Hull Design for Fast and Efficient Round-Bilge Pure-Displacement Craft using RANS CFD and Non-Linear Optimisation

Abstract

Fast round-bilge displacement hulls capable of operating above displacement speeds have long been important in the design of warships. The size of the vessels makes planing impractical due to the power and speeds that would be required to produce sufficient hydrodynamic lift. These vessels are uncommon when compared with both slow displacement and planing craft, the available design data is limited and their development has traditionally relied on the experimental testing of series of hulls. While vessels of the dimensions of ships can adopt slender forms and these are beneficial for reaching higher speeds, smaller vessels must maintain sufficient stability and can’t be designed with such narrow hulls. The hulls of small craft designed to operate above traditional displacement speeds evolved towards semi-displacement and planing hull forms, where hydrodynamic lift can reduce the effective volume of water displaced and the wetted surface of the hull. This is conceptually simple and can lead to reduced resistance, but at the added cost of producing such lift. Producing hydrodynamic lift in the transition speed range is largely ineffective because the flow velocity and resulting forces are insufficient in relation to the mass of the vessel. Designing fast displacement hulls for such small vessels is more difficult because stability requirements largely preclude using fine and narrow hulls, but it should be of great interest. The author uses a gradient-free search and optimisation algorithm coupled to a parametric modeller and the Star-CCM+ RANS flow solver to optimise the entire shape of complex hulls under displacement and stability constraints in a free-to-trim and free-to-heave condition. The process is computationally efficient in a large multi-dimensional solution space. It is first used to produce a model hull and test it in the towing tank of the Australian Maritime College in Launceston, Tasmania. The problem of obtaining the resistance of full-scale hulls is then considered and the same methods are applied to design a substitute hull of the same displacement and initial stability as an existing 20-metre hard-chine semi-displacement fishing boat. The author demonstrates the possibility of economical operation at 150% of hull speed. When this optimised hull is compared to hard-chine semi-displacement hulls, much better fuel economy at all speeds can be demonstrated and this difference increases even further when the hulls are required to carry payloads. Superior results are also illustrated in comparisons with existing fast displacement hull designs, such as the NPL hull, even when their length/beam ratio is more favourable.