Experimental Study of Active T-Foil Control of Longitudinal Motions of a Trimaran Hull in Irregular Waves

High-speed ships often experience over larger ship motions in waves. Therefore, highspeed ships without motion control hardly have practical commercial values. Recent years saw wider adoptions of T-foils on high-speed ships, which greatly improve ship performances in waves. In this study, we designed a series of model tests to study the motion-reducing effects of T-foils in waves. Heaves, pitches, and bow accelerations are quantities to be measured in the experimental tests. The study shows that the reduction effects of T-foil-controlled longitudinal motions of a trimaran model are prominent. Comparing with naked model tests, it is found that active T-foil controls may reduce motions of trimaran ships in irregular waves by about 20–30%. In the most favorable case, the reduction is as high as 51%. This study shows that installation of T-foils may dramatically reduce the motions and, thus, provide an efficient control tool to mitigate ship motions in waves. The main aims of the present study were to provide an experimental setup for model testing of an active T-foil control system and determine the control equations. Although the control and actuation systems designed in this study are simple, they do produce the required effects. The model-tested control equations can be used for high-speed foil-type ship design directly if proper similarity ratios are used to project the model test results to real ships. Increasing the length-to-beam ratio is one of the most effective ways to reduce wave-making resistance so that high speed can be achieved (Ackers et al. 1998). High-speed ships with very slender hulls are, however, not stable. Placing two small side hulls on the port and starboard with displacements less than 10% of the total ship displacements improves the ship's stability greatly. This simple consideration led to the fast development of trimaran hull forms in recent years (Coppola & Mandarino 2001; Degiuli et al. 2003; Oh et al. 2005). Encouraged by the successful sea trials of the 127-m-long littoral combat ship Independence at the speed of 50 knots, researchers move forward to search for new technology to improve the motion performances of trimaran hull forms in waves (Li et al. 2002; Hebblewhite et al. 2007; Jia et al. 2009). This is because high-speed ships are faced with the big challenge of severe oscillations in waves. Too large motion responses in waves would counterbalance the high-speed benefits. Therefore, motion control is crucial for high-speed trimaran ships.