Ship waves: The relative efficiency of bow and stern

Abstract

It seems fairly certain that one of the main causes of difference between theoretical and experimental results is the neglect of fluid friction in the calculation of ship wave, and further that the influence of fluid friction may be regarded chiefly as one which makes the rear portion of the ship less effective in generating wave than the front portion. The process may be pictured, possibly, in terms of a friction belt or boundary layer whose more important effect is equivalent to smoothing the lines of the model towards the rear. Some calculations were made from this point of view in a previous paper, the purpose then being to show how such an asymmetry, fore and aft, reduced the magnitude of interference effects between how and stern waves. We may also describe the frictional effect as a diminution in the effective relative velocity of the model and the surrounding water as we pass from how to stern. This is not very satisfactory from a theoretical point of view; but, on the other hand, it leads to a comparatively simple modification of expressions for the waves produced by the model. From a formal point of view, we may regard the modification as an empirical introduction of a reducing factor to allow for decrease in efficiency of the element of the ship's surface as we pass from how to stern. There are now available experimental results, for wave profiles as well as for wave resistance, which made it possible to attempt such a comparison. The following work is limited to a few simple cases, and the assumptions are made in as simple a form as possible for the purpose of the calculations; these deal with the wave profile and wave resistance of a model of symmetrical form, and also with the difference between motion how first and motion stern first for a simple asymmetrical model.