Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 17-10-2013, 02:28   #1
Registered User

Join Date: Oct 2012
Location: Richmond, VA
Boat: 1975 Dufour 31
Posts: 273
Replacing the cockpit with Plywood

So I need to replace the cockpit on my sailboat and will be using plywood coated with marine epoxy.

here is the question. I called around looking for marine grade plywood and had no luck. I finally got in touch with a boat repair place and the guy told me to just use exterior plywood which can be found at Lowes.

Now the marine epoxy is being given to me by a buddy that does these kinds of things. He says the problem with using treated plywood is the epoxy takes FOREVER to dry because of the treatments used. He told me to just use regular plywood and by coating it with the epoxy it would seal it for use....however not sure if this is factual.

Appreciate the help!
-Aaron
__________________

__________________
AirBoyzT is offline   Reply With Quote
Old 17-10-2013, 02:46   #2
Registered User
 
salticrak's Avatar

Join Date: Jul 2011
Location: palmwoods qld australia
Boat: wharram tiki 26
Posts: 739
Re: Replacing the cockpit with Plywood

Exterior plywood comes in many grades. If you gonna go to all the trouble,why not use the best materials? Marine ply has more layers than c.d. ply and possibly a lot less voids. Marine ply is glued together with waterproof glue. Also marine ply will be lighter in weight. Marine ply has not been ''treated'' and epoxy will bond to it nicely. Use a piece of plastic or squeegee to spread epoxy.
__________________

__________________
salticrak is offline   Reply With Quote
Old 17-10-2013, 02:58   #3
Senior Cruiser
 
Talbot's Avatar

Cruisers Forum Supporter

Join Date: Jun 2004
Location: Brighton, UK
Boat: Privilege 37
Posts: 3,579
Images: 32
Re: Replacing the cockpit with Plywood

One of the main differencs with real marine ply is that there are supposed to be no voids within each layer - that is each piece of wood in a single layer is tight against its neighbour, and the use of a decent waterproof glue.

High quality exterior grade ply is supposed to have the same sort of glue, but is not packed together as tightly, therefore there are weak spots in the regions of the voids.

I used exterior grade on an engine cover, well soaked in resin, and it lasted several years, but I would not have wanted to do the same in an area of high use and critical to the boats water-tightnesss.
__________________
"Be wary of strong drink. It can make you shoot at tax collectors - and miss."
Robert A Heinlein
Talbot is offline   Reply With Quote
Old 17-10-2013, 03:13   #4
Registered User

Join Date: Aug 2012
Posts: 959
Re: Replacing the cockpit with Plywood

It is not the glue adhesive but the amount of voids which distinguishes 'marine' from exterior. The designator 'marine' simply means there are less voids in the ply. You can make marine grade by filling the voids, commonly on the edges but wherever you detect a void.

The glue is mostly aliphatic resin (Elmer's or if you remember the kid from grade school eating the same in the USA) or urea-formaldehyde. This is no different than sheathing used for roofing in home construction. To be clear here I am talking of the adhesive. The grade of wood for these purposes are designed to be overlaid by roofing materials.

What you want is the species of wood used for the plies with an A/B rating. Birch wood is what you are looking for. If there are voids, fill them with epoxy or equivalent. Viola, 'marine' plywood.

But know this, no ply is gonna last as much as solid wood. For one, ply succumbs to expansion from solar radiation differently than solid wood. There will be the expected warpage as the plies react in different planes plus whatever moisture is entrained in the laminate. Stress in the plies relieve itself through strain which you will see as buckling. You may back cut the plies to help with that but I don't know if that will work for you.
__________________
Richard5 is offline   Reply With Quote
Old 17-10-2013, 03:33   #5
Registered User

Join Date: Aug 2012
Posts: 959
Re: Replacing the cockpit with Plywood

Aaron, to a builder there ain't no such thing as "regular" ply. Different plywood have different purposes. Ask 10 diff people what "reg" is and you will get 10 diff answers according to the intended purpose. For your purpose what you want is exterior but with a grade of A/B. Avoid C or D or heavens to betsy CDX (x meaning exterior)

X means exterior but C or D is the bottom of the grades which you don't want. And like I said what you want is Birch. So Birch A/B is what you are looking for. To make it 'marine' grade you just fill the voids with epoxy or equal.

As far as using ply consider this, all wood expands and contracts due to heat and moisture content. Without getting into the ray and plinth of wood grains, the wood will mostly move across it's grain. Perhaps you have seen how a wood board would 'cup' across it's width. This is the wood trying to relieve the stress caused by expansion as it absorbs heat from solar radiation. Wood is absorptive which means there is a certain amount of moisture in the wood grain. A 6" wide board would cup more than a 2" wide board. A 12" board would cup more than a 6" board.

Now you have a single sheet of ply which is, say 30" wide. Sure, the individual plies are only fractions of an inch thick but still a single ply is that 30" wide. It too experiences expansion and contraction in the elements. It has to relieve the stress caused by that expansion. But it is bonded to other layers in the ply. Over time all of those plies will succumb to the expansion resultant from heating/cooling. The wood plies are trying to relieve the stress of expansion. You will notice this as buckling.

There are some things you can do to slow this process. A favored method is to coat the wood. Painted epoxy or keeping the ply immersed in water or shading are some ways. Still, the wood wants to move (expand and contract according to radiation and moisture content) so you will have to come to expect that. Backcutting of ply is also an option. Backcutting involves grooving a kerf in the backside of the plywood to allow for the movement of the plies..to help relieve the stress in the wood. The saw kerf would cut into the wood but not through the thickness of the plies. Another tried and true method is to rip the plywood into thinner widths...say 2" widths. You commonly see this on wood decks although typically these are solid wood.

I apologize if it seems I am talking down to you.
__________________
Richard5 is offline   Reply With Quote
Old 17-10-2013, 04:12   #6
Registered User
 
aboutgone's Avatar

Join Date: Jun 2009
Location: Marathon, Boot key harbor
Boat: CSY 44 w/o hull# 158 S/V Leighward
Posts: 252
Re: Replacing the cockpit with Plywood

why not use divinacell , its not cheap but its the best
__________________
Never start vast projects with half vast ideas
aboutgone is offline   Reply With Quote
Old 17-10-2013, 05:58   #7
Registered User
 
Delancey's Avatar

Join Date: Oct 2011
Location: Miami, FL
Boat: sunk by irma
Posts: 3,464
Re: Replacing the cockpit with Plywood

Wow, surprising amount of crazy talk for a simple plywood thread.

Ureau formaldehyde and aliphatic resin glues are not used in marine plywood construction. Per the American Plywood Assocuation UE is water resistant NOT waterproof. There are waterproof aliphatic glues such as Titebond 2 but it takes far too long to cure to make plywood. Phenolic glues are used for both exterior and marine grade plywoods, identifiable by the sort of burgundy-colored glue line visible between plys on the edge.

As before the same glue is used in exterior and marine plys, the distinction between the two lies in the voids. Depending on the classification, marine grade should have none or very few. Besides weakness induced by the void, the main concern with voids is when used below the waterline where the void itself becomes like a magnet for water.

Birch, while a fine wood in its own right, has very poor durability and is rated at low rot resistance and should absolutely be avoided for structural use in boats. Maybe interior cabinetry at best.

Lastly, plywood is an excellent material. The fact that it is made from multiple layers with an alternating grain orientation means that it is dimensionally much more stable than solid lumber. The multiple plys mean that the effect of individual defects such as knot holes are minimized when the strength of the panel is considered.

If you are in North America, ABX Southern Yellow Pine or Douglas Fir is probably adequate for your needs as long as the finished product is sealed with epoxy and painted. Spending more for a APA marine grade Douglas Fir panel will get you only a slightly better product.

Whatever you do, avoid buying fancy marine plywood made from rainforest trees, especially when it is going to be covered up and painted. It's just not cool, leave the poor trees alone. If you buy domestic plywood it is very likely not old growth and even more likely not poached to finance some war in the jungle.

http://3s.lcmtdv.com/sites/default/f...aldehyde_0.pdf

Silver Birch | The Wood Database - Lumber Identification (Hardwoods)

Wood glue - Wikipedia, the free encyclopedia

Plywood - Wikipedia, the free encyclopedia
__________________
Delancey is offline   Reply With Quote
Old 17-10-2013, 07:02   #8
sitting on the dock of the bay

Join Date: Mar 2006
Posts: 1,513
Images: 6
Send a message via Yahoo to gonesail
Re: Replacing the cockpit with Plywood

forget lowes exterior plywood. douglas fir is good but i would get the best plywood for the deck: 3/4" okoume. seal it on all sides with epoxy.
__________________
sometimes the things that may or may not be true are the things a man needs to believe in the most.
gonesail is offline   Reply With Quote
Old 17-10-2013, 07:35   #9
Registered User

Join Date: Oct 2012
Location: Melbourne Florida
Boat: Columbia 24
Posts: 98
Re: Replacing the cockpit with Plywood

Birch will not stand up to the moisture and humidity anywhere on a boat, even inside the cabin.
__________________
Eumelia is offline   Reply With Quote
Old 17-10-2013, 08:47   #10
Registered User
 
Sailorman Ed's Avatar

Join Date: Jun 2010
Boat: Polynesia 40/42
Posts: 685
Images: 15
Send a message via Skype™ to Sailorman Ed
Re: Plywood

Excellent thread for a common component!!

Eumelia -birch even with a complete coat of polyurethane or epoxy? if not birch inside, then what do you suggest?

AirBoyzT- I replaced my aft deck and all of the hard bimini with a product that 3M just purchased: **3Mô Honeycomb Core H8PP, 20 mm, 4 ft x 7 ft

Basically a PP core with a fiberglass scrim that I covered with glass and epoxy. I was very pleased with the results. After removing the waterlogged 1" plywood (who knows what grade), my stern came up 1"
Ed
__________________
Sailorman Ed is offline   Reply With Quote
Old 17-10-2013, 09:12   #11
Registered User
 
Delancey's Avatar

Join Date: Oct 2011
Location: Miami, FL
Boat: sunk by irma
Posts: 3,464
Re: Replacing the cockpit with Plywood

Okoume makes for some real pretty plywood if you sleep well not giving a crap about the planet and you support the government of a country considered by some to be the "rape capital of the world" which has a history of human rights abuses aimed particularly at women such as female genital mutilation.

Democratic Republic of the Congo - Wikipedia, the free encyclopedia
Women in the Democratic Republic of the Congo - Wikipedia, the free encyclopedia

If you are interested in using plywood for boat building, you might want to look for a more durable species like Douglas Fir because the rot resistance and strength of Okoume is comparatively less than DF or even SYP.

Aucoumea klaineana - Wikipedia, the free encyclopedia

Questionable forestry practices as well, one third of the harvesting being apparently illegal.

Congo Brazzaville tropical wood export statistics forest Christelle Sassou Nguessou SCTB VPA exhaustion sustainability subcontract logging tax concessions China Rimbunan Hijau Taman Industries Vicwood okoume sapelli logs sawn wood grumes sciages bois

Don't be a jerk, buy domestic plywood. Paint it and feel good about yourself for doing your part in your own small way.
__________________
Delancey is offline   Reply With Quote
Old 17-10-2013, 09:27   #12
Registered User
 
Delancey's Avatar

Join Date: Oct 2011
Location: Miami, FL
Boat: sunk by irma
Posts: 3,464
Re: Plywood

Quote:
Originally Posted by Sailorman Ed View Post
Excellent thread for a common component!!

Eumelia -birch even with a complete coat of polyurethane or epoxy? if not birch inside, then what do you suggest?

AirBoyzT- I replaced my aft deck and all of the hard bimini with a product that 3M just purchased: **3Mô Honeycomb Core H8PP, 20 mm, 4 ft x 7 ft

Basically a PP core with a fiberglass scrim that I covered with glass and epoxy. I was very pleased with the results. After removing the waterlogged 1" plywood (who knows what grade), my stern came up 1"
Ed
When I was growing up there was a newish (in the eighties) sloop in the yard that was built in Russia, how it got there I don't know. It was interesting to check out because of the extensive use of Aluminum, where we would normally expect to see stainless steel, as well as the use of Baltic Birch plywood.

Generally it struck me very much as a "people's" boat in terms of the quality of the build, well below our standards for yacht construction. This of course makes sense because, well, it was built in Russia, and they have lots of Aluminum and lots of Birch trees.

Baltic Birch as a generic type can be very some very good plywood. You'll know it by the Burgundy colored glue line as distinct from Birch cabinet plywood that has a clear UF glue line. This plywood is used as Die Boards and for concrete forming. It has many plys, low voids, and is very strong.

However, sadly it is still Birch at the end of the day and it will rot in the long term if exposed to water. This reality can be tempered if the principles of the Wood Epoxy Saturation Technique are applied. To learn more, check out this excellent resource-

The Gougeon Brothers On Boat Construction-

http://www.westsystem.com/ss/assets/...k%20061205.pdf
__________________
Delancey is offline   Reply With Quote
Old 17-10-2013, 09:59   #13
Registered User
 
Delancey's Avatar

Join Date: Oct 2011
Location: Miami, FL
Boat: sunk by irma
Posts: 3,464
Re: Replacing the cockpit with Plywood

Quote:
Originally Posted by Delancy View Post
Okoume makes for some real pretty plywood if you sleep well not giving a crap about the planet and you support the government of a country considered by some to be the "rape capital of the world" which has a history of human rights abuses aimed particularly at women such as female genital mutilation.

Democratic Republic of the Congo - Wikipedia, the free encyclopedia
Women in the Democratic Republic of the Congo - Wikipedia, the free encyclopedia

If you are interested in using plywood for boat building, you might want to look for a more durable species like Douglas Fir because the rot resistance and strength of Okoume is comparatively less than DF or even SYP.

Aucoumea klaineana - Wikipedia, the free encyclopedia

Questionable forestry practices as well, one third of the harvesting being apparently illegal.

Congo Brazzaville tropical wood export statistics forest Christelle Sassou Nguessou SCTB VPA exhaustion sustainability subcontract logging tax concessions China Rimbunan Hijau Taman Industries Vicwood okoume sapelli logs sawn wood grumes sciages bois

Don't be a jerk, buy domestic plywood. Paint it and feel good about yourself for doing your part in your own small way.
Sexual violence in the Democratic Republic of the Congo - Wikipedia, the free encyclopedia
__________________
Delancey is offline   Reply With Quote
Old 17-10-2013, 10:10   #14
Senior Cruiser
 
Cheechako's Avatar

Cruisers Forum Supporter

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 19,369
Re: Replacing the cockpit with Plywood

Exterior plywood has the same glue as marine plywood. (It used to be the Resorcinol type) As mentioned Marine ply is supposed to have nearly no voids. Not sure if this is big deal or not... if glassing over, probably not. You dont want plywood treated for rot (green) due to the chemicals used. However, you are doing a big job, using the best materials is nice. OF course some Marine ply sheets may cost you 4-6 times what 3/4" exterior will so it does matter.
If you go with Exterior, get at least A C grade. (one side A grade, one side C grade), I'm not sure you can commonly find any better than AC in regular Fir plywood.
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard











Cheechako is offline   Reply With Quote
Old 17-10-2013, 10:26   #15
Registered User
 
Celestialsailor's Avatar

Join Date: Nov 2006
Location: In Mexico, working on the boat
Boat: Hallberg Rassy 35. and 14ft.Whitehall pulling skiff.
Posts: 8,013
Images: 5
Re: Replacing the cockpit with Plywood

I would like to add that I would glass/epoxy the ply, inside and out. Just epoxy coating a cockpit area is asking for trouble down the road. The cockpit has way too much traffic with people jumping in and out constantly. with that and the constant expansion/contraction of the area, eventually the epoxy would fail in a crevice or seam, allowing water egress. You might be able to get away with just glass/epoxy taping the seams.
__________________

__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"

http://wwwjolielle.blogspot.com/
Celestialsailor is offline   Reply With Quote
Reply

Tags
cockpit, plywood

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 01:33.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.