On the use of lifting surface theory for moderately and heavily loaded ship propellers

It is usual to subdivide the loading range of a ship propeller, in which it developes a thrust in the direction of advance, into light, moderate and heavy loadings. The division is based on the degree to which the flow is influenced by the action of the propeller. For the heavily loaded propeller no generally reliable computation model is yet available. Often a computation model for the moderately loaded propeller is used,in which the effects corresponding to the deformation of the wake are missing. In this study it is investigated,for a propeller model with infinitely many blades, in which way the characteristic flow phenomena of the wake react on the flow at the location of the propeller. For that purpose a consideration,indicated by Von Karman and Burgers, of the hydrodynamical equation of motion is applied. In doing so the non-linear terms and the pressure term in the equation of motion are partly considered as secondary forces and partly derived from an acceleration potential. It is proved in this way that the centripetal pressure build-up, due to the rotational velocities in the wake, can be considered as a secondary force on the propeller model and induces an axial velocity. The contraction of the wake also induces velocities at the location of the propeller model, which cannot be associated with secondary forces on the propeller model, however. The effects found are, in modified form, also assumed for a propeller with finite blade number. They are introduced into an available madel based on lifting-surface theory. The results of the analysis of some propellers show a good correlation with experimental data. This correlation leads to the conclusion that the mean axial velocity at a propeller blade section is practically independent from the load and equal to the product of the rate of rotation and the pitch.Applied Science