restoring old mahogany small boat

I am sure the structure is capable of supporting a 20hp outboard. If you want to install foam as a double botom it would have to be shaped and glued down with epoxy, not a work you want to do. For buoyancy just put some EPS foam under the seats.

 

I am thinking to put it sprayed and leveled în order to have some strength on the bottom. Now i am sorry not putting 9 mm bottom plywood. I am stressed from two points of view. The boat îs to narrow 4.3 ft wide, and the bottom îs to thick only 6mm plywood and two layers of 6 Oz, 12 Oz în total fiberglass... And i put it only on the outside. I am thinking that a 20 hp it will be Over powered... Any advice are welcomed.

 

If you use spray foam you will be rebuilding the boat very soon, PU spray foam holds water. The only way to do this is using XPS shaped to the stringers and epoxy glued, making absolutely sure there is no woid anywhere.

The bottom is thick enough, if it breaks you can always laminate more plywood on the outside. Just build the interior and go boating, nothing will happen unless you hit a log or sandbank at speed. For flotation take EPS blocks, lightly glass and paint and fix them in unused spaces (under seats, deck, etc.).

 

Your boat is plenty stout. Don't overdo it, it's way stronger than it needs to be for 40 hp. 6mm ply with 2 layers of 6 oz cloth and the framing that is in there is way stronger than you can imagine. At this point all you would be doing is adding weight. If you wanted to make it bullet proof add a layer of 6 oz on the inside of the plywood, but that's probably overkill too. 20 hp is low for that boat as it is now. I'll run fine, but you could put 30 or 40 on it and it'll run very nicely. Good job enjoy it.

 

What you want to think about is how you connect the transom to the cockpit side rails. That is where all of the transom strength comes from. The original design used the covered area around the motor to transfer the motor forces to the sides of the cockpit. Recreating that would be good idea. The "knee" supports transfer motor forces (that are pulling aft on the top of the transom) to the bottom, and that's not where you want them. All that does is bend the bottom up and makes the bottom hook upward and that is not a good thing. It's much better to transfer those forces to the sides of the cockpit and that will keep the bottom flat. What you want to do is add a some material (like a 2x2) AFT of the existing transom outside of where the motor will clamp and add a 1x2 to the cockpit sides and then you can put a piece of 6mm ply that looks like the old motor area "deck" and that will transfer the transom bending loads to the cockpit sides and you can put 50 hp on the boat and you'll be fine.

 

I will build a motor area deck as it was original from mahogany 10 mm thick. The structure for transferring the transom force to the sides looks like în picture 2, 3, 4. I think it is enough.
The transom îs 20 mm thick with tha mărgins doubled by a 20 mm thick and 100mm wide. The central part of the transom where the motor îs clamped (an area of aprox 400x400mm) is made by 3 layers of mahogany în total of 40mm thick.
Thank you for your advice. Please be welcomed to comment or give some advice. Is my first wood boat that i build. Any advice are welcomed.

 

Sorry i made a mistake. The transom îs 13 mm or 1/2 inch thick not 20 mm.