A PRACTICAL DESIGN PROCEDURE OF CONTRAROTATING PROPELLERS

A practical design procedure for contrarotating propellers is presented. Two methods are used. The first is the optimal design method using the equivalent operating conditions and design charts for single propellers. The other is a calculation method for estimating the influence of the nonuniform flow on the performance of the contrarotating propellers. Next it is shown how calculations and model tests for a contrarotating propeller designed for an 80,000 dwt tanker demonstrate that in comparison with a single propeller whose decrease in efficiency due to slipstream rotational flow is estimated at 12%, the efficiency improvement of a contrarotating propeller under the same load is 8%. The difference is mostly due to additional viscous loss. They also show that vibratory shaft forces having frequencies other than integer multiples of the blade frequencies appear by the interaction between the two propellers rotating at the same or different speed of revolutions.