Thursday, December 22, 2011

Musing on concave

I had a chance to try out the new board at a great flat water spot about 4 hours north of Sydney called Old Bar. We had great wind there and so got to try the new board out on the flat water as well as in the surf.

Overall very happy despite the rocker table mess up the board feels great despite being different.

One thing I noticed is that the narrower tips seemed to compensate for the lack of rocker when it came to handling the chop. I working theory on tips is that because when your edging the hard the board is more vertical than is it horizontal, the curve of the rails toward the tip is works like the rockerline in the case the board is being ridden fairly horizontally. The net result is the the board rode over the chop nicely didn't through up much spray.

The other plus of the narrow tips is that it carved really well on the waves. Because there is more pressure on the wider mid section of the board its easy to balance your weight and pivot the board about the mid point rather than drive the carve from the back fins. This meant it turned on a dime!

I rode this board and my other board back to back and I could not really detect any difference as a result of the absence of concave. This has got me thinking about whether concave is worth it when working with stiffer core material.

My understanding is that concave does 2 , maybe 3, things:

i) It adds stiffness 'artifical thickness' which the BE equation analysis shows a doubling of stiffness when the concave is equal to the thickness of the board although the maximum stresses on the top and bottom layer as a result of the concave are very different (the top experiencing greater stresses - not good when it comes to compression). The stiffness is important for foam cores where the core itself contributes nothing other than to keep the laminate separated and locked relative to each other. The stiffness of the core means this is not so critical.
ii) It flattens the rocker line down the center of the board relative to the rails. This means earlier planning.
iii) Concave also means that the angle of attack of the rail is greater than that at the middle and so the rails bite into the water better and there is the possibility that can reduce the losses associated with cross flow on the underside of the board resulting in more lift on the toeside edge (helping dig the healside in) and possibly more forward momentum if the water flow is redirected towards the tail. However, I don't have any sense of how large these effects are.

On the negative side:

i) Concave adds drag because of the increased angle of attack.
ii) The increased angle of attack means your more likely to catch the toeside edge when you land and have the board too flat. This is the reason that some boards end up having slight bevel on the rails or take then edges from concave to convex as they approach the edge.
iii) Although there is a weight penalty, the stiffness issue can be addressed with additional reinforcement over portions of the board (say the middle)

So what's the so what of this?

i) For wood cores concave may be less effective of desirable than foam cores boards that use it as artificial stiffness.
ii) In wood cores, maybe the edge holding benefit of concave might be replaced with simpler carved channels close to the rails.
iii) The early planning advantage may be partially addresses by the rockerline design. The current rockerline I'm trying is like the NHP where most of the curvature is in the middle 25% and from there to the tips is quite flat. This gives a good planning surface towards the tips rather than the middle so may require a different stance to get started but should benefit early planning potential.


  1. Hi Matt,

    It would really be cool if you could put a comparison picture of the three boards outline and rocker, would be more easy to follow and perhaps comment.


  2. Thanks Mirsad. Thats a good idea. I've just been lazy not wanting to draw pictures but maybe I'll take some photos.