The drag of windmilling propellers

Abstract

One would naturally think that a free-wheeling propeller in a fluid such as water, for instance, induces less drag than a locked one in the same fluid under the same conditions. A practical example where this drag problem arises is in the case of a vessel, which for some reason or other stops using one its propellers. Should this propeller be locked or allowed to rotate freely, in order to induce the least drag possible? Another important practical example where this difficulty arises is in the case of a racing yacht which when racing can only use its sails. Today technology has provided us with variable-pitch type propellers which at any required instance could be feathered in the direction of the fluid flow so as to produce the minimum of drag when such a propeller is not being used. However, the fixed-pitch type of propeller is still commonly used. The aim of this project was to investigate the drag of windmilling propellers (the term 'windmilling' was applied to free-wheeling propellers) and compare the results with those obtained by the same propellers when in a locked position. The tests in both cases were carried out under the same conditions. The characteristics of the windmilling propellers are plotted as graphs of the Drag Coefficient CD versus the Advance Ratio J, while for the locked propellers, CD versus Velocity V graphs are plotted. Instead of moving the propellers through a stream of water, the propeller is attached to a specially-built rig and the fluid - in our case water - is made to flow past the propeller, the latter being placed in a downstream position.