Cavitation on Marine Propellers

Abstract

Abstract The author defines terms used In connection with cavitation of marine propellers and hydraulic turbines. He points out that cavitation is a problem of growing importance where higher powers and speeds are required, and when cavitation occurs that thrust, torque, and efficiency are reduced and erosion of the hardest metals occurs. Theoretical and experimental research into the mechanics of cavitation and its effect on thrust, torque, and efficiency is reported. The author reports that the standard or usual equations for thrust and torque are merely special cases of a general thrust and torque theorem which he develops. He points out that cavitation on marine propellers is a phenomenon different from anything occurring on aerial propellers. The author discusses the following items in detail: (a) In the general case, thrust and torque coefficients are functions of slip ratio, rotary speed, and pressure, and are not merely functions of slip ratio alone as is generally assumed. (b) For thrust and torque coefficients to be treated as functions of slip ratio only, the propellers must operate at low rotary speed or under high-pressure conditions. (c) Marine propellers operate under low-pressure conditions. (d) A simple equation indicates where burbling cavitation will begin on the back of any blade element, and what can be done to delay the appearance of such cavitation. (e) A vortex, in which the water particles move in approximately helical paths around the axis, is prerequisite to laminar cavitation. (f) A laminar cavity is not in contact with the propeller blade, but is separated therefrom by a wall of water. (g) Both kinds of cavitation can actually reduce thrust and torque with increasing rotary speed, and not merely reduce the thrust and torque below the values which would have been attained in the absence of cavitation, as is usually assumed. (h) After either type of cavitation completely covers the back of propeller blades, thrust and torque will increase again with increasing rotary speed, although at a lesser rate than in the absence of cavitation. (i) Both kinds of cavitation result in an increase of pressure on the cavitating side of the blade in excess of the pressure which would have existed in the absence of cavitation, and this accounts for the relatively small effect on thrust and torque when cavitation occurs on the face of the blade. In conclusion, the author shows that previously reported explanations are inadequate to account for all the observed effects of cavitation on thrust and torque. He then develops his own theories to explain such effects, and shows that insight into actual physical phenomena has frequently been obscured by the otherwise useful custom of plotting results in the form of dimensionless, or quasi-dimensionless, coefficients against dimensionless products such as Reynolds’ numbers or slip ratio.