Oscillatory testing of ship models

In order to determine the directional stability of ships and submarines it is necessary to know the hydrodynamic forces due to the various motions which might occur. It is commonly accepted that very simple expressions based on “slow motion” derivatives are adequate for this purpose, and oscillatory tests are finding increased favour as a means of measuring them. However, this simple theory is not adequate when applied to the more severe motion of the oscillatory test itself. This paper reviews the theory of oscillatory tests and their application to the measurement of “slow motion” derivatives. In particular it is suggested that the effects of past history of the motion might be represented by expressing the forces as the solutions of differential equations with constant coefficients. The relations of these coefficients to the “slow motion” derivatives which we seek are established.