Non marine ply in salt water?

[Tim Good]

I’m building a flipper stopped from a piece of thick plywood I have kicking around. The final version will be sealed with epoxy flow coat but I don’t want to go to all that hassle until I’ve tested the design and size to determine if it’s effective.

My question: if I submerge a piece of non marine ply for an evening to test it out, will this be enough to permanently damage it, rendering it useless to then epoxy and use as the final version?

 

[Bru]

There's a chance you'll end up with a lump of soggy delaminared veneers

Even if it survives initially, it's likely to weaken the glue bond between the laminates (unlikely that the glue is salt water resistant*)

And chances are the ply has voids* in it which are likely to retain moisture and lead to structural failure of the ply later

So for my money, don't do it with the piece of ply you intend to use for the final version

* Two of the key features of genuine marine ply is that the glue must be salt water resistant and that the laminates must be free of significant voids. Most structural and decorative plywood has neither of these attributes, unfortunately a lot of so called "marine ply" actually doesn't either as I;ve found out to my cost which is why I now pay the premium to order my ply from Robbins)

 

[KellysEye]

If you test it without sealing it it will be damaged, the highest quality marine ply is the Dutch Kuyper.

 

[rogerthebodger]

If the ply is WBP, Water boil proof and you seal the edges with penetrating epoxy it will be OK

If it not WBP it will delaminate and if you don't seal the edges before immersion water will get into the plies and will be trapped in when you finally seal with epoxy flow coat

 

[ganter]

Build the 'prototype' out of cheap ply. Work on it to get correct size and shape. Then you have your pattern to make "the real thing" out of top quality marine ply

 

[macd]

* Two of the key features of genuine marine ply...I now pay the premium to order my ply from Robbins)

All well and good, except there is no agreed international standard for marine ply and no agreed definition to which sellers must adhere. (Although there is a British standard, BS 1008, I think the OP's boat is overseas). As you suggest, buy only from a reputable vendor. This isn't easy in many parts of the world.

Northcave: I assume you're making a triangular flopper-stopper with off-centre ballast? If you're doing this from on-line first-hand guidance, I doubt you'd need to do much by way of experimenting other than, perhaps, the amount of ballast. Don't expect it to be particularly effective in short "local" seas. Anyway, slapping on a coat of epoxy isn't going to cost that much money or time.

 

[Tim Good]

All well and good, except there is no agreed international standard for marine ply and no agreed definition to which sellers must adhere. (Although there is a British standard, BS 1008, I think the OP's boat is overseas). As you suggest, buy only from a reputable vendor. This isn't easy in many parts of the world.

Northcave: I assume you're making a triangular flopper-stopper with off-centre ballast? If you're doing this from on-line first-hand guidance, I doubt you'd need to do much by way of experimenting other than, perhaps, the amount of ballast. Don't expect it to be particularly effective in short "local" seas. Anyway, slapping on a coat of epoxy isn't going to cost that much money or time.

Yes it’s for long rolling seas in the Canaries. It’s more to test for size actually. My boat is 17t and the triangle I have is around 80cm along its ends.

 

[macd]

Yes it’s for long rolling seas in the Canaries. It’s more to test for size actually. My boat is 17t and the triangle I have is around 80cm along its ends.

I've not used the type you're making, just an improvised, weighted drogue. Size is less critical than you might suppose for worst-case rolling since the forces needed to prevent rhythmic rolling initiating are slight. In shorter seas, though: almost useless.
Good luck.