So, the first thing Mike did after getting the boat up on the hard (and BTW, that’s the owner of ProCarpentering based at Boat Lagoon in Phuket) was have his guys take away the teak from the side decks, which had become spongy over the past year. And then he dug a hole into it. Well, that was an eye opener. The decks and hull are made of a balsa sandwich, which is a layer of balsa wood, coated on each side by a ton of fibre glass. This was common practise back in the late seventies and early eighties when fibre glass was a relatively new material in the industry, and the boat builders didn’t really trust its strength. And the majority of seasoned cruisers I know of (and that’s a lot, given my work) prefer these old builds to the new flimsy fibre glass hulls which you can see the light through, and which bend if you kick them. That said, this deck was bending if we even just walked on it….
Now, what happened is that ten years when we lifted the boat, we noticed some bulging around the side stays (they’re actually called shrouds on a yacht, but I’m a beach cat sailor by origin, so bear with me – od habits die hard). So we lifted the teak, and found the core was rotten. So we cut out the decks and replaced them with a core of expanding foam. That I just found out. I believed until 30 seconds ago that it was plywood. Hm. Well, that clearly didn’t work, cos there’s nothing there now. So we’re going to replace it with proper stuff… Marne ply I think, which is the usual practice. I had actually thought we used plywood (I guess I should have been paying more attention back then, but it wouldn’t have made any difference back then since in those days I knew next to nothing, and left the decisions to the Captain while I went out t work to pay for the boat)!!
Baljit Pawar
So, all in all it looks like I might have been just a little bit right when I suggested that perhaps when we do this we will find lots and lots of other things to replace as well as the teak deck…
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Saucy Sailoress 
I would really recommend you not use plywood coring. Use properly installed end grain balsa or dense closed cell foam. Properly installed end grain balsa is one of the best materials you can use for that purpose but some prefer dense closed cell foam.
As a follow up: end grain balsa is installed with the fibers of the wood vertical to the surface. They have been cut into small blocks and glued to a scrim sheet. The channels between blocks need to be filled properly to keep water from migrating from a leak area. Plywood has the fibers running parallel to the surface. Any water intrusion will wick up the fibers and eventually saturate the whole plywood core, not just a small few inches, as will happen with balsa. Let google be your guide on how it needs to be installed and for alternative foams.