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.

Paddling
Paddling

While paddling, the flow still moves off the rails because higher pressure is created under the board as the board moves forward. 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, it is still beneficial to use 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.

Take Off
Take Off

At take-off, the stagnation line is still perpendicular to the centerline of the board. However, 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. This is 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.

Straight Ahead
Straight Ahead

This picture shows the board running across a flat spot in the wave and trimming toward the next section. Most the flow is running nose-to-tail but the streamers near the rails are still flowing transversely. Scott’s CFD analysis shows that the Camber Surfboards bottom and rail shape in this mode are still more efficient than a conventional short board bottom with a modest concave because most of the lift comes from the transverse flow and not from the flow running nose to tail.

Paddling
Paddling

While paddling, the flow still moves off the rails because higher pressure is created under the board as the board moves forward. 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, it is still beneficial to use 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.

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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…