Experimental Study on Porpoising of a High-Speed Planing Trimaran

Abstract

Porpoising is defined as unstable coupled heave and pitch motions. In this study, seven test conditions with various longitudinal locations of the center of gravity and the moment of inertia were designed to study the porpoising phenomenon of a tunnel-type planing trimaran. After comparing the lower speed limit for porpoising, this study revealed that moving the center of gravity forward could increase the lower speed limit of porpoising, but it could also increase the amplitude of heave and pitch. Furthermore, the reduction of the moment of inertia of the planing hulls could effectively reduce the oscillation amplitude of planing trimaran porpoising and improve the longitudinal stability of planing trimarans. In this test, condition A5 had a 15% reduction in the inertia moment compared to A3, but the amplitude of the pitch oscillation in porpoising decreased by 60%. Unlike conventional trimaran towing tests, this experiment studied the effect of speed on porpoising after exceeding the lower speed limit for porpoising and found that with an increase in speed, the amplitude of porpoising first increased, then decreased, and then increased again. This study defines the speed at which the amplitude of porpoising increases for the second time as the second critical speed of porpoising. The porpoising generated by a planing trimaran after reaching the second critical speed is defined as the second porpoising. Finally, the limitations of the conventional longitudinal stability limit curve method to predict the porpoising of a planing trimaran are discussed.