The Fundamental Assumptions in Ship-Motion Theory

Abstract

Paper prepared for International Symposium on the Dynamics of Marine Vehicles and Structures in Waves, London, April 1974.&&Some of the fundamental assumptions of ship_motion theory are examined for the purposes of elucidating the success of the heuristically derived strip theory of Korvin_Kroukovsky and of recognizing some inadequacies of that theory. The formal approach employed is a systematic slender_body expansion. In the zero_speed problem, both far-field and near_field views can be used generally to recommend the assumption that frequency of oscillation be taken as "large, " in the sense that the corresponding waves have wavelength that is comparable to ship beam. This assumption leads to considerable trouble in the head_sea case, however, and this special case has not been fully solved yet. Force and moment on a ship can be computed, even in the head-sea case, through use of the Khaskind formula, but computation of the load distribution necessitates solving the diffraction problem, or, possibly, solving two_dimensional near_field problems involving the Helmholtz equation. Rationalization of the short_wave assumption is not really successful in the forward_speed case, except in terms of the observed accuracy of the motion predictions. The most thorough analyses to date of the forced_motion and head_sea diffraction problems are based on disparate assumptions about the orders of magnitude of the characteristic wavelengths, although both require the product of speed and wave frequency to be large. Some discussion is presented on the interaction between ship oscillations and the steady_motion perturbation of the incident stream.