Custom about 72mm centre to centre. I went with M4 to try get the profile as thin as possible but then you are also dictated how thin you can go by the Ti plate size and because the Ti is expensive you want to get that as close as possible. A standard hole spacing on something this would just not work you would not be able to torque down the fin to the board and might end up with the tips of the fin slightly raised.
True, I found this when working out fin placements - result was I bought new Fins for my regular board and am using 2 of my old ones which i sanded flat so they are flush against the board. Did think about sanding the tip profiles to fit the fins (a lot of work as I needed to get each end identical as board is flat with fibreglass skins - board is bi-directional) and also manufacturing some fin profile spacers.
Another thing you look at is failure at high speed you don't want super strong M6 bolts holding the fin down because if hit a rock or debris at high speed you would rather want the fin to break away from the board and not take the tail with it, you would want the fins to shear away before it destroys the board.
Good point but also depends where you are riding - most locations I normally surf at, are sandy bottom with no rocks or stones. Only problems could be something floating in the water, something left burried in the sand which gets uncovered by the movement of the water or have the fins worn down by the sand - a good reason to use SS ot Ti.
My board is just a R&D project at the moment and has kept me busy on no wind days (contruction) - if the board works well then great. If not its back to the garden shed for modifications or total redesign when there is no wind.
Would need to find out if the new official WSSRC ruling on minimum water depth also applies to all the GPS runs ?
if so - running a ground is not a major issue.
A few guys are using M4 bolts. To torque it down real good I'm going to use some cup washer to spread loads and I'm using some decent quality high grade Aluminum PRO-BOLTS .
You try getting 3/16BSW aluminium bolts, or any kind of BSW bolts - Total Nightmare... ended up buying replacement set of F-One fin bolts because they use a non standard fin thread.
Also I dont have access to a lathe to manufacture my own.