How to shape in concaves

After thicknessing my blank and cutting the outline, my blank seems to have an almost perfect curve. The only problem is i want some concave from the mid point to the back fin with the deepest bit in front of the front fins. Should i just run the planner down the middle a few times then take some off the rail line near the nose and tail so its flat in those areas? or just try and dig some foam out where i want the concaves? It just looks as if the curve was designed to be a flat bottom. Hoping someone can help me become less confused! maybe a sideon pic of the rocker? cheers

You haven’t exactly said, but there’s a wood stringer?

Handplane along the stringer to almost the depth you want, but not quite. Then use a piece of heavy grit sandpaper wrapped around a firm sponge or some other kind of flexible foam (not surfboard blank foam) to create the concave you want. This will inevitably leave the stringer just a little proud of the foam around it - which is why you left it a little high to begin with. So the last thing is to go back with the handplane and take the stringer down to its proper depth & to match the surrounding foam.

Maybe this may help you, although the concave in this case is a longboard “spoon” under the nose. But the technique should be more or less the same:

Thanks for the replies, if only i could use a planer like that! im only really comfortable planning with a set depth at the moment.

You can always sand more… but you cant “unsand” the board. Take it slow and easy, pay attention to detail and it will come out well. Good luck