Hardwood vs laminated plywood?

Working on a dagger board trunk for my boat, the plans call for oak as specers for the trunk sides, but was wondering if pressure treated plywood might be just a good if not better? Anyone here know how it holds up in salt water?

 

Pressure treated plywood is an absolute error. There is no standard for them to remove moisture after the dip/treat so if you try to build with it; it will almost certainly dimensionally change and have adhesion issues with almost all glues.

Do NOT use it. You are welcome.

I did use it once for a carpeted unattached wet deck, but uses on boats are very few.

trunks are especially trouble under dimensional change. My brother had a centerboard trunk where the sides of trunk case seams opened and a wee bit of ingress and the centerboard sticks. We had to vee the seam; dry it out and seal it.

 

So, if your half-inch P.T. plywood is sitting in your shop, side by side with a half-inch marine grade fir plywood sheet for six months, are they both equally likely to move or is there something about P.T. that makes it more unstable even after acclimation?

 

Well, the main problems.

1. Noone waits 6 months for it to dry.
2. if you wait 6 months, the stuff moves and is not flat while it dries, always

PT ply is intended for contacting concrete and a few other uses. It is truly not a boat building material ftmp. I offered a carrot as to when I have used it, but no way does it belong in the dagger trunk.. An epoxy encapsulated wood is best. Or any wood that is stable or stabilized.

 

You can buy kiln dried PT plywood, it's what the production marine industry uses.

 

One of my first jobs was work in the yard at a building supply company before there were Lowes or Home Depots. I then worked general construction after graduating a furniture design and fine woodworking trade school program. The biggest problem I saw with P.T. was in how it was handled after it left the mill. Because it was P.T., it was left outside, in the rain and weather, in banded stacks, where non-P.T. wood was covered up from the elements or brought inside.

In my woodworking program, one of my instructors described walking past a construction site in the rain. He said the pallet of bricks was covered in a tarp, but the dimension lumber was left out, uncovered to soak up the rainwater. The wood was stamped on each piece, "KILN DRIED".

 

Lets look at an easy summary.


Marine Plywood vs Pressure Treated Plywood – LBI Fiberglass Products https://www.lbifiberglass.com/marine-plywood-vs-pressure-treated-plywood/#:~:text=Marine%20plywood%20is%20specifically%20manufactured,will%20absorb%20moisture%20very%20easily.

 

The point about voids, leaching, and glue are well taken. However, the point about softer wood is not so much.

P.T. plywood is most often made from S.Y.P. which is a category of woods widely used in boat building and is considerably harder than fir and other softwoods that boats often use.

 

Another problem with pt ply is the treatments contain copper and copper reacts with aluminum.

https://www.buchnermfg.com/pressure-treated-wood-and-aluminum

 

There are various grades of KD PT plywood, I have customers that buy truckloads of it. You can get the older style less corrosive versions if needed.

It's not used to 'build' a boat, it's used as a localized core or as structural supports in locations that are appropriate for its properties.

Yes it gets misused, or used with terrible attention to detail, plus the box store wet (waterlogged) product is used as the 'PT' standard, this is where the horror stories come from.