164 VARIATIONS OF SHAFT HORSE-POWER, ETC., 



The models were carefully made and all were fitted with the same cast stern 

 frame, which included the stern bearing for the propeller shaft. The stern frame 

 had the rudder cast with it. The whole frame and rudder was fitted to each of the 

 four models before the self-propulsion experiments were undertaken, and, together 

 with the propeller shaft, propeller, and dynamometer, was transferred from one 

 model to the other as the experiments with each model were completed. 



The dynamometer consisted of a small direct-current motor, the armature shaft 

 of which was directly connected with the propeller shaft by means of a flexible coup- 

 ling. The armature shaft was free to float fore and aft in its bearings about 7/16 

 inch in an axial direction. The armature shaft was connected to a calibrated spring 

 by means of a thrust bearing, so that the axial displacement of the armature shaft 

 gave a measurement of the propeller thrusts. Similarly, the frame of the motor was 

 mounted so as to rotate in independent bearings. The torque developed by the motor 

 acted against a calibrated spring so that the deflection of the spring 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 bear- 

 ings, 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 dififerent speeds, and the 

 propeller shaft, without propeller, run at the range of revolutions 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 

 10 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 guilding points attached to the car- 

 riage, 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 con- 

 trolling 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, start- 

 ing 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 dis- 

 carded 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 



