For some time it has been recognised that the aerodynamical theory of the screw propeller, as at present presented, leaves considerable room for improvement. The leading specialists in this branch of aerodynamics, such as Mr. Fage, Mr. McKinnon-Wood and Dr. Watts, have given recognition to the all-important fact that a velocity of inflow can only, be produced by the mutual interfering action of the propeller blades upon each other. Dr. H. C. Watts was, I believe, the first person to point this out clearly, and to show that the hitherto accepted theories of inflow rested upon a highly empirical basis. By “ velocity of inflow ” is meant the additional velocity required to modify the method of propeller analysis, known as the blade element theory, originally enunciated by S. Drzewiecki in 1892. The accepted and standard inflow theory of the airscrew, as, for example, given in Professor L. Bairstow's “ Applied Aerodynamics,” rests upon the empirical assumption that the ratio (velocity of inflow)/(velocity of impressed slip) is a constant for all propellers and for all radii along the propeller blade length.