refurbishing rotting plyword under fibreglass

[banjobolt]

Hi
I have recently acquired a 32' SANDAWANA sailing yacht mono hull that is about 15-20 years old but has never yet been put on the water. It is a fiberglass hull with plywood / fiberglass finishings, deck etc.
The boat is about 60% complete and I am now refurbishing it, strengthening the fiberglass, cutting out rotten wood where necessary and painting it etc.

On the fore deck the top layer of paint and resin has cracked and allowed a bit of water to seep in over time. Everything still feels strong but I notice in the front cabin that it has leaked a sticky black substance through the fiberglass in one patch, leading me to believe that the marine ply wood above is probably rotting to a certain extent. Now, if I am to start digging out rotting wood I might end up with no fore deck whatsoever and then have to start from scratch again with new ply etc Again, it is all really strong although it may be slightly spongy in a couple of places.
Do you think it would be fine to just break out the cracked fiberglass and re-fiberglass the entire deck, thus strengthening it and sealing whatever dry rot in?
I was told that dry rot has to have access to air to feed it and that basically the wood was only there as a shuttering effect, that the real strength is in the fiberglass. Hence if I were to strengthen the fiberglass it wouldn't matter if all the wood completely rotted away.

Is this correct? What do you guys think?

Ben

Measure the humidity in the hull first! (or get it done) If the entire GRP layup has soaked water, and the hull was all the time on the hard, you most probably have a case for the landfill. Therefore no other step is sensible.

Sorry, for that, and I hope for you it is not the case, but that would surprise me.

Regards
Richard

[gonzo]

You have to cut out the rot. From your description it looks like an amateur construction. Have someone knowlegeable inspect it. The plywood is the core, if it fails, the whole laminate comes apart and fails.

[missinginaction]

I hope that the engineers chime in here. In my experience it depends on how the boat is constructed weather or not the "core" is carrying the load or just acting as a form for the fiberglass. When I made my stringers I used a foam core since in that application the loads were all carried by numerous layers of fiberglass. In the case of the decks on my boat I replaced the original decks and stayed true to the original design. The foredeck is 1 1/8" laminated douglas fir sealed with epoxy and covered with a layer of 4 oz/sq. yard fiberglass cloth. The glass on the deck is there to improve durability but certainly not to add strength.

So I think you have to consider how the structure is engineered.

One tip: If you're going to remove that fiberglass from the wood do yourself a favor and buy a nice variable temperature heat gun and a couple of hefty scrapers, say an 1 1/2" and 3" wide. You can heat up the surface and the glass resin will come off pretty easily. You'll probably smell styrene. That's your clue that the boat was built with polyester resin. Epoxy will work better.

Best of Luck,

MIA

[gonzo]

If the fiberglass skiin cracked there are more serious problems than just the core. It is an amateur construction and the quality is probably poor.