PURPOSE: To improve the accuracy of under water acoustic observation by reducing the propeller cavitation of an observation vessel resulting in a noise source as low as possible.

CONSTITUTION: The underwater noise generated from a propeller 1 is detected by a sensor 2 and is input to a processing section 3. Simultaneously with this, the existent number of propeller rotations is detected by an axial tachometer and input to the processing section 3. The propeller noise is recognized and evaluated by the processing section 3. When the noise generated by the propeller 1 is increased in comparison with a preset allowable value, a propeller pitch angle reduction signal is transmitted to a speed controller 4 from the processing section 3 through a mode selection switch 8. Then, a signal synthesized with an external command 6 is transmitted to a propusion unit and the propeller rotation speed is reduced. As a result, the propeller noise level is reduced and the obstacle in acoustic observation is decreased.