transom replace: Lumber vs plywood?

[jdcoombs]

Well, y'all probably knew the transom was rotted on my 14 ft '70 Ouachita tri-hull, but were waiting for me to discover it myself, eh? The deck was rotted and foam drenched, but I was hoping it was OK. It wasn't.
Anyway, I read how to replace, etc,
But I was wondering why everyone uses 3/4" plywood? I can't get it here (just 1/2 and 5/8. But plywood just wicks water up through the end grain.
Why not use 2x's (treated, of course) put together in a butcher-block manner? Gotta be stronger than plywood, and will be harder for water to penetrate.
Anyway, that's what I did. Looks pretty good. Will glass it next weekend.
Any comments?

 

[DJ]

Re: transom replace: Lumber vs plywood?

Transoms flex. Regular lumber does not always handle flexing well.

Plywood does handle flex.

"Wicking" is easily overcomed by sealing the ends and holes. Its strengths far outweigh its shortcomings, with a lot less work.

 

[External Combustion]

Re: transom replace: Lumber vs plywood?

Even if you are very picky and only use quarter sawn two bys, edge gluing them will not give the stiffness that plywood will. Also there is almost no chance of getting quarter sawn two bys from any normal lumber yard. Standard sawn lumber will cup when it gets wet. Several cycles of wetting and drying will permanently cup them. Glueing them sucessfully, especially if pressure treated is generally an aquired art.

If you try the two by four route, use a daddo blade or a router and add splines to help the finished article.

Personally I would recommend plywood for a transom. It is significantly "stiffer" and that is what is needed for a transom. As DJ said, the end grain can easily be sealed and it will last a long time if one side can remain open to the atmosphere so the wood can dry after being wet.

 

[Grits]

Re: transom replace: Lumber vs plywood?

What kind of lumber did you use?

Grits

 

[jdcoombs]

Re: transom replace: Lumber vs plywood?

With all due respect, Lumber has about the same stiffness than plywood, more of a factor of type of wood used, part of the reason I went w/ it. Plywood is "tougher" meaning it'll flex and take a beating better than lumber before failing, like DJ said, a trait that is likely important w/ large craft and a large engine. With my li'l ol' boat, and a 40 hp w/ a 4 ft long transom, I figger it's fairly moot. I was more concerned about moisture getting in since plywood rots easier, to which my transom was a testament.

I used PT doug fir. All 2x3's. I "glued 'n' screwed" it w/ stainless screws between edge pieces to draw it together. I used the smaller cuts, 2.5" ain't too wide, and alternated grain centers to overcome the cupping problem. I Used Gorilla glue. It is basically the same chemically as the foam used in the hull, but foams less (some, to fill gaps) and has a phenomenally stronger adherence. Won't emulsify at all,like most glues. Yeah, edge glueing won't give as good of a "unit" action. Hadn't thought of that. Dado or T&G would have helped I guess.

I had wondered if it was an "ease of construction" thing for the initial building. Reason I did lumber was, the way I put it in, it was *much* easier than making it out of plywood. Either way, I imagine the transome will outlive the hull at this point... ;-) Given this info, if I ever do a big one, I'll use plywood.

Thanks for the info.

 

[jcsercsa]

Re: transom replace: Lumber vs plywood?

All right guys , I am right in the middle of my transom rebuild and I posted a thread here awhile ago about useing 2x12s and plywood , nobody said i souldnt us the 2x12 , I am slip joint the 2x12 and then 3/4 ply wood over that , I was also thinking of Gorilla glueing the joints together, then epoxy over the boards, then glass, then plywood after i epoxy that, then glass and epoxy them together, when it will be done i will router it to fit . The 2x12 I have planned down so they are about 1 1/8 inch thick,and with the plywood ,it will be a little over a 1 3/4 thick ,so i will have to router about a little over 1/4 of a inch on the top of it ,to get the top to fit. Now these boards i have had for over 10 years , so they are dry, really dry , they should really suck up the epoxy , So will I get it out on the water and have it fall apart?? or will it just rott in a year or two ?? and oops did bring up a good point , didnt in the old days use solid wood ???? and i am going to epoxy the heck out of it , dont think they used that back then ?? and how long did did the ship last ???? john

 

[jdcoombs]

Re: transom replace: Lumber vs plywood?

That sounds pretty stout. I'm sold on Gorilla glue. If you use the 2X12's, rip them into 4 pieces and flip them around. They will cup, check and warp if you don't.
I saw a link around here somewher w/ the pour-in transom fix. I like that one.

 

[jcsercsa]

Re: transom replace: Lumber vs plywood?

yea the gorilla glue will work, I hope ,lol thats what i did, i have 4 pieces of 2x12 , i fliped every other one , planed them and then lap jointed them , 1 1/2 inch in and about .675 deep just glueing them now !! so we will see john

 

[Robj]

Re: transom replace: Lumber vs plywood?

Use plywood it is stronger in both directions, as opposed to dimensional lumber which is only string in one direction. i.e. it is easy to break a piece of laminated 2x4's with the grain, but not against the grain. The grain of the vineers is alternated so it is strong in both directions. If you are worried about rott, use PT ply, it will last much longer than you will probably want to keep the boat.

Have a great day,

Rob.