Non-linear Behaviour in the Seakeeping of High Speed Craft

Non-linear behaviour in high speed craft motions can be attributed to high Froude number, increasing wave height and the variable above water-line geometry; wet deck slamming will add to the non-linear phenomena. When comparing wave piercing catamarans (WPC) to mono-hulls, conventional or semi-swath catamarans and trimarans it may be expected that the non-linear behaviour will vary with vessel type. Little data has been published to date on this behaviour possibly due to the difficulty or expense in determining the non-linear effects with limitations on software meaning the only viable alternative is model testing with the subsequent expense. Model tests of a typical WPC show strong non-linear behaviour in head seas at high Froude numbers with increasing wave height. The effect can be seen in non-linear pseudo transfer functions or response amplitude operators (RAOs) where the resonant peak of the heave transfer function reduces as the wave amplitude is increased. The pitch transfer functions show similar non-linear behaviour but not to the same extent as the heave. Little non-linear behaviour is seen at lower Froude numbers.