OF SINGLE-SCREW CARGO SHIPS. 129 



indicated the torque of the motor. In addition to this there were suitable means 

 provided for measuring the revolutions of the shaft. 



The order of procedure in making the tests was as follows: — The shaft and 

 dynamometer were carefully lined up and the whole run for a sufficient time to 

 warm up the bearings and reduce the bearing friction as much as possible. Owing 

 to the fact that the dynamometer was placed very close to the stern, but a short 

 length of propeller shafting was necessary, and this was supported by two self- 

 aligning bearings, one at the stern bearing and the other at the forward end of 

 the stern tube. With the propeller shaft in place and everything working freely, 

 the model was towed in the Model Basin beneath the towing carriage at several dif- 

 ferent speeds, and the propeller shaft, without propeller, run at the range of revo- 

 lutions to be covered in the course of the experiments. The propeller was then fitted 

 to the shaft, and cards for torque and thrust and revolutions per minute were taken 

 with the model self-propelled at different speeds. In these tests the model was 

 guided by two plates about lo inches in width placed at either end of the model 

 so as to steer it in a straight course. The guide plates floated between the guiding 

 points attached to the carriage, but the towing carriage did not exercise any force 

 on the model in a fore-and-aft direction. Starting at low speeds corresponding to 

 about 5 knots for the ship, the towing carriage was adjusted to run at a uniform 

 speed. The rheostat controlling the speed of the propeller dynamometer was then 

 adjusted so that the thrust of the propeller would just keep the model running as 

 fast as the towing carriage, without striking the stops, which were placed at an 

 interval of 6 inches. Thus, starting with the model in the mid position, it was free 

 to gain or lose a distance of 3 inches as compared with the towing carriage before 

 striking either stop. When the propeller was running at the proper speed to keep 

 the model up with the towing carriage, the record of thrust, torque and revolutions 

 per minute was taken. If, in the course of the run, the model struck either stop on 

 the carriage, the run was discarded and another run made. Having obtained the 

 desired data at the lowest speed, the carriage speed was increased for subsequent 

 runs and similar data taken at higher speeds. The range covered corresponded to 

 speeds of 5 to 13 knots for' the ship. About forty different runs were made with 

 each model, giving a corresponding number of points for plotting the torque, thrust 

 and revolutions per minute curves. 



The armature of the propeller dynamometer was especially designed to reduce 

 to a minimum the amount of magnetic thrust. This thrust increased with the torque 

 and amounted to 0.17 pound when the armature was displaced ^-^ inch and the 

 torque delivered to the shaft was 16 pound- inches. Neglecting this at higher powers 

 would have caused an error in thrust measurements of about 1.4 per cent, but 

 would not have caused any error in the power measurements. However, this mag- 

 netic thrust was separately calibrated, and corrections for it were made in working 

 up the results of the experiments. 



Immediately after completion of the self-propulsion tests on the model the pro- 

 peller was removed, and the runs to obtain the shaft friction and the thrust without 



