[sailingdog]

I would highly recommend doing the repair from the topside, not from the bottom. First, getting a properly laid up laminate without voids is much more difficult from the bottom—since gravity is working against you. Second, you have to really be careful when glassing inside the boat or you can make a really amazing mess...that will take forever to clean up—which isn't the case when working from the top down. Third, it will go much faster if you work from the top down. You also do need to put a layer of fiberglass on the side you're working from—since that is what gives the cored laminate its strength, as others have stated.

The way a cored laminate works is that the two layers of fiberglass act as the top and bottom of an I-beam effectively, one in tension and one in compression with a stress web, the core material, between them. This makes it far stronger than solid laminate of the same weight would be. Not glassing the interior will result in a one-sided I-beam...which is pretty weak.

For core material, I'd go with divinylcell or Airex foam rather than balsa. It is probably going to be a lot easier to work with. ContourKore balsa is great to work with, but have you priced it recently???

 

[sailingdog]

Yes, but ease of working with it may more than make up for the price difference.

 

[sailingdog]

Balsa and Plywood don't bend—foam does, at least Airex does, Divinylcell less so. Most cabin tops aren't flat, so some curves are needed. IIRC, ContourKore balsa is pre-cut with kerfs in it so it can be somewhat more flexible than plain balsa would be.

 

[sailingdog]

LOL... while this sounds simpler, you'd have to make the fiberglass far heavier than the cored deck would be to get the same strength and stiffness.

Heck Trevor just cut the top off, take it home and make a new one with the fittings you need where you need them, then tab and glass it back on and non-skid the whole thing.
Custom boat with a solid deck to walk on, lighter than the original.

 

[sailingdog]

Well - you sound like you and your posse are ready to take on a big project so I'll wish you well. A couple of suggestions...

Don't pull the entire underside ot the deck off at one time. The boat will flex and you'll be epoxying a few bends and twists into the hull that shouldn't be there. Do it a small section at a time.

This is very good advice.

Think about using an alternate method. Cut 2 1/2 inch longitudinal channels out of the existing underside. Lay in a 3/4" stringer (wood/foam/carbon fibre/whatever) and then cover the stringer with several layers of roving. Once you have a complete structure of stringers, finish it off with some trnasverese ribbing done the same way. Then core between the stringers - use Airex. Cover everything with a bit of roving.

This is more work, but probably a better choice in the long run.

Remember NOT to use balsa in areas where hardware will pierce the deck. It is not strong enough from a compression standpoint. Use plywood. Make epoxy plugs on each hole.

Make the sections where the hardware comes through the deck solid glass that is tapered to meet the cored areas so you don't get a hard transition line if you can avoid it. Solid glass is a much better solution than pads of plywood, since the plywood can rot.