Marine plywood VS. pressure treated (wolmanized) plywood - SailNet Community

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  #1  
Old 02-13-2006
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Marine plywood VS. pressure treated (wolmanized) plywood

I need to rip out and replace the plywood under my cockpit floor. Since marine plywood is expensive, and hard to find, I was wondering if using pressure treated plywood (for outdoor decking) would do the job. Please correct me if I'm wrong, but I thought the only real difference between marine and regular plywood is that the marine ply used a non water soluable glue, and regular ply does not. Now for the assumption (yeah, I know), wouldn't pressure treated plywood also use a non water soluable glue, AND also have the pressure treatment to make it withstand rot even better than marine plywood? This is my thinking for using the pressure treated as opposed to marine plywood. If I'm wrong, PLEASE tell me before I make a mistake. Thanks.

Russ
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Old 02-13-2006
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Well I don't know much about the differences in the glue used, but I redid my entire interior on my 22 Southcoast last summer and just used regular old 3/4" plywood, so I'm sure no one here would recammend that in the least. But before any of the wood went into my boat, I absolutely saturated it in WEST sysem epoxy, then glassed them in. So far through the Keys, the Chesapeake Bay, and now Buzzards Bay, MA they have stood up wonderfully, and have survived all the leaks my cheap little boat has. But...mine is a trailer sailor so it gets to dry out...though when it rains it still leaks a bit w/ no adverse effects.

I guess it depends on your intended use, ext ext. For me it rested on budget and I wasn't willing to spend the money for marine plywood.

Hope that helps a bit.
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Old 02-13-2006
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Guys, make your own mind up regarding whatever floats your boat. Personally, if it's not rated for marine use, it serves no purpose on my boat.

I'm not a Naval Architect, but being a building architect, I've seen what can happen to so-called "exterior glue" plywood when directly exposed to the elements for a while. It warps, buckles and delaminates . . . regardless of that label. You never know when the cabin may take on some water.

Hate to use the overused cliche' "penny wise, pound foolish" . . . but it holds true with plywood. Use only marine rated plywood, with mahogany core plys on boats. Anything else is a serious compromise and not worth the $30.00 to $50.00 +- savings.

Edit - I forgot to mention that pressure treated plywood contains high levels of arsenic. Not familar with boat-building use regulations with this stuff, but in building construction practice, It is illegal to use PT wood indoors without the ability to ventilate the outgassing of poisons.

Last edited by TrueBlue; 02-13-2006 at 08:56 PM.
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Old 02-14-2006
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My understanding (based on nothing I remember :-) is that there are other specifications for marine plywood as well as waterproof glue. Standards concerning the number and size of voids in the interior layers. As much as it pains the pocketbook, stick with the marine stuff.
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Old 02-27-2006
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I've been doing some research on marine ply recently, as I'm replacing some of the interior wood in my seafarer 30. Apparently Marine Ply uses the same waterproof glue as exterior ply, the main difference is the lack of voids in the wood. For structural use I agree, I wouldnt mess around with the cheap stuff.

What are people's thoughts on plywood for interior use? (cabinets, galley, etc.) Is marine plywood necessary? The wood I removed is 3 ply 1/2" with a teak veneer. I'm thinking the teak veneer isnt really necessary, I'll just stain the wood to a similar colour.
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Old 02-27-2006
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Marine Plywood

Sean C. is absolutely correct regarding the glue used. There is no difference betweem marine and exterior. The difference is in allowable core gaps. Exterior has gaps and the core fits flush in marine. Unless your needs are severe structural such as hull planking, exterior should be O.K.
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Old 03-02-2006
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Poisonous salts

Articles in Woodenboat and on their website fault Pressure-treated lumber for leaching poisonous salts into their surroundings, especially on boats where it can be wet for long periods. (No time to dry out and stabilize the poison -its always wet and always leaching out.) Beyond this, pressure-treated wood is often of lower-quality softwoods that soak up the poison better than stronger, harder woods. The poisons in it prevent decay, so it doesn't have to be as strong. Decks attached to a house are one thing, decks on a boat are something else. On a boat, marine ply is the best route, but I'd go with regular ply before I went with pressure treated in any case.
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Old 03-02-2006
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Read the usenet group rec.boats.building. Most of those guys have built an assortment of boats for years out of exterior ply and have gotten good service out of them.
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Old 03-03-2006
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I agree with the majority here, treated lumber is normally the cheapest you can get, it's soft and wicks water throughout, the price is in the treating.

Most modern treated wood uses copper azole, which has been pushed in many states as better healthwise than the old style, but still has the same materials as the same states are trying to ban from bottom paint.
With the copper based treatments you need triple coated screws and have to observe all the same disssimilar metals rules, Just as though you were bolting various metals together.

cabinet grade plywood is great for interior projects on a boat because it is normaly a nice surface on both sides, and while NOT cheap, still cheaper than marine grades.

-Normally- the need for void free plies is when you have water contact, it can hold water and delaminate, or when using it to build a hull or other curved surface the voids are weak spots and either cause uneven bending or potential breaks.

For soles I'd be happy with a good one face exterior ply, if one face is hidden, why pay for two good faces?


Ken.
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