RoyStuart.biz: Rampant's surfboard design: why the concave hull?

As shaping of the 'Rampant' surfboard progresses, the interaction of its curves emerges visually, including the full length single concave. It's a good time to investigate what benefits this feature has for the rider.

The interaction of geometrically generated design elements

The shaping story so far: starting with a close tolerance 'blank' hollow wooden hull, the primary facets were first created with a belt sander, and then faired by hand. Secondary facets halved the angle between the primary facets and were shaped using a hand held sanding block.

So, what is the purpose of the concave hull?
What is the surfboard design theory behind it and, more importantly, how does it feel in the water?

Hull concave as it rests on the stand

Surfboards will go faster if the water is less disturbed

The basic theory is well known: as a planing hull moves, water flows outwards. This is inevitable and reduces efficiency. Ideally the surfboard would travel without disturbing the water at all. If this could be achieved there would be a significant parasitic drag reduction and consequent gain in both acceleration and speed. That is why ice yachts can be much faster than those which move in liquid... they don't disturb the ice as much as a hull in liquid does. (The fastest accurately recorded ice yacht speed is 84mph, whereas the highest recorded water speed for a wind powered vessel is only 74mph.)

Helical effect of parallel profile with concave

Concave works by containing the spanwise water flow.

( tweet this) The surfaces adjacent to the rail face partially inwards, preventing at least some of the outwards water movement. This increases lift, reduces drag and thus improves the lift/drag ratio. The bottom of the board is like part of a pipe, and it directs the flow in the same way as a pipe. ( and in the case of our tunnel finned craft actually becomes a complete enclosed pipe in the tail area. Tunnel fins aka 'annular wings' have been proven to have extremely low induced drag ).

Close-up of tail with flattening s-rocker

Planing hull 'S' rocker ideal for a tunnel fin .

When turning however, the concave hull has another advantage ( in terms of water flow includes trimming along the wave face without apparent change of direction, which is effectively a constant turn). due to the rate at which wetted surface area is reduced as the board rolls on the fore and aft axis. As can be seen from the simplified drawings below, the concave hull reduces wetted surface area faster as it rolls during turns, particularly in the initial stages of the turn when the angle is low.

First the flat hull at a low angle of roll:

Now the concave hull at the same angle:

More surface area has been lifted out of the water simply by changing the bottom contour. What this translates into is: less wetted surface area; more speed and acceleration through turns when trimming or 'pumping' down the line and a more positive and immediate control response through rail to rail transitions.