The goal is to generate more lift with less drag (improved Lift to Drag Ratio, ie. L/D) to increase speed. The first step is to identify the flow under the board. Paddling out we typically see a rider on a wave trimming or turning with the spray sheet starting at the stagnation line/spray root and coming off the shoreside rail. We need to look under the surface to see what is taking place behind the spray root.
The above photo shows a board on a right taken with the Camber Cam mounted at the top of a tall center fin. The flow wrapping up and around becomes the spray sheet. The streamers behind and parallel to the spray root illustrate the stagnation line showing the flow moving transversely across both rails. The flow across the outside rail is the most obvious. The flow across the inside rail and back up the wave face is less obvious but still very important.
The surfer at this point is basically flying the bottom of a glider wing across the water. On the shore side of the stagnation line the flow up the face the wave is reversed and flows toward the shoreside rail. On the wave side of the stagnation line the flow moves transversely across the bottom toward the wave side rails due to the momentum of the water flowing up the face of the wave. However, since the forward velocity of the board is high in relation to the now slowed transverse flow, the flow appears to be mostly parallel to the centerline of the board on the wave side. In addition, since the fluid is accelerated away from high pressure regions under the board, toward low pressure regions near the rails and out from under the board (to near atmospheric pressure), the flow accelerates toward both the wave side and shoreside rails. Both rails essentially function like the trailing edge of a glider wing. The Camber Surfboards bottom is designed to slow the motion toward the rails and create higher pressure under the bottom of the board.
How do we improve performance now that we know the direction of flow? A flap at the trailing edge of wing is commonly used to slow down the flow and increase pressure under the wing thereby increasing lift.
The exit angle, area and shape of the bottom area adjacent to the rail are more important than the depth of the concave. For example, a board with a one-inch deep concave that has an arc from rail-to-rail will actually have a relatively shallow angle at the rail that may not be very effective. That is one of the reasons why a Camber Surfboard has a flat center section.
The orientation of the board is constantly changing relative to the wave and, therefore, the direction of water flow over the bottom is also constantly changing. The inside rail becomes the outside rail, and vice versa, as the surfer changes direction and puts pressure on one rail or the other to turn. The challenge is to design a bottom configuration that works well over a wide range of flow conditions to achieve the best lift to drag ratio (L/D).
This video slowed down 50% shows how the transverse flow moves from the inside rail to the outside side as the rider bottom turns, top turns and then does another bottom turn.
While paddling, the flow still moves off the rails because higher pressure is created under the board as the surfer moves the board forward with each stroke. While paddling, the stagnation line is
perpendicular to the centerline and just under the nose. Although the pressure differential is small when paddling because the speed is low, there is still a very slight benefit to using a bottom shape with a flap to slow down the escape of pressure off the rails. This helps the board plane-off and allows the rider to catch waves sooner.
The rider in the photo has his hands on the rails as he pushes to stand up. At take-off, the stagnation line is still perpendicular to the centerline of the board because the rider is heading in the same direction as the wave toward shore, but the speed is higher because the board and rider are accelerating down the face of the wave. At this point the flow is mainly moving from nose to tail but the flow near the rails is moving transversely across the rails because flow in the high pressure area under the board is accelerating to an area of lower pressure (atmospheric pressure) outside the bottom of the board.
As soon as the rider starts to angle across the wave and/or bank the board into a turn, the stagnation line and the flow behind the spray root starts to become more parallel to the centerline of the board. When this happens, the performance improvement of a Camber Surfboard rail and bottom becomes very noticeable.
The obvious question after seeing the direction of the streamers in these photos is, what about the streamers that are still running nose to tail (axially)? We had the same question about how much lift was being generated by the transverse flow versus the axial flow. The Computational Fluid Dynamics (CFD) program has the ability to separately analyze the lift generated by the axial flow and the transverse flow. The results indicate that the majority of the lift is coming from the transverse flow, and a smaller part of the lift comes from the axial flow. Once you know that transverse flow is creating the majority of the lift, the objective then is to maximize the benefit of that flow. The Camber Surfboards rail and bottom contour reduces the velocity of the transverse flow and generates higher pressure under the board. A flat bottom board, or even a board with a conventional shallow concave, allows increased flow to escape off the rails resulting in lower pressure under the board. The goal is to increase the pressure on the bottom of the board to increase lift. More lift allows the board to carry the weight of the rider with less drag. Less drag lets you go faster…
Good surfers instinctively use transverse flow without thinking about it. Two examples are pumping a board to generate speed in flat section or doing a quick top turn to get up to speed. Pumping a board in a flat section rolls the board up on a rail to make the flow run transversely across the bottom to generate more lift. As the board is pumped in a turn, the board is aligned parallel to the wave crest which results in the flow coming up the wave face moving transversely across the bottom. Doing a quick top turn also rolls the board up on a rail and it aligns the board parallel to the wave crest which both create more transverse flow. Obviously, it works on flat bottom boards as well as boards with moderate concave, but it works better on a Camber Surfboard with a higher exit angle at the rail that functions like a flap to generate higher pressure under the board.
We use Computational Fluid Dynamics (“CFD”) to predict lift versus drag (L/D) for various bottom contours with different design conditions. A CFD program 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 yielding a color-coded pressure distribution map making 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 images below illustrate a CFD prediction of the difference between the pressure distribution on identical boards. The difference is that the board on the left has a typical concave 0.18 inches deep with the deepest point located 18 to 24 inches up from the tail which is typical for a conventional short board, whereas the CAMBER Surfboard on the right has a concave of modest depth and optimized contour in the area adjacent to the rail. The CFD runs for each board were done at the same pitch, roll and yaw angles as well as the same wave speed and wave contour. Purple represents the highest pressure and blue the lowest pressure. It is easy to see that the board on the right is generating far greater lift based on the colored pressure contours particularly near the stagnation line region.
The CFD image above is representative of a condition where the flow is quite parallel to the centerline of the board and with a bit of fore-aft trim. The stagnation line is clearly parallel to the centerline of the board, similar to what you would experience taking a fast line in a wave.
If you look closely at the cross-section images below you will notice that the magenta high pressure region on the Camber Surfboard bottom is larger and extends closer to the rail of the board. Higher pressures on the bottom of the board of course create greater lift which allows for the generation of greater speed by allowing a reduction in trim angle and/or reduction of wetted surface of the board. For the above condition, the CFD prediction of the total lift on the Camber Surfboards bottom is 13.7% higher than the same board with a conventional concave bottom. In general, the higher the flow angle across the board, the greater the increase in lift for the Camber Surfboard.
The next set of cross sections is a bit farther back along the board. Again, on the Camber Surfboards bottom you’ll see the magenta and red regions near the spray root are a bit wider than the conventional concave board. More noticeably, the yellow region extends closer to the rail and the green region is narrower showing increased pressure on the bottom and a larger pressure gradient at the rail.
So what does all this mean when you are out surfing? The Camber Surfboards bottom should allow you to plane-off and catch waves a bit easier since the rail is more effective at maintaining the pressure under the board. Once you have taken-off, the increased lift allows you to generate more speed because the higher pressure under the board allows the rider to reduce the trim angle of the board in relation to the wave face thereby decreasing drag. You should also be able to maintain a bit higher line in a wave and make some sections that you otherwise wouldn’t. When you immerse the forward rail in a turn, the lift generated by the rail and bottom helps draw the nose of the board up the wave face, resulting in a more maneuverable board. Because you don’t have to put quite as much weight on your rear foot to carve the same line up the face, you maintain more speed.
We hope this explanation takes some of the mystery out of what is happening under your board. The more you know about how your board functions, the better you should be able to ride your board.