PURPOSE: To reduce the effect of impulsive pressure of wave on the outside plate of a half-sunk high-speed catamaran, by fixing a rugged elastic mat to the outside plate.

CONSTITUTION: The mat 18, which is made of elastic material such as FRP, rubber, plastic, etc. and has straight grooves, is fixed by a bonding agent 17 to each of the outside slats 1W4, outside bottom plate 5 and outside lateral plates 7 of the half-sunk catamaran. The dead rise angle effect and the flare angle effect of the grooves of the elastic mat, the air enclosing effect between the grooves and the impact absorbing effect mat act together to reduce the impulsive pressure of wave on the voyage. Since the impulsive pressure can be reduced only by fixing the rugged elastic mats to the outside plates, it is not required to make a double boat structure resistant to the impulsive pressure.