A Theoretical Approach to the Design of Hydrofoils

An investigation has been made into the application of the theory of thin sections to the design of hydrofoils having high cavitation speeds. Consideration is given to both symmetrical sections, which themselves are suitable for struts, and camber-lines, which, when used with the symmetrical sections, lead to cambered sections which are suitable for lifting surfaces. In all the aim has been to keep the peak local velocities to a minimum, and the sections developed differ from' low-drag' aeroIoil sections mainly in that, being hydrofoils, the sections have sharp leading edges. The theoretical optimum section consists of an elliptic symmetrical section superimposed on a logarithmic camber-line. Typical practical sections will cavitate at a speed lower by about 5 knots than the theoretical optimum section of the same thickness/chord ratio and at the same lift coefficient. For strut sections it is shown that sections having high cavitation speeds at zero incidence tend to be inferior to other sections at incidences as small as 2 deg. For lifting surface sections it is shown that although a high cavitation speed demands a low design lift coefficient, a high loading at cavitation demands a high design lift coefficient. Operation above cavitation speeds or over wide ranges of lift coefficient are not considered. 1. Introduction.-The theoretical approach to the design of hydrofoil sections follows very closely that of aerofoils with good compressibility chaSacteristics. There are, however, a few characteristics peculiar to the hydrofoil which make separate consideration necessary. In this investigation consideration is given to the design of symmetrical and cambered sections which have high cavitation speeds. Such sections are suitable for struts and lifting surfaces respectively. It will be assumed that the hydrofoils are sufficiently immersed for the undisturbed flow to be treated as a uniform stream of infinite extent.