Transom plywood choices - amount of plys per thickness

[silentbravo]

I am rebuilding my transom and need to get to 1.5" thick. There is no marine plywood available near to me so I am planning to use some good structural plywood, AC variant.

I can do 2 pieces of 3/4" combined, or 3 pieces of 1/2" combined. They call the 3/4" as 5 ply, which it is really only 4 ply with a very thin surface veneered 5th ply. The 1/2" is 4 ply, but the same deal with 3 plys similar sized and a very thin 4th ply.

So combining the plys, 3/4" gives 10 ply (8 equal sized plys) and 1/2" gives 12 plys (9 equal sized)

The original transom was 9 plys of equal thickness, so not really losing or gaining much in strength as far as the wood goes, unless "marine" plywood is super special.

It should only take 1 sheet to do it, either way. 1/2" will obviously be slightly cheaper, only difference in construction is more time and more glue. Which way should I go or does it matter?

This is for a 16ft Starcraft with a Johnson 48 SPL and 10hp kicker. I'll have more in depth posts when I start the build log, but need to start ordering stuff, especially the sheet of plywood soon as it just keeps getting more expensive as time goes on.

 

[lckstckn2smknbrls]

2 layers of 3/4" BCX properly sealed will last years and years and years.
I like the old timers formula to seal the plywood others will say to use fiberglass epoxy resin.
Marine plywood will rot just as fast as if not sealed.

 

[gogittum]

This brings up a thought I've had many times. Regular lumberyard 1/2" & 3/4" plywood seems to have lots of voids, knots and repairs, it's quite rough finished and doesn't inspire a lot of confidence in me. I've looked at furniture grade plywood which I "think" is imported Birch plywood from Scandinavia and a piece of 3/4" I'm looking at right now has 12 plys and they are tight with zero voids. The stuff is smooth finished and rock hard, too.....seems like a Very superior product to me...BUT....

How resistant to rot would it be ?? Looks like it should seal well.....?? I've never really worked with it. I'm sure it must be much more expensive but I'd much prefer it if it were My transom.

 

[RaisedByWolves]

I used two 3/4” layers of Oak plywood for mine so it has a hard surface, probably a smidge stronger too.

Factor in that your bonding layers should have a filler added (I used west systems filler, but microballons is the same thing) and you wind up with a finished product that is thicker than the sum of its parts.

An additional layer of whatever will just make it thicker yet.

 

[MrGiggles]

This brings up a thought I've had many times. Regular lumberyard 1/2" & 3/4" plywood seems to have lots of voids, knots and repairs, it's quite rough finished and doesn't inspire a lot of confidence in me. I've looked at furniture grade plywood which I "think" is imported Birch plywood from Scandinavia and a piece of 3/4" I'm looking at right now has 12 plys and they are tight with zero voids. The stuff is smooth finished and rock hard, too.....seems like a Very superior product to me...BUT....

How resistant to rot would it be ?? Looks like it should seal well.....?? I've never really worked with it. I'm sure it must be much more expensive but I'd much prefer it if it were My transom.

Probably pretty good stuff, but it most likely doesn't have exterior rated glue like exterior ply.

Of course if it is sealed, the glue should never get wet, but you never know.

 

[silentbravo]

I used two 3/4” layers of Oak plywood for mine so it has a hard surface, probably a smidge stronger too.

Factor in that your bonding layers should have a filler added (I used west systems filler, but microballons is the same thing) and you wind up with a finished product that is thicker than the sum of its parts.

An additional layer of whatever will just make it thicker yet.

The plywood aren't true 3/4 or 1/2", actually slightly thinner so adding a coating shouldn't swell the dimensions to much.

I was leaning towards epoxy sealing the whole thing, but I don't necessarily want to spend a fortune on this either. Plenty of ways to dump money into the boat already lol

 

[gogittum]

The epoxy is fairly expensive, tho' for just a transom it shouldn't take all that much. Store it right and the leftover will be good for years on the shelf. Done carefully and thoroughly, the epoxy will more than pay for itself over the long run.

 

[CedarRiverScooter]

You will find other uses for the leftover epoxy, so it won't be a waste of money. Small quantities can be had for around $25. If you thin it 25% xylol, it soaks in much farther.

 

[RaisedByWolves]

I used two 3/4” layers of Oak plywood for mine so it has a hard surface, probably a smidge stronger too.

Factor in that your bonding layers should have a filler added (I used west systems filler, but microballons is the same thing) and you wind up with a finished product that is thicker than the sum of its parts.

An additional layer of whatever will just make it thicker yet.

The plywood aren't true 3/4 or 1/2", actually slightly thinner so adding a coating shouldn't swell the dimensions to much.

I was leaning towards epoxy sealing the whole thing, but I don't necessarily want to spend a fortune on this either. Plenty of ways to dump money into the boat already lol

Between the laminating process and two coats of epoxy sealer I had a hell of a time getting mine back in place.

If you dont encapsulate your transom its not going to last.

 

[silentbravo]

Pretty much set on epoxy, covering the whole thing and edges extra. I see everyone mention that only smaller amounts of epoxy will be needed for this. I'm looking at the RAKA UV resistant 3 qt kit for this. Sounds like I will have plenty of extra for other projects?

Probably going to go with a 1/2" sheet, should be about $10-15 cheaper than the 3/4" sheet, and I will have enough room to make 3 pieces on it.

Now I see that I need temps of at least 50F for the epoxy to cure right, that may be a problem for a while.... Still getting cold at night here.

 

[LDUBS]

This brings up a thought I've had many times. Regular lumberyard 1/2" & 3/4" plywood seems to have lots of voids, knots and repairs, it's quite rough finished and doesn't inspire a lot of confidence in me. I've looked at furniture grade plywood which I "think" is imported Birch plywood from Scandinavia and a piece of 3/4" I'm looking at right now has 12 plys and they are tight with zero voids. The stuff is smooth finished and rock hard, too.....seems like a Very superior product to me...BUT....

How resistant to rot would it be ?? Looks like it should seal well.....?? I've never really worked with it. I'm sure it must be much more expensive but I'd much prefer it if it were My transom.

Probably pretty good stuff, but it most likely doesn't have exterior rated glue like exterior ply.

Of course if it is sealed, the glue should never get wet, but you never know.

Unless things have changed true baltic birch (not to be confused with birch veneer ply) uses waterproof grade glues. Pretty nice stuff but pricey. Additionally, last time I bought some the biggest sheet was 5' but that was many moons ago. If still the case it might not be wide enough for a transom.

Just my opinion but when considering plywood for a transom I see no need to go with hardwood veneers other than for looks. I would advise to not use oak or birch veneer cabinet grade ply. It is for inside use.

 

[gogittum]

Now I see that I need temps of at least 50F for the epoxy to cure right, that may be a problem for a while.... Still getting cold at night here.

I think 50F is an absolute minimum - higher is better.

I built a 13 ft Sam Devlin design Black Brant in the winter in Port Angeles, WA some years ago (early '90s) and it were cold. I put an eye in each end of a 16 ft 2x4 and hung it with ropes from pulleys in the rafters, so I could raise and lower it, then put a socket every 4 ft and one more on each end for a total of 5. Screwed heat lamps into those sockets.

(I hafta back off a bit here - memory is failing me. Can't remember for sure if I put those lamps at 3 ft or 4 ft intervals. Seems like I had 6 of them)

It worked out very well. While working on it, raise the heat lamps to a comfortable level - more pleasant for me in the cold - then when finished, lower them down close to the work surface to keep it very warm and speed the cure. You can believe I monitored the surface temp very closely at 1st to make sure it got warm enuf but not too hot.

Prob'ly didn't do my electric bill too much good, but the boat came out beautifully.

 

[DaleH]

I wouldn't wa$te the $$ on UV-proof epoxy, just buy the regular Raka epoxy and paint or varnish it ...

 

[silentbravo]

The UV is about the same price maybe even cheaper from a seller on ebay, than direct from Raka.