Trimaran Vessels and Parametric Roll

Parametric roll can occur in case of trimaran vessels, in particular when the outriggers have limited draught and/or limited transversal separation. Due to the significant nonlinearities of the restoring moment and due to the often complex shape of the metacentric height in waves for this type of ships, parametric roll clearly shows quite peculiar characteristics. Starting by the description of a simplified 1-DOF nonlinear mathematical model for parametric roll in longitudinal regular waves the paper then describes how the Floquet theory can be directly applied to the linearized model in order to determine the instability regions for the upright position. An example of calculation of stability map for the upright position is provided as well as an example of prediction of rolling amplitude in the nonlinear range to highlight the peculiar aspects relevant to the considered trimaran configuration. In both cases experimental data are also reported for comparison. Finally the paper describes how the position of the outriggers can be optimally chosen in order to minimize the variations of metacentric height in waves.