an air vent that passes from the deck of a surfboard through to its bottom . the vent may have a wide variety of shapes . the trailing boundary of the vent &# 39 ; s lower portion is preferably inclined forward toward the surfboard &# 39 ; s nose . the inclination serves to prevent water flowing upward . instead , air is entrained by the water flowing across the vent &# 39 ; s bottom exit and pulled downward . a portion of the entrained air flows rearward out of the bottom exit along the surfboard &# 39 ; s bottom . this entrained air forms an air film between the board &# 39 ; s bottom and the surrounding water , thereby reducing friction .