I have to make a new frame for the rear seat, and I wanted to try and use something lighter in weight than the Marine Ply that was used previously,

I was thinking Balsa wood maybe but not sure how it would hold up
this is going in a 1987 Baja Force 235 so Im trying to save weight anywhere I can that is affordable,

also looking to do the rear engine hatch cover over as well, and was thinking about Balsa wood

anyone have any suggestions

Thinking about this myself as I'll probably be rebuilding my rear seat and engine hatch this winter as well. My wellcraft hatch must weigh 100 lbs and I think I could build a sufficiently stiff hatch noticeably lighter using a sandwich construction. I thought about kledgecell as a core but it's expensive. I thought about using wood frame matrix with thinner skins, but I'm not sure if I have enough thickness but that would not have good longevity over time I fear. Thinking about this though for a winter project as it would be nice a bit lighter every time I have to raise it and I'd rather spend money saving weight than buying bigger gas shocks for it.

Some years back I had a friend who worked in the RV industry. We made a sort of solid cockpit cover for a runabout we towed down to the keys. He sandwiched together a 4x8 sheet of I think 1/2 foam board, the common exterior insulation blue board or such, and put a sheet of Luan on each side. These were contact cemented together. I was pretty surprised how strong it was. KI recall standing on it an it only bowed a bit.

You could make a sundeck out of this if you had the perimeter solid wood to give you attachment for hinges and stapleing points. The force uses that fiberglass underliner andis very heavy. The lighter sandwich could possibly be made from a plastic type sheet but it may not have the strength that the luan did. I have contemplated this myself to drop some weight. If the sandwich was not strong enough support bars could be put inder the hatch for it to rest on laying horizontal on over the valve covers and one each on outboard edges. Outboard edges could be L channel or something.

Or find a composite board designed for this.

You could do something similar

Starboard and Starboard light is a plastic. You will want the light if you can find it the stuff is HEAVY!!!

Baltek has a product they call Decolite. It's end grain balsa covered by a thin layer of fiberglass on each side. Looks like a sheet of plywood and comes in different thicknesses. I would rib anything that needs any strength.

BINGO JUST saw this Mrv8outboard and myself replaced my motor cover in 95 or 96. used 1 sheet of DECOLITE cut off the extra and used it for the turn down in front and did ribs with balance also cut and glassed in a pro stock scoop primed it with a heavy primer filler to seal the ends lots of sanding so it looked one piece and you could jump on it .I DID NOT USE THE UNDER LINER that was made out of glass its weigth was hugh. so my hatch weights about 30 lbs to attach it I use just to latches that have a pin going threw them so no more loose hatch, and you can keep the elecrtric piston. for a back seat I got rid of the slant piece just made it 90 degree did the base to. then paint make sure you get 1'' what I would do different is dig out a 1/2'' of balsa all around the edges and fill with resin and fg filler re sand that way water can't get in the core ever after 10yrs I had a spot on the corners any other questons artieps in the baja sec look under pics of my boat you can see the scoop.

Look into Azek board.

Air Time,
So which product was it from there spec sheet?
http://www.cattcousa.com/images/pdf/...ct%20Guide.pdf