Contra-rotating propellers 1

Abstract

After a brief description of the velocity field induced by a propeller, the results obtained from open-water tests with systematically varied contra-rotating propellers of the B3-65 type are discussed.

The results of these experiments are given in a ${\mathit{B}}_{\mathit{p}}-\mathit{\delta }$ diagram. With the aid of this diagram it is possible to derive a satisfactory survey of the efficiencies of contra-rotating propeller systems. This diagram also supplies data concerning the optimum diameters and the pitch ratios of the front and rear propellers.

An analysis with the aid of circulation theory of the results of the experiments shows that with a distance of $0.37\mathit{D}$ between front and rear propellers the axial induced velocity generated by the rear propeller is, in way of the front propeller, about 20 per cent. of the value it has infinitely far downstream of the rear propeller.

Finally, a method based on circulation theory is developed by which contra-rotating propellers can be rapidly designed, provided they are of optimum diameter.