The Thrust of a Marine Screw Propeller

Abstract

Part 1 of the paper puts forward the proposition that since the phenomenon of development of thrust is much the same in all screws, there should be a common basis by which it can be represented.

The characteristics of aerofoils are considered, first in flight singly, and then the general effects which arise from the interference of adjacent blades. It is concluded from this work that for any blade there are two essentials to be fixed, the slip angle, and the slope of the lift curve as affected by aspect ratio and, or alternatively, cascade. This should afford a basis for plotting screw data.

Part 2 of the paper is an examination of the results obtained with over 100 model screws. The data are given in the form of the thrust constant k_T/sp to a base representing blade width. All the results plot reasonably well together, and mean curves for screws with various numbers of blades are given. These curves enable the designer of any screw of normal type to form an estimate of its thrust for any speed of advance and revolutions per minute. Methods are given in the paper for detecting eddymaking in the screw, and it is shown that a little roughness of blade surface, although having a material effect on efficiency, does not affect the thrust prediction.