Effect of boundary layer on thrust deduction

Methods of computing thrust deduction usually ignore viscous effects and assume that the flow field of the ship and propeller is irrotational. The present work was undertaken in order to compare the computed values of the thrust deduction with and without the boundary layer and wake. A streamlined body of revolution was selected, and a sink on the axis behind the body was used as a simple mechanism to simulate the suction at the stern induced by the propeller. A measure of the accuracy of a calculated irrotational-flow pressure distribution for a body in a uniform stream is the nonzero magnitude of its pressure drag. It has been found that, for calculating thrust deduction, the errors of available methods were acceptably large. A procedure for obtaining greatly increased accuracy was developed and applied to calculate the thrust deduction for irrotational flow. When the boundary and wake are present, the thick boundary near the tail of the body is first calculated by a previously developed method, in which the equations of a thick boundary layer and wake are solved numerically by finite differences, and the outer irrotational flow is obtained as the solution of an integral equation. An iteration procedure in which the inner and outer flows are successfully adjusted converges to the desired solution.