Rotten balsa core - major repair

[smj]

Originally Posted by boatpoker

Wow !

First thread I have ever seen with nothing but excellent advice.

Stop right now and don't read another thing posted on this thread.

👍🏻👍🏻👍🏻👍🏻👍🏻

[Chenega]

Originally Posted by Norado

Why would you choose this approach instead of laying all the core and then glass over at a later time (in sections)?

You'll end up with more prep when you add the top layers of biaxial cloth if you let it cure (doing all the core at once). The putty will ooze out and need to be sanded down because it'll cure. Place the glass after doing a section and any excess thickened epoxy holding down the core will become part of that top laminate. Keep the job in control by doing sections - not that hard.
Understand, there's a hundred ways to do this job - I've done many over 25 years in Marine trades. This method has worked best for me. Be very particular rolling out air and bubbles.


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[Mike Banks]

Don't mess about trying to save time and money. Replace the entire deck--and use epoxy. Nothing else will do as well.

Whatever foam you use--buy the heavy duty stuff.

Use ply sandwich wherever there are deck fitting and winches to be added--and use a LOT of ply/glass layers of reinforcing below the deck to prevent flexion.

Two layers of glass is better than one heavy layer--so only use the really heavy stuff where major stress points occur, such as deck to coaming, coving joints, deck to dinghy davit foundations etc. Use heavy reinforcements below decks and plywood sandwiches under stress points, taper the plywood to foam joints to avoid concentrating forces and use extra glass tape over transitional areas. Foam sandwich does not like point load compression or concentrated loads--the load must be well spread.

A lot of trouble--but once accomplished you will never be troubled by leaky decks ever again, and if a massive sea crashes down on your fordeck--it will withstand it admirably..

[Norado]

I've cut, prepped and sanded say 60% of the deck at this point. I have left areas where I figured I could just inject epoxy as the deck is still structurally sound, although with a considerable moisture content still.

Now I'm leaning towards doing as you say Mike, replace the entire deck. The job I'm doing now will hopefully last this boat for ever (if all future hardware is installed correctly). The areas that still have questionable balsa might delaminate in 5-10 years, and I'd hate to have to revisit those areas having to pull off the synthetic decking. I will first do the areas I've already opened to build some confidence. By then I could easily estimate the time needed to do the rest of the deck. Cutting up the whole deck at once would compromise the structural integrity of the deck, especially if I were to move about and work from the deck.

The guys at the yard recommended not using anything lighter than 600 g/m2 (18oz I believe) to rebuild the deck. I'm trying to work out a layup schedule, but it seems to be a number of ways to do this.

Regarding the areas where I have cut too close to the portholes not leaving room for a wide bevel I'm thinking I will use a 3 inch wide marine grade plywood along the cabin. Cross section:

[Mike Banks]

Norado--If your diagram is to scale--there is not enough taper between the ply and the foam. Make the ply a little wider and the taper more shallow--it will finish better and be stronger.

If you find epoxy a little more expensive, it is possible ti use Vinylester resins instead, but epoxy is my resin of choice for my work on account of its ease of mixing and controllable curing periods depending on which hardener is used. Accurate proportions are essential however.

I use 3mm plywood marine grade, which is quite cheap in comparison with other thicknesses of plywood, and I use it as a base if I have no mold or table from which to work. One can get it melamine backed if one wishes to coat it with release compound and peel it away later, or just leave the ordinary bare plywood dress surface to get an apparent timber surface to the ceiling. Make up a frame and do a small panel first to see it the method suits--I have used it to make all sorts of things from cabin tops to shower cubicles, using the melamine side to the resin/glass after first coating it carefully with Carnauba wax and release compound. Used carefully it would be possible to pre-form all of your deck panels in a smaller area, them move them to the boat and bond them to the deck beams. If one was careful one could make all of the panels and them just cut them to shape and finish the edges later. If you recover the melamine ply instead of leaving it there as part of the structure, the same pieces can sometimes be re-used for other purposes.

Have fun!!


Anyway, I fit my plywood in place and lay my first medium to heavy glass covering over it. This is the mat that will be in tension when the deck is loaded. Once that has set, the ply will be far more rigid than it was. Then another good coat of slow-set resin is rolled on and the first foam panel is cut to shape and laid in place and weighted down. I find bags of washed gravel work well here--they stay in place, are not to heavy and do not provide any point loads. Once you have the panel n that and weighted down, you can roll out the excess resin from beneath lifting bags and replacing them as required. Roll not too heavily. just firmly, from one side to the other to expel all air. Do not rush the job, take it a sheet of foam at a time, once the foam is glued to the glass it gets a little bit stiffer again.

If you use slow set resin you can stop the job at any time and mop up any excess resin. If you use a medium speed hardener you will have less time to work the job, and the gain in pot life is an advantage too good to pass up.

When you come to the top layers of glass--I use peel ply cloth to save me having too much messing about afterwards. Two layers is a good surface if the glass is heavy enough. If you use heavy glass mat, a slow set resin is essential to be able to get it properly saturated. Peel ply is cheap and saves a lot of work. Only a few spots will need the grinder, and the surface will take a good epoxy undercoat.

Once that ir ripped off after the resin has cured, your deck should be extremely strong