Question on repair of balsa cored deck--what material

[John F.]

Crows Nest has an after market f'glass deck consisting of 3 pieces--port, center, and starboard. Probably Glass-tech. The port and starboard deck both had rot in the balsa core where the engine box hinges screw down into the deck. The rot spread around a little over the years, and I have about a 6" x 9" spot on each deck that needs repair.

I removed the decks and cut the f'glass out from the underside of the decks. I dug out the rotten core, and am going to epoxy in some new core and then f'glass it back up. The repair will only be visible from the underside of the deck, so I shouldn't have to do any paint work. I was looking around and found that I have marine-ply and pvc board that I can use for coring. I was planning on marine ply, but was wondering if pvc board would work (and wouldn't rot). I know pvc board isn't structural, but neither is balsa. Anybody use pvc board as a coring material? Anybody just think its a bad idea and I should use marine ply?

And I know something like coosa may be preferable, but I don't have any, don't know where to get it, and don't know if I can get small pieces of it.

[Lars]

Hi John,
Divinycell is one of the very best materials for replacement of balsa sandwich core. You don’t have to buy huge quantities, and can get it from amongst others Jamestown Distributors. Good luck with your project!

[Yannis]

John,
I dont know about divinycell, it either doesnt exist or they call it differently here, but my point is dont get too overstressed about what material to use.
For such a small addition, I would use the first material I could lay my hands on and this is marine plywood by far.
Consider how many years it took the balsa wood to rot...the plywood will take ages more...you will not even notice!
Now, if you manage to prep it ala Bob Lico at his strut supports...then not even your grandkids will ever notice.
Use sikaflex in the new holes, ss bolts with nice washers underneath and forget about it for life.

[Bruce]

John,
For those small parts you can mix some glass reinforced epoxy resin to a paste and fill the spots. This will allow the screws to bite into some thing.

[Pete Fallon]

John F,
I would use the marine plywood that you have on hand. Like Bruce said mix some epoxy and fiberglass strand into the patched area for better holding for screws,thru bolts and patch. Use large fender washers to back the thru bolts. Mix epoxy and fiberglass strand together to the consistency of loose peanut butter for the area between the old glass in the deck and the new marine plywood, clean the area to be fixed with acetone or denatured alcohol and make sure all the rotted balsa core is out. Allow it to set up for a day or so then grind it flat, you can use bronze nut inserts that screw into the plywood for even more strength, they make them up to 1/4-20 in size, a little overkill but if you have the lift back engine boxes it won't hurt there is a lot of strain on the hinges and the deck area, especially when they are open.
Pete Fallon

[Tony Meola]

John

It is a small area, follow what Bruce said. If it gets to be a larger area, stay away from plywood. Too dam heavy. I had mine repaired when we did the repower. had to replace about half the balsa on the middle section. I told the guy that fixed it for me to use the left over Coosa board I had. He did not listen and used plywood.

I curse him (god rest his sole, was a nice guy) every time I have to pull it out. Glad I have only done it once since he redid it 9 years ago. That piece is heavy as hell and it is only half plywood.

[John F.]

Thanks for all the advice. I appreciate it.

[CamB25]

Nothing sticks well to PVC. I would use coosa, divinycell, foam, or plywood in that order. I have ordered lots of material from these guys over the years. They have regular 20% off sales:

https://www.fibreglast.com/category/Foam

I will check my scrap bins for coosa, etc. I might have a few small pieces.


Cam

[John F.]

Cam-

Thanks for the offer on the scraps. I'm good. The rot has been dug out and core cut away, the area has been cleaned and primed, the marine ply has been cut, shaped, primed and epoxied in, and I'll be putting a few layers of mat down within the next few days.

John

[Rawleigh]

John: Just glass in some 1/4" or thicker pieces of aluminum or stainless steel plate and then drill and tap them for the hinge machine screws. use Tef-gel on the threads when installing. This way you never have to worry about the screws loosening up. Probably cheaper and easier than the foam with glassed in screw holes. Plus you can get it locally. No shipping!