On the planar motion mechanism used in ship model testing

Abstract

In the linear theory of small departures from steady reference motions of submarines and ships it is standard practice to employ the idea of hydrodynamic ‘derivatives’. These derivatives permit the magnitudes of fluid forces and moments to be specified. In recent years it has become common to measure the derivatives by means of a - planar motion mechanism ’ which is essentially a device for oscillating a ship (or submarine) model while it is being towed in a testing tank. The derivatives referred to in the maritime literature have invariably been * slow motion derivatives ’. The theory of the planar motion technique is recast in terms of ‘ oscillatory derivatives ’—or, better. ‘ oscillatory coefficients ’, since they are more appropriate for use where the mechanism is concerned. The idea behind these quantities is borrowed from aeronautical practice, but it requires some adaptation because ( a ) ship models work at the water surface, and ( b ) ships and submarines are subject to significant buoyancy forces. There can be little doubt that the planar motion mechanism is a powerful tool and a reappraisal is perhaps timely since the first mechanism of this sort to be installed in the U.K. has recently been commissioned (1968).