A Generalized Optimum Condition for Wake Adapted Screw Propellers Available to Purchase

A screw propeller design problem is considered for two cases: 1) both circulation distributions along chord and radius are pre-set; 2) chord circulation distribution is pre-set and radial circulation distribution is to be optimized for maximum propeller's efficiency. In both cases a Generalized Lifting-Line (Linear) Model (GLM) is used. This model supposes the existence of an undeformed helical vortex sheet behind each blade. Though, differing from orthodox linear vortex models the pitch of the mentioned helicoid is assumed to be a model's free parameter serving for filling calculated integral characteristics (thrust, torque, efficiency) experimental data. An algorithm is given for the calculated determination of this parameter with account for radial non-uniformity of axial and circumferential wake velocities. On a number of examples the efficiency of the proposed. The algorithm is demonstrated and it was implemented in the computer code SPD-96 (Abbreviation for Skew Propeller Design) capable to carry out the hydrodynamic design of propellers (including those with high skew) with prescribed circulation distribution according to the lifting surface theory. A design example for a 72 deg. skew propeller is given. Within the framework of the lifting-line theory in its GLM-modification a generalized optimum condition (GOC) had been obtained earlier (Achkinadze, 1985). This condition takes into account axial and circumferential wake non-uniformity and is also characterized by a more consistent account of profile loss. It is pointed out that many known optimum conditions can be deduced from the GOC and are valid only for usual or generalized linear models. A numerical solution of the integral equation obtained on the basis of the lifting-line theory coupled with the use of induction factors allows one to obtain very accurate optimal circulation distributions together with corresponding values of propeller efficiency al given thrust or torque. The influence of viscosity was taken into account according to A1ishkevich. Some examples are given of successful design of single and contra-rotating propellers. It is shown that at some sufficient wake swirl the obtained optimum circulation distribution is close to that corresponding to the so-called unloading distribution which is usually used for decreasing levels of cavitation noise and vibration. The described screw propeller design method is based on the traditional linear concept but due to proper choice of the free vortex pitch is capable to assure sufficient accuracy in many cases being important for practice. In addition, the method possesses the following advantages: 1) design is guaranteed of wake adapted propellers with full account for axial and circumferential wake velocities; 2) optimization problems are well-posed and can be partly solved analytically; 3) reliable evaluation of propeller efficiency including the case when the blade tip sections are overloaded.