The object of this note is to draw attention to a misconception of the theory of screw propulsion which appears in much good work now being done.

This misconception lies in the combination of the Froude inflow theory and the Drzewiecki blade element theory used in conjunction with aerofoil characteristics obtained from tests in an air tunnel.

To explain further one must refer briefly to the various theories which are put forward to account, wholly or partially, for the behaviour of a screw propeller.

The theory and method of design in use for aircraft propellers is that due to Drzewiecki and modifications of it. The method in its simplest form consists in considering a tangential element of the blade as an aerofoil.