Experiments and CFD of a variable-structure boat with retractable twin side-hulls: Seakeeping in waves

Abstract

The form-changeable boats have received much attention due to its far-ranging applicability, strong combat capability and the low operating cost. This paper constructs a variable-structure boat whose mainhull is planing hull form and twin retractable deep-V side-hulls are placed on both sides of the mainhull, aims at assessing its seakeeping performance in the monomer-form state (MFS) and the trimaran-form state (TFS) by model tests, numerical simulation and sea trials. Regular head wave tests were conducted at volume Froude numbers (Fr∇) ranging from 0.44 to 2.67 for the MFS and TFS models. Verification and validation of the CFD method were performed, the influences of side-hull draft on the seakeeping of TFS model under different wave configurations were analysed. Sea trials of the full-scale vessel (USV) in irregular sea were also carried out at the design velocities. The results show that the TFS has different effects on motion responses and vertical accelerations of the model at different speeds and wavelengths, the applicable range of side-hulls with gain effect is in the diaplacement and semi-planing stage, and the maximum velocity is suggested not to exceed Fr∇ = 2.22. For the USV at Fr∇ = 1.33, the TFS mainly cut down its roll in irregular head sea, followed by following and beam sea; when the Fr∇ increases to 1.78, the roll gain declines, but the pitch gains more, the better side-hull draft ratios are 1.0 and 0.67, corresponding to the above two design speeds, respectively.