The Unified Slender Body Theory for Lateral Motions of Ships with Forward Speed

Abstract

This paper describes a new slender-body theory for the sway and yaw motions of ships, moving with forward velocity in calm water. In analyses for vertical plane motions of heave and pitch by use of the Slender-body theory, the unified theory established by Newman and Sclavounos is known as a powerful method. The present study is an extension of this unified theory to the lateral motions of ships. The unified theory emphasizes that it is reasonable to include an expression for three-dimensional interaction effects in the so-called strip theory, although the strip theory for lateral motions has been considered so far to be valid for all frequencies of practical interest. Computations are also made for the added-mass and damping coefficients and then results are compared with experiments for a container ship model in order to investigate the relative importance of three-dimensional effects on the hydrodynamic sway force and yaw moment. As a result of comparison, it is revealed that three-dimensional effects are not so significant and that another formulation, independent of the usual strip-theory concept, must be considered in order to incorporate the forward-speed effect in the inner problem of slender-body theory, particularly for relatively high Froude numbers.