and M. E. Ps. and used in place of the original observations. 



Plate 64 shows the original observations plotted and the northerly and southerly curves, 

 uncorrected for wind or tide, faired through them. 



DERIVATION OF FINAL CURVES. 



To these original observation curves were applied corrections for wind effect and tidal 

 or current effect, as follows : 



The effect of the wind on the speed of the ship is proportional to the square of the rela- 

 tive velocity of the wind by the ship. For example, at about the time of runs 2 and 3 the 

 wind was estimated at 15 knots. Run 2 speed was 7.5 knots against wind and tide; run 3 

 speed was 9.5 knots with wind and tide. Relative wind speed "against wind" was 22.5 

 knots ; relative wind speed "with wind" was negative, or helping the ship, and of an intensity 

 of 5.5 knots. The normal air resistance should be against the ship and equal to the speed of 

 the ship through still water, or about 8.5 knots. 



Against; proportional to 22.5 = 507 



2 



With; proportional to 5.5 = 30 

 Normal; proportional to 8.5 ^= 72 

 Change from normal — Against 435 



With 102 



Ratio . 4}i 



