Seakeeping Analysis of a SWATH-type Trimaran Using Potential Flow

Abstract

Potential flow methods tend to over-predict ship motions, especially the lightly damped, viscous effect dominated modes, such as roll. For SWATH ships, it is not uncommon to also over-predict heaving and pitching motions because of the strong viscous effects dominating the energy dissipation mechanism due to significant vortex shedding generated by the submerged hull(s). Care should be taken whenever such viscous effect become a significant part the total damping. Such fact appears to render the potential flow approach less useful when analyzing the restored response of SWATH ships compared to conventional ships, such as monohulls. The Atlantic Center for Innovative Design and Control of Small Ships (ACCeSS), under ONR sponsorship, has been studying a hybrid trimaran hull form with a small waterplane area center hull, so-called the Tri-SWACH. Extensive towing tank testing has been undertaken with the intent to characterize the behavior of the Tri-SWACH and to provide data for the verification and validation of predictive tools. For previous work, the scale models of the Tri-SWACH were identical below the waterline, but lacked any realistic cross-deck structure above the waterline. This setup is ideal to capture the nonlinear ship motion behavior in future studies since the scale and numerical models were made identical above and below the waterline, and include a representative cross-deck. For the current study, the linear transfer functions and short-term statistics are obtained using the potential flow code WASIM, and are partially validated using experimental results obtained from the United States Naval Academy and Stevens Institute of Technology.