The flow on the bottom of a surfboard moves transversely off the rails. It is most obvious on the shore side rail behind the stagnation line/spray root. Less obvious, but still very important, is the transverse flow off the the wave side rail that runs back up the wave face.
The Camber Surfboards bottom shape next to the rail is designed to function like a flap at the back of an airplane wing. This maintains higher pressure under the bottom of the board creating more lift with less drag (L/D) by slowing down the transverse flow escaping from the high pressure area under the board and flowing toward the rails where the pressure is lower.
The exit angle, area and contour of the bottom and rail are more important than the depth of the concave. For example, a board with a one-inch-deep concave that uses an arc from rail-to-rail will have a relatively shallow angle at the rail and will be less efficient. That is one of the reasons why a Camber Surfboard has a flat center section.
We use Computational Fluid Dynamics (“CFD”) to predict lift versus drag (L/D) for various bottom contours at various design conditions. The images below show a board with a conventional shortboard bottom and the same board with a Camber Surfboards bottom. The images were generated using a CFD program that divides the water into millions of cells and then solves the basic fluid dynamics equations for each cell. The CFD results include the flow velocity and pressure in each cell which yields a color-coded pressure distribution map that makes it possible to compare different bottom pressure contours and resultant forces at various pitch, yaw and roll angles as well as different speeds and rider weights. Bright purple represents the highest pressure and blue represents the lowest pressure. The board on the left has a typical shortboard bottom with a modest concave. The Camber Surfboard on the right has much higher pressure and greater lift because the bottom shape at the rail increases pressure on a large portion of the bottom of the board. Higher pressure with less drag results in higher speeds.