Grafton, T.J.; (2007) The roll motion of trimaran ships. Doctoral thesis , University of London.

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Abstract

This thesis reports on research conducted into the roll motion of trimaran ships. After reviewing the relevant literature to determine the state of the art of roll motion prediction for both monhull and multi-hulled ships a hypothesis is set out that:- Accurate trimaran roll motion predictions can be obtained using linear Potential Flow Seakeeping theory with the roll damping term either obtained from a roll decay experiment or augmented with empirically based theoretical roll damping components developed for monohulls. This hypothesis underpins the work of many of the existing researchers who have investigated the seakeeping performance of trimaran ships, although none have formally proved it to be true. After conducting theoretical, experimental and combined theoretical and experiment studies using a single trimaran model this hypothesis is subsequently disproved. This leaves a problem: How can accurate trimaran roll motions be deteraiined The focus of the remainder of the thesis is to understand why the hypothesis is incorrect, investigating in turn each of the assumptions that underpin it. Finally, as a recipe for future researchers, a series of experimental and theoretical investigations has been devised to explore the physics of trimaran roll motion from first principals. This work has shown that, for a trimaran with significant flare above the waterline on the side hulls, roll decay coefficients cannot be measured from free decay experiments if the motion is characterised by a single degree of freedom roll equation with constant coefficients. Furthermore, it is postulated that heave and roll are strongly coupled for all trimarans. It is shown, using the results of model experiments, that this coupling breaks the assumption of linear theory where an input sinusoidal wave of constant amplitude and frequency leads to output motions which are also sinusoidal with constant amplitude and frequency.

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