Surfing Waves

I want to build an Alaia, I find a lot of information about it on the internet but I can't really find how to make the concave.
so my question is: How should I build the concave on an Alaia?

Lay a ruler across the bottom mark the outer edges of where you want the concave to be along the bottom work out depth and shape template for points along the centre line out to that line you marked around the edge that is side to side, carve away the wood with your device of choice to the template line and smooth though the length between them it also helps if you have a nose to tail template.
It's an interesting and challenging skill getting concaves right. Do you understand what concaves do and how they work on Alaias?
That might help you design your templates.
Good luck, pictures as you go would be great

Hey thanks alot man, im gonna use this for sure, and im not sure but i think they are for better balance and to avoid the spinning , correct me if im not right.