Hello Tom,

This has actually come up before, and there was one guy who was building one (Actually his was total SOF with wood slats on the deck) but I believe he never finished. The skin bottom would have a few drawbacks. One is it would give a little and even the slightest give would be bad for a surfboard. Also if you did just a skin bottom how would you attach the skin? It seems gluing would be risky. Maybe route a groove for a stringer and compress the ends of the skin into that? Also, and believe me I am a fan of SOF kayak and actually have a project going right now for a 17' greenland SOF, but I would be cautious to use it on a board. Although a wood board will have some buoyancy if it gets holed, it won't have enough to get you out of the surf zone. I've nearly drowned on big waves with a foam baord. My board was even being held under by the wave. I'd hate to be out there with a skin bottom and snag a coral head and be stuck on the reef with a waterlogged board. With a balsa bottom even if you got a failure you'd at least have that much more buoyancy to the board.

What I do is build the bottom almost solely out of balsa, and skin the bottom with 4 ounce glass. With balsa 4 ounce is about as little as you want to go. I've been wanting to try that Raka 3.6 oz tight weave- it might work. With 4 ounce over balsa you still have some give when you push in hard with your thumb, but it's durable enough for the job it does. Since this board will get a lot of use I am going to go with what I learned on this forum and do two layers of the Raka 2.3 ounce over the entire board. An 8-foot board weighs in the range of 20 pounds. I'm shooting for 30 pounds on this 11 foot SUP because I used a lot of hardwoods on the deck. If you wanted to go really light you could use a plywood bottom. Personally for me I just like the look of a balsa bottom with a few mahogany strips for accent. I may one day do a bottom with 1/8" ply and skin it with a light glass- probably a short board where weight is much more of a factor. (because they have less buoyancy) Swiss-cheesing the frame saved me 1.25 pounds, which in my book is a lot. The thing to remember about weight is every bit makes a difference. Little things like making the fin box recess fit exactly perfect rather than cramming it with fiberglass (which a lot of people do) make a difference.

One area people miss when saving weight is glue. A standard bottle of glue weighs 8 ounces. I use maybe one bottle on a board this big. I'm very careful with glue and any drips on the inside get cleaned up. I have seen boards that the inside looks like there was an explosion in a glue factory! Of course it's not going to cause a whole lot, but it could add half a pound, and that helps. A lot of guys on the wood surfboard forum are using Gorilla Glue. I never understood why. It makes a total mess, and being expanding it isn't good for edge gluing. Apparently the quick curing time is one reason, and some use it where they say they want gap-filling. I say don't have gaps! Anyway, the problem with Gorilla glue is you get this horrible mess on the inside of the board that is hard to clean up without tearing stuff up, and a lot of people just seal it up inside- out of sight, out of mind, but NOT out of wieght!

All that said, if you do a SOF bottom I'd love to see it and know the results. I would certainly suggest putting some extra longitudinal stringers on the bottom. I would choose a board with a flat bottom rather than a concave. Most of the wood boards in kit form are flat-bottomed. Hey by the way, I just thought of something- you still would need a wood block in the tail for a fin box.