The influence of stern vortices on ship manoeuvring

Abstract

This thesis is concerned with the steering and manoeuvring of surface ships. Recent developments in hull design have seen the introduction of the pram stern. This hullform has proved less directionally stable than more conventional stern shapes. Recent theoretical developments include the effect of stern vortices in a slender-body treatment of the ships hull. The inclusion of these vortex effects explains the discrepancy between the distribution of force and moment coefficients along a hull obtained experimentally compared to those determined using slender body theory alone. As yet this approach requires the position and strength of the shed vortices to be determined by other means. The theory also offers a means of explaining the relative instability of the pram stern. An experiment has been designed to directly test some of the predictions of this theory. The experiment entailed oblique towing of a set of 7 hullforms with segmented stern sections. The first group of five models are based on the British Bombardier with a variety of conventional and pram stem configurations. The remaining models were based on a simple elliptic hullform. The experiments provide detailed data of the longitudinal distribution of force and moment sway derivatives of the stem regions of these models. The experimental evidence provided support for the theoretical predictions, although some results were inconclusive because of the effect of additional flow phenomena. The physical insight provided by the generalised slender body theory has generated the impetus to use a semi-empirical approach to predict the manoeuvring derivatives from basic hull geometry. The equations developed for the linear velocity derivatives use predictors based on the physical phenomena and are formulated to recognise the interdependency of these derivatives. The new equations are statistically more satisfactory than previous analyses of this type.