Abstract
The speed characteristics of the propeller noise of ship are measured in an experimental water tank. Instead of a model propeller, a rectangular iron bar is rotated by a 3kWD. C. motor. Ultrasonic noise is received with a 17. 5kc magnetostriction transducer, and audio ferquency noise with a moving coil type hydrophone with a thick rubber diaphragm. No noise except that due to mechanical vibration is detected at the speed under a critical speed of about 8m/s at the end of the bar, and a violent noise is generated when the speed is raised beyond the critical speed. The amount of bubbles produced by cavitation is measured at each speed, and its variation with the speed is similar to the noise voltage. If the depth of the rotationg bar is shallow that air is sucked into the water, some noise is detected even under the critical speed. For the investigation of the cavitation noise due to the roughness of outside surface of a ship at high speed, a wooden drum is replaced for the rotating bar, and several models of shellfish or head of rivet are attached on the cylindrical surface of the drum. The critical speed for these cases is 9-10m/s(17. 5-19. 4 knots). The author suggests, in conclusion, that a propeller and water can generate no noise without the presence of cavitation or bubbles.
