RUDDER TRIALS, U. S. S. STERETT. 327 



B = Force to overhaul lead B (starboard side) A and B are assumed to be 

 equal. 



R = Force required at end of tiller yoke to overcome rudder friction. 



T = Force required at end of tiller yoke to overcome moment of water- 

 pressure against rudder, when under way. 



PF= Force required at end of tiller yoke to overcome moment of water- 

 pressure against rudder, when moored alongside pier. 



NoT^.- — Owing to the double purchase in A and B, a force in either lead, 

 forward of the sheaves, is equivalent to a force twice as great, at 

 the end of the tiller yoke. 



The cases considered are: — (i) Helm moving to starboard. The force 

 in dynamometer (D) equals force required to overcome the friction of over- 

 hauling lead B plus one-half force to overcome friction of rudder in its 

 bearings, i. e.: 



Di^B+^R (curves I and V, Fig. 17) (i) 



2. Helm moving to port. The force in dynamometer D2 equals force 

 to overcome friction of overhauling lead A, i. e., 



Di = A=B (curves II and IV, Fig. 17) (2) 



:.Dx-D, = B+\R-B = \R (curve IH, Fig. 17) (3) 



On the tests afloat, the value of Di was found to be not greatly different 

 from, although somewhat less, than that obtained in dry-dock. The value 

 of I>2 afloat, however, was found to be considerably greater than in dry-dock. 

 There are several factors which may serve to explain this increase in D^ 

 with the consequent decrease in Di—D2 = ^R. 



1. The tests in dry-dock were made in a heavy rain, with decks, chains, 

 fair leads, etc., wet. Therefore we might naturally expect the friction of 

 deck-leads to be less than on the second day, when the deck was dry and 

 gear probably somewhat rusted. 



2. When the ship is afloat, the rudder is partially supported in its 

 bearings by its own buoyancy, amounting to some 1,660 lbs., and is lubri- 

 cated by the surrounding water. This should give a smaller value for 

 rudder friction (R) with ship afloat than with ship in dry-dock. On the 

 other hand there was probably some additional force (W) required to move 

 the surrounding dead water; this force as well as the effect of any tidal 

 currents has been neglected. 



In view of the above considerations, and since it is believed that the 

 tests afloat represented more nearly, in every way, the conditions which 



