Experimental results and numerical simulations on strongly non-linear rolling of multihulls in moderate beam seas

A trimaran and a pentamaran ship have been investigated to study the effect of transversal hull separation on roll motion in waves. Four different configurations for the trimaran have been tested in beam waves and two for the pentamaran, all using the same main hull. Typical outriggers with small beam and draught have been used. The experiments in regular waves revealed the extreme sensitivity of these hull typologies to roll motion in beam waves, even in mild sea conditions (wave steepness = 1/60). Two configurations of the trimaran and one of the pentamaran exhibited a multivalued roll response curve in the low frequency range. This is, to the best of the authors knowledge, the first experimental evidence of bifurcations in roll motion of multihull ships, which are usually designed and selected for their superior stability characteristics and low rolling. Three mathematical models have been used with the aim of reproducing the experimental roll response curve. Two models are based on a partially semi- empirical extension of the standard linear seakeeping equations in the non-linear range, while one model is based on a non-linear one-degree-of-freedom modelling of roll motion. Two models are based on a relative angle approach for the non-linear restoring, while one model is based on an absolute angle approach for the computation of the non-linear restoring.