Marine Plywood & Bulkheads - Page 2 - SailNet Community
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post #11 of 20 Old 03-29-2011
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We have a couple of boats and I am slowly restoring a catalina 22. The PO used AC grade plywood good on one side for some of the covers to storage areas. I painted them with a gray latex paint and they have held up just fine. The covers are in the interior of the boat for the six years we have owned the catalina. She is not the prettiest girl at the dance but a lot of fun to sail even with the exterior plywood storage covers.
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post #12 of 20 Old 03-29-2011
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Exterior ply and marine ply both use waterproof glues. But the strength, stability, and ability to take a finish (if the marine ply is not fir) are vastly different.

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Exterior Plywood

Exterior is the grade given to any plywood which is bonded with 100 percent waterproof adhesive and is intended for permanent outdoor exposure. Exterior plywood must be graded a C or above, meaning knots and knot holes of up to 1 inches are allowed. The species of wood can also vary and often different species are combined. Core gap size for Exterior wood cannot exceed 1 inch wide. Exterior wood is good for most all wood needs, and can be purchased to look almost identical to Marine plywood at a much lower cost.

Plywood graded C-D or CDX is often mistaken for exterior plywood, but is not approved by the APA for outdoor exposure. CDX plywood is considered Exposure 1 plywood where it can encounter high moisture content, but needs to be sealed for complete outdoor use.

Marine Plywood

Made entirely of Douglas Fir or Western Larch, marine grade is one of the highest designations which can be given to a piece of plywood and is considered to be "premium." In order to be marine grade plywood, the outer plies must be graded at least a B or better. B grading means that the wood may have some knots, but no knot holes. In order to be A grade ply, there cannot be any knots or knotholes present in the layer. The maximum core gap size allowed is an 1/8 of an inch, and both outer panels must be sanded, Medium Density Overlay or High Density Overlay. The durability rating must be Exterior and fully waterproof structural adhesive is used.

Read more: http://www.doityourself.com/stry/mar...#ixzz1I2B6FoGZ
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post #14 of 20 Old 03-29-2011
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The best marine ply is BS1088 certified. It is usually occume. The 2 more popular makes are Joubert in France (occume which is African Mahogany) and Hydro-Tek (Meranti which is a mahogany) made in Malaysia. Both are BS1088.
Aqua-Tek is made in Malaysia and is BS 6566 certified.
3/4" Joubert is 9 ply, 3/4" Hydro-Tek is 13 ply and 3/4' Aqua-Tek is 11 ply.
The Joubert ply has no voids allowed in any ply, patched or otherwise. Easy to work with, very stable and strong, and takes any finish you like.

Domestic ply in the better grades has deteriorated in quality over the years. When I was young you had trouble telling which side was the bad side of good one side ply. Now you have trouble telling which is the good side.

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post #15 of 20 Old 03-30-2011 Thread Starter
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Thanks so much for all of the replies (heh) everyone. I think I will go for the marine ply. With the amount of time and effort I am putting into this project, it would be silly to skimp on something as important as the bulkheads. The fact that I am replacing them because they rotted away also leads me to want the more rot-resistant choice. Once the job is done, I think I will be happy I spent a bit more for the higher quality wood. This is the same philosophy I am going with for deck painting by using Perfection instead of Brightsides. I am going through countless hours in preparation, I might as well do it right.
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post #16 of 20 Old 03-30-2011
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With a boat like yours, I'd say that's the way to go! With a cheapy like mine, wouldn't be worth it!

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post #17 of 20 Old 09-23-2012
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Re: Marine Plywood & Bulkheads

I have done some research regarding the use of marine grade plywood vs. exterior grade for interior boat work. It seems to me that exterior grade is acceptable from a longevity perspective. Granted that it usually is not pretty, but I plan to skin all of mine with a veneer (type of wood TBD), so the "look" of the plywood is not really important in my case.
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post #18 of 20 Old 09-23-2012
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Re: Marine Plywood & Bulkheads

we used regular plywood but inspected it well for flaws and then thoroughly penetrated it with epoxy and wrapped it with fg cloth and resin. is my structural bulkhead forward. has been in place 2 yrs and is solid. same with deck backing plate we replaced due to excessive wet rot.....is awesome.


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post #19 of 20 Old 09-23-2012
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Re: Marine Plywood & Bulkheads

Quote:
Originally Posted by LandLocked66c View Post
Exterior Plywood

Exterior is the grade given to any plywood which is bonded with 100 percent waterproof adhesive and is intended for permanent outdoor exposure. Exterior plywood must be graded a C or above, meaning knots and knot holes of up to 1 inches are allowed. The species of wood can also vary and often different species are combined. Core gap size for Exterior wood cannot exceed 1 inch wide. Exterior wood is good for most all wood needs, and can be purchased to look almost identical to Marine plywood at a much lower cost.

Plywood graded C-D or CDX is often mistaken for exterior plywood, but is not approved by the APA for outdoor exposure. CDX plywood is considered Exposure 1 plywood where it can encounter high moisture content, but needs to be sealed for complete outdoor use.

Marine Plywood

Made entirely of Douglas Fir or Western Larch, marine grade is one of the highest designations which can be given to a piece of plywood and is considered to be "premium." In order to be marine grade plywood, the outer plies must be graded at least a B or better. B grading means that the wood may have some knots, but no knot holes. In order to be A grade ply, there cannot be any knots or knotholes present in the layer. The maximum core gap size allowed is an 1/8 of an inch, and both outer panels must be sanded, Medium Density Overlay or High Density Overlay. The durability rating must be Exterior and fully waterproof structural adhesive is used.

Read more: Marine Plywood vs Exterior Plywood | DoItYourself.com
Good advice. True marine ply is hugely expensive - up to 10X the cost of good exterior ply. If you are building a dinghy hull or something it is worth spending the extra money but for interior rough construction on a "big" boat it is not worth the price unless your budget is unlimited.

I have never been on a production boat that used it AFAIK.

I have always used good exterior ply and sealed the edges with epoxy - never a hint of a problem.

I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.

Last edited by SloopJonB; 09-30-2012 at 02:58 AM.
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post #20 of 20 Old 09-24-2012 Thread Starter
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Re: Marine Plywood & Bulkheads

just an update to this since it got bumped

I've been using some high quality Fir plywood, though not marine grade, for the bulkheads and such. However, I did coat each cut piece with 3 layers of epoxy (6 layers on the edges) before mounting. I figure the epoxy treatment would make them as impervious as possible.

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