Removing and Replacing Damp Plywood + CSM Framing in a FG / GRP Boat

Hello Everyone-
My 21' (6.4m) trailor sailor has some 'damp' in the tabbed plywood (3/4" or 19mm thick w/ CSM and PE resin 3/16" or 4.5mm thick) that makes up the 4 floors and the connecting spacer/keelson.

The original glassing was not sealed enough to prevent pooling/standing water (root cause was a neglected cockpit drain hose, turning the dry bilge into a kiddie pool) from ingress into the plys.

For the most part the tabbing in CSM was intact, including under the keel bolts, except for 2 lateral cracks along the tops of 2 of the floors, which was suspected to be from the moisture swelling the plywood.

I am trying to do a decent repair with best practices, it is my personal boat and I have done enough other things in terms of fixing it that I do not wish to have it expire on the trailer for this concern. I have laminated about 10 gallons of epoxy and 2 gallons of PE resin and a matching amount of plain weave fiberglass, carbon fiber, etc. on other boat and water toy projects but I am not experienced with the heavier reinforcements like roving, CSM, and biaxial. I made some test panels with CSM and PE resin and biaxial+mat and epoxy this weekend to start to get a feel for the materials.

The plan is to remove and replace, taking care to make all wood products are sealed. The question is whether to replace like for like or to take advantage of different materials such as timber instead of ply, epoxy instead of 'the Esters' (not my turn of phrase but I like it), and 1708/1808 biax w/mat instead of plain CSM. Understood that most CSM's are for PE resin due to styreneated binders.

Any best practice suggestions, links, discussion of material choices/combination, filleting, etc. would be appreciated. I am not looking for anyone's advice to be a 'sign off' in terms of liability.

One of my sources has suggested replacing the CSM with +/-45 biax w/mat but did not offer any sort of conversion ratio of thickness between CSM and biax, just that the biax is 'stronger' since there would be long fibers crossing the corners at 45 degrees. They also mention adding a putty fillet where the floor tabbing meets the hull as somw these spots show poor wetting and some resin draining towards the bilge currently.

View of the area in question, approx 4 feet (1.3m) long and 3 feet (.9m) across. Thanks for reading!



PS Shoutout to @fallguy who has been helpful as well!

 

I’d try to get away from using wood entirely. Either grind it and it’s fiberglass clear away and layup the area THICK, or use a structural C shape of G10 to support the keel bolts.
Pretty sure that any material you use will eventually get wet, so use something that won’t be destroyed by moisture.

 

That is a good point and some good options, thanks kapnD !

McMaster-Carr https://www.mcmaster.com/c-channels/structural-frp-fiberglass-u-channels/

 

So, in my case we think the water ingress which caused the plywood to expand and delam and some of the CSM to crack. The rest was intact and the only problem signaled was two weepy keel boats .
If a waterproof lightweight core was used, like Coosa board (Bluewater 26?) and it was glassed over and tabbed to the hull and each other with some biax and epoxy gusto, the composite becomes the load-bearing structure and the core would be inert and along for the ride, yes?

 

The core must have tremendous compressive strength, and the inner skin as well to protect the core.
Keel load, even at rest is great, carrys keel weight and bolt torque , and is multiplied and modified under sail, and vastly spiked by grounding.
The boats bottom is obviously of sufficient strength to carry the keel without adding anything on the inside, and should do so well, as long as the bolts are evenly torqued.
If it were mine to do, I’d remove the core, add a few stepped layers of heavy biaxial glass over the area, and extending out over the hull a bit, and eliminate a problem area.

 

Thanks KapnD!

I remembered this video from a while ago showing a crew adding floors with biax and epoxy using a non-structural core to create forms for deep c-channels.

 

In the Sailing Uma video, was there a keelson? I only saw them replacing the floors.

The keelson could be made from any number of materials, but what it is doing is evenly distributing keel loads across the hull versus a dozen bolts and washers. So, it can't be made from a couple layers of glass because all that is doing is increasing hull thickness to a better, but still unknown shear rating.

In the Uma video, they used 60 pounds of glass and 90 pounds of resin. If you used half that amount, the resin cost would be about $1200.

Their floors do not take the keelbolts. Any structure that takes bolts cannot have a foam core like the one they show. Pressure on a fastening would be uneven and potentially crush the hull at the weak foam. Your floors do not take the keelbolts either, so you could build solid glass floors, but the keelson cannot be made with light foam or just a couple layers glass. Big no-no. Keelson needs to be strong enough to distribute the 500# keel across entire dimension of keelson...

I agree the cores must be removed.

I am wondering if @rxcomposite would be willing to offer any wisdom. I trust his laminate plans over any.

 

Thanks.
I ran out the 4 floors and 1 keelson for 4 staggered and overlapped layers Dbm1808 45 w/ mat in a spreadsheet and came up with 8 yards or 12 pounds at 24 something oz per sqyd. So bare minimum would be a 2 gallon kit, I would probably order a 3 gallon kit as I am down to my last quart on 2 different brands. I will also need some fillers for the fillets.

I appreciate the time people are taking to read and reply.