Marine vs Pressure Treated Plywood

[George H260]

What is the difference between marine and pressure treated plywood, if any?The reason I'm asking is that I need to add a wood spacer piece between the hull and a new thru hole fitting nut. There are wood spacers under all the thru hole nuts on my Hunter. I assume they are there so there is a flat surface for the thru hole fastening nut to work against.I dont want the wood to swell so much that it puts excessive perssure on the fitting.ThanksGeorges/v Windswept

 

[Steve Dion]

Have not heard about pressure treated ply!

George:I have not heard about pressure treated plywood. I had thought about using Polyboard instead of plywood. So I was wondering if anyone out there has any opinions about this.

 

[Ian Cameron]

Pressure treated resists rot

Marine grade fir plywood is a thing of the past, except for special orders. All fir ply is now made with UF109 or the equivalent, and is water resistant. Ply from other sources (Brunzeel from Holland, IE) comes in different grades, and marine quality must by specified. (Note that Brunzeel marine is excellent quality ply.)Pressure treated ply, like pressure treated dimensional lumber, is treated with arsenic or copper salts to resist rot. It's used for foundations and swimming pools. Another alternative is Crezone, which is 1/2 inch 5 ply fir coated with a Kraft paper surface. It was designed for making road signs in Canada to resist 40 below to 40 above, and is very impressive. Multi hull builders swear by it.

 

[Kevin]

Plywood

I have used regular 3/4 inch ply wood which was soaked in Epoxy resin, allowed to dry and then 5200 to the hull. The wood is water proof and will out last the boat.Kevin

 

[red coles]

Same thoughts

While I used exterior ply coated with epoxy the last time, I also thought about using poly board. A small cutting board should yield enough for a couple of fittings. Any comments? Good luck red

 

[Tony Z.]

Too much has been written but....

FIY Good quality marine plywood has zero voids and can warreented up to a lifetime (depending on the manufacturer) regarding delamination.Typically PT is not the first choice in a marine environment....The adhesives used to laminate marine plywood are serious stuff.PT ? not sure...Woodenboat.com may have additional info....

 

[Dan Bryant]

Why Plywood?

I've used thick oak for my thru-hulls. Its exceptionally strong, has no surprises, and once its embalmed in 5200, I have a hard time beleiving it will rot.