Bottom Contours such as concave are used in conjunction with rockers, to direct water through a straighter path along the bottom of a surfboard. They can add significant glide and hold, and, at the same time, give a board squirt through turns. It is important to understand that vee and concave alters the rocker. The average board these days has two rockers – one along the rails and one along the stringer. Part of personalizing a board is learning where to place concave and vee along the bottom to create that magic, lively feel that a particular surfer is looking for. This is the most challenging and exciting part of designing a surfboard.