Modelling trailing vortices from a slender ship hull for manoeuvring calculations

Abstract

A particular problem that has been encountered in modelling the forces and moment acting on a manoeuvring ship, has been the correct estimation of the distribution of side force along its length. If traditional slender body theory is used, reasonable agreement can be obtained between theoretical and experimental result over the forebody of the ship. However, moving aft, the two increasingly diverge until there are significant differences at the stem. For this reason manoeuvring coefficients cannot be accurately predicted by this approach. In a number of studies, the reason for the discrepanciesh as been attributed to the influence of trailing vortices that develop along the hull. The conclusion is consistent with sensitivity analyses carried out with augmented slender body theory incorporating vortices of specified location and strength along the ship. The present thesis is concerned with modelling trailing vortices along a ship in drift motion so that they can be used in the calculation of the associated distribution of forces and manoeuvring coefficients. A numerical model based on the Discrete Vortex Method has been developed for the analysis of vortex flow around the ship which is representedb y slender body approximation. The trailing vortices are modelled by a series of transverse two-dimensional multi-vortex solutions marching longitudinally down the hull. Results are presented for six different hull types; a flat plate, the Wigley hull, a block hull, a Series 60 hull, the British Bombardier and the British Bombardier with a pram stem. The effects of varying drift angles are also investigated for each hull types. Good qualitative agreement is shown between the predicted velocity and vorticity fields and results from experimental studies. The distribution of side forces and yaw moments along the hull is also well predicted. The results explain manoeuvring phenomena occurring for the hull forms considered that have been observed experimentally and at full scale.