Challenger 7.4 Sailboat 1973. Mast deck stepped construction issue,

My boat was purchased with a post added years prior to me buying it. There is a whole cut in the cabin so the post is below deck fiberglass. Now… is there supposed to be some type of support, Balsa wood or something sandwiched between the deck fiberglass and the cabin roof fiberglass. Is it possible to close the hole and fill the void with expanding foam?
or should I use another method?

Any suggestions

It is hard to tell what your photo shows. There is not supposed to be a void under the mast or above the compression post. The whole path between the bottom of the mast and the keel should be solid. It there is a void in the top of the cabin, it would probably be best to fill it in with plywood that has had the edges epoxied to stop water ingress. Foam would be too soft and spongy to serve this purpose.

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Hi PaulK, thanks for getting back to me.
I have more pictures, I could only upload 1.

I was thinking of a 2 part closed cell. 16 pound. High tensile strength stuff. I wasn’t thinking about foam; like cushion foam, I should have specified the foam…Or are you thinking that the foam would not be strong enough. The stuff I’m thinking about; It has break strength of 750 lbs p/sq inch.

Physical Properties 16lb foam

Parallel Compressive Strength: 580 psi

Tensile Strength: 450 psi

Shear Strength: 230 psi

Flexural Strength: 750 psi.
Seems like strong enough stuff? I’m not an engineer.

I was also thinking about plywood as well but what about the curve of the Deck? Would I cut a big hole from underneath and apply several thinner ones together and expoxy glue them together?
Or as solid as I can go?

Thanks for your help
Here’s another picture


Did a quick research on plywood…
Plywood Strength

Mechanical Properties Metric English
Tensile Strength 27.6 – 34.5 MPa 4000 – 5000 psi
Modulus of Rupture 0.0483 – 0.0689 GPa 7.00 – 10.0 ksi
Flexural Modulus 8.20 – 10.3 GPa 1190 – 1490 ksi
Compressive Strength 31.0 – 41.4 MPa 4500 – 6000 psi

Though the thickness isn’t specified, It would seem to be substantially stronger than your foam.