Stability of ships with forward speed

• Main Author: Darvazehnoie, Akbar Shahrbaf
• Published: University College London (University of London) 2008
• Subjects: 623.81
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582631

The initial stability of a ship is currently evaluated by empirical formula based largely on a static approach. Evidently stability is affected by speed that causes variation of pressure distribution on the wetted surface of a ship's hull, and generated waves on the surface of water by the vessel's motion. Forces and moments resulting from bottom pressures, as the speed of the ship changes, are significantly different for the ship at rest and in a seaway. The principal aim of the research is to investigate the effect of variations of forward speed on stability of a ship in calm water. The thesis presents theoretical and experimental approaches of the research. The novelty of the research results leads to the conclusion that although increasing speed may improve the stability of a ship in some cases, it also depends on heeling angle or on any asymmetry of the wetted area of a hull. Taking into account asymmetry effects, unbalanced pressure distribution acting on the wetted surface of the hull, in some cases, hence, decrease of stability is also possible. On a moving, partially immersed body, hydrodynamic and hydrostatic forces predominate. These forces arise owing to interaction between the body's motion and its weight with respect to the surrounding water. In order to calculate these forces, the panel method of Computational Fluid Dynamics (CFD) has been applied to identify the velocity and pressure distribution on the wetted surface of a ship's hull. The mathematical model adopted is based on the source distribution on the ship's hull, known as the Kelvin source. The model for a body travelling with steady forward speed, where its motion does not disturb the free surface, is known as double body theory. For consideration of waves generated by the motion of the ship on the free surface, a three dimensional linearised potential flow solution has been utilised. Comprehensive tests conducted in the UCL towing tank have established a better understanding of the significance of variation of ship's stability associated with forward speed in calm water. It is shown that an accurate judgement regarding the ship's stability cannot be made if only the effect of forward speed, as a single parameter, is considered. The heeling angle is another important parameter that must also be taken into account. The effects of a combination of both variables have been investigated, and are reported herein. The research is presented showing that the applied CFD method may be developed as an alternative method to assess stability of a ship in seaway, but there is a long way for the CFD approach to replace towing tank testing. At present, CFD may be used for consideration as a precursor to improve ship's stability during the design stage, for modification, and before operation. It is hoped that in future the findings of the experimental approach of this form of research could be used as additional guidance to be incorporated within the stability documentation for individual ships at the design stage, and ship trials.