Design and hydrodynamic performance of trimaran displacement ships

To meet the demands for increasing the speed and improving the seakeeping behaviour of commercial and naval ships, this thesis investigates a new type of ship configuration - The Trimaran Displacement Ship, it features a slender centre hull and two small side hulls. The design methodology and hydrodynamic performance of this new ship concept has been investigated through design studies, model experiments and theoretical analysis. Potential advantages of the trimaran ship are, lower wavemaking resistance at high speed, larger deck area, and improved seakeeping behaviour compared with existing ship types. Firstly, existing marine vessel types, both monohulls and multihulls, are reviewed with regard to their advantages and limitations. The new trimaran concept and its background is then described from its initiation by a desire of inheriting advantages and avoiding limitations of these existing ship types. Namely, achieving the low resistance of slender monohulls and large deck area of multihulls, whilst eliminating the speed limit of conventional monohulls and the stiff roll motion of catamarans. The review of the trimaran ships design studies shows the potential applications of this new concept in commercial and naval roles. The feasibility of the new concept and the methodology required for its design are investigated through the concept design studies of. a trimaran fast ferry (Figure 1) and the hull form design for a trimaran model ship (Figure 2) for seakeeping experiments. This provides an initial view on the design procedure and basic design considerations for the trimaran ship. The parametric study in the trimaran ferry design gives the basic parameters for trimaran hull forms. Hydrodynamic performance of the trimaran ship has been investigated through model experiments and theoretical analysis on seakeeping, resistance, and manoeuvrability. Good agreements between theoretical predictions and model experiments have been achieved. This shows the merit of the computer programs developed during the investigation so they can be used in future trimaran ship designs for hydrodynamic performance assessments. A three dimensional theory is used in the trimaran motion analysis. Roll damping characteristics of the trimaran ship has been examined by including viscous effects in roll damping which can be derived either by simulating damping data from free decay experiments or by direct computation, that has been shown to improve the roll motion predictions. Systematic investigation into the wavemaking resistance of the trimaran ship reveals the relationship between the side hull configuration and the resistance performance. Wave cancellation effects can be achieved when the side hulls are appropriately located to further reduce the wavemaking resistance of the trimaran ship. A manoeuvrability study concentrated on the effect of the side hulls on the turning ability of the trimaran ship as well as the effect of side hull propulsion. Following the hydrodynamic analysis of the trimaran ship, the design procedure and general considerations in trimaran ship design distinctive from other ship types are proposed and discussed with particular reference to stability and hydrodynamic performance. Trimaran hull form options are also discussed alongside some other design considerations. The thesis concludes that the new trimaran displacement ship shows superior characteristics in some hydrodynamic aspects over existing marine vessels, particularly in resistance and seakeeping, and therefore there is no reason why this novel concept can not be translated into real ships.