Core repair ?s

[buckdrew]

I've dug into an area on the deck with rotten core. Access from below seemed like the more evil way to attack this so I removed a chunk of deck skin for access. I'm looking for suggestions on:
material to use for repair (looks like balsa versus the ply that I used to rebuild hatch covers and bulkheads - OK to use ply?)
suggestions for best practice on this repair and refinish - hoping I can use the piece I removed to seal it back up but wondering if I need to grind/bevel a few inches to glass the seams? Or just epoxy the piece back in and fill my seams with gel coat?
any thoughts?

 

[PA228G]

Skip the wood and go with closed-cell foam. You can order it from places like jamestowndistributors. Where are you located - I may have some laying around the shop depending on thickness - or you can plane it down if you have the tools. Don't worry about getting a good overlaminate for structural purposes other than making it stick and look good - the outer gunnel and cockpit sides are carrying all the load in that area.

 

[richie rich]

you can use either the wood or the foam core, but if you go with foam, you may need to add a layer of glass on the bottom side (facing the bilge) as GW uses only a thin layer of glass on the inside and a thicker layer on the gelcoat side (not sure why..maybe cost/good enough)....since the foam has little strength on its own vs plywood, the glass sandwich needs to be strong to maintain stiffness. I would prep the inner glass nice and clean, add a layer of 17oz biax, bed in the new core and epoxy the skin you removed. For the joint, I'd just clean it up with a dremel tool and gelcoat the cut seam. Depending on what you get for core thickness, it may actually be easier to dry fit the new core, glass the top skin on it first nice and flush, then when bedding, use thickened epoxy underneath to make up any irregularities and squeeze out the sides...then finish the joint.

 

[gw204]

I would at least bevel the edges a bit and lay some mat or light cloth before you fair and gelcoat.

 

[BobP]

Nice cut lines!

I guess Grady didn't caulk those screws and fitting at the factory.
That's the deck pump out for the waste tank.

If the seams open up, namely the long ones, you will have your work cutout for you.
Next time check here or at ClassicMako.com on where to make the cuts.

The entire bridge and liner is built as a self supporting structure that lands on very few points in the hull. On that side below the head floor, and the bulkheads closer to the centerline under the entrance to the lower berth. Then at the rear of the bridge before the step up.
Then the entire side hull which is cored for a reason. 3/8 inch balsa.

Just walking in the walkaround stresses the sides to flex in.

I would have made the long cuts 1.5 inch closer to each other from both sides, then pick out from underneath the remainer of rotted wood. Then the bevels are easy and flat
You will find out the difference should you have to make the joints on the inside corners. Plus the walkaround is not so easy to get at.
If the bottom skin is flat the lowest cost core is wood, you can use the project plywood usually sold in smaller pieces like 2 x 4 feet at the Depot, avoid treated ply like sheating, then marine plywood, then non-wood substitutes.
Each step up is twice as expensive as the previous, the first one up 3x as expensive. Marine ply only comes in 4 x 8 sheets I recall. Marine ply is exact dimensions.
Project ply may not be truly 1/2 inch as the balsa is, so you can get 3/4 inch project and hand router it down to exact thickness.
All of them will equally get the job done right to the same hardness as original.

Make sure you seal the pump out fitting and screws when going back in.

Good luck.

 

[buckdrew]

thanks for all the advice here. it was a big help.
i'll post some repair shots soon.