More plywood discussions

Hey everyone. This is my first post on this site so I guess I should give a little background. I am building a 16’ flats boat. The interior (deck, frames, stringers, etc) are all specified to be ½”(12mm) marine ply. I have done a couple cedar strip canoes/ kayaks and a kayak using stitch and glue but this is my first “real” boat.

My question is on marine plywood. I have been doing a lot of research on this and trying to find the right material at a reasonable cost. I have a local supplier that has Fir marine plywood however it is expensive and not the best quality. To order what is needed of Okume or Meranti is going to cost a fortune in shipping/crating charges. I know there have been thousands of discussions about marine plywood and that it is the ONLY suitable plywood to be used in boat. If the local supplier was selling Okume or Meranti, I would just buy that but I hate spend a lot of money on Fir which I am told really finishes bad.

So after all my research I still can’t understand why you have to use marine ply if you are completely encapsulating it in epoxy. Builders specify using balsa in decks and that seems to be just fine. So why not use a good quality interior plywood (either fully encapsulated just in epoxy or epoxy/cloth)? If I have to use Fir marine plywood, what is the best way to finish it. I plan to fully coat all parts with epoxy no matter what. Is that enough or do I need to add a layer of cloth with the epoxy to get it to finish well?

Thanks for any help.

 

If your boat is low power..a row boat.... with no slamming loads, then you could use non marine...non structural plywood with epoxy.

If its a fast boat and you use non marine ply, it will fall apart.

One half inch ply sounds like you are building a fast boat.

Marine ply is not only waterproof glue, it is the structural quality of the wood plys and the sheets construction.

What was the price of doug fir marine ? Normally its the cheapest marine grade.

When covered with glass cloth fir is OK.

 

Interior plywood usually uses glue which is not waterproof. So if used in a boat and water gets to the glue due to any gaps or openings in the epoxy, such as around fasteners, then delamination can occur.

Interior plywood is typically is manufactured to look good on one or both faces, but the core construction may not have the same strength as good quality "marine" plywood.

Where are you in Indiana? Johnson's Workbench in Charlotte, Michigan south of Lansing sells several varieties of marine plywood. http://www.theworkbench.com/pdf/plywood.pdf If you don't have access to a truck or trailer to pick it up yourself they deliver into Indiana with their own truck and also ship. http://www.theworkbench.com/delivery.php

 

do not use interior plywood, it is nothing but trouble, moisture will eventually seep in and it will fall apart. You might see if you can get exterior grade 1/4" AC or AB plywood and epoxy glue two layers together to make a reasonably good quality 1/2" plywood. Just make sure you clamp it up so there is even pressure over the whole surface to get a good bond without any voids. this will give you 1/2 plywood with six layers, put the poor quality faces together and than you will have the good sides out.

You can also use 1/2" CDX plywood but there are a lot of voids and the surface finish is not very good, also it is not as strong so you would want to use a heavier grade. You would cut each peice and carefully fill any voids around the edges. this would make an inexpensive boat but do not expect it to last very long, several seasons perhaps. I have done this for a fun quick knock around boat, but do not expect it to last very long.

The other thing you might consider, though at more effort, is to build a strip built hull where you cut all the strips from purchased (or salvaged) lumber, separate the defects out (or use them where it will not show). Than you fiberglass inside and out. this will make for low cost materials.

 

Use Aquatech (brand name) plywood. It's a BS-6566 marine standard and very good stuff (meranti), for the price. Exterior grades do have WBP glue, but the low veneer count and construction of these panels, make it hard to recommend them for much more then a small skiff. A 16' flats boat will have a minimum of 12 sheets of plywood, assuming some compartmentalization.

There are several locations in Indiana to get marine grade stock and shipping is typically about $100. One or two sheets makes this costly, but if you get all of your plywood at the same time, the cost per sheet is greatly reduced. A half inch "XL" grade of Douglas fir will have 5 veneers and costs about $60 full retail. A 1/2" meranti Aquatech sheet will run about $70 full retail, so 12 sheets of good quality Douglas fir is $720, plus shipping and the Aquatech is $840 plus shipping. Of course you can use higher grades for the boat, but I'd recommend a BS-6566 sheet as the minimum, for a tough, durable build. I just talked to a buddy that happens to sell plywood and he can ship you 10 sheets from Florida, for something like $150. Yep, that does mean it's $15 bucks more per sheet, but that's not so bad really, especially once you add up the cost of an outboard, controls and steering, for that puppy.

The reason you use marine grade plywood is the quality of the panel. Epoxy and a light sheathing doesn't add much strength or stiffness, just abrasion protection, so a good plywood panel is necessary, especially as noted on high speed craft. A 1/2" exterior grade (APA) sheet will have 3 veneers and lots of internal defects. This just doesn't hold up, tends to break, doesn't take bends well, etc., etc., etc. A quality APA sheet will have 5 veneers which is much better, though the internal defects will still be high. A BS-6566 or BS-1088 sheet will have 7 veneers and nearly perfect internal veneers. In fact these sheets look so good, you'll feel guilty about painting them.

 

You sound as if you are looking for a clear finish. If so, I think if you arent going to use FG cloth with the epoxy, you are better off just using a good varnish.

You have to use varnish on Epoxy anyway if it is clear, and without glass, epoxy is just an more expensive coating that is much harder to repair as cracks and fissures develop over time.

 

I disagree in that encapsulation of plywood is the safe bet, for durability, even if you plan on a bright finish. I don't agree on solid stock, but plywood really can use encapsulation, even the good stuff.

Cloth doesn't make clear finishes any better, though the surface is more durable with it in place. If you do use cloth and want a bright finish, use 4 ounce or less, other wise it'll show through the clear coats.

 

My experience is that thin unreinforced epoxy eventually develops small cracks and fissures from expansion, impacts etc that absorb moisture, discolour, and are hard to sand back to keep the finish.

 

Do not even consider interior plywood, even if you encapsulate it with epoxy and it succesfully keeps the moisture out it will still fall apart just from heat buildup in any sealed compartments. A peraplegic friend built a little 10ft Marples trimaran out of decent looking underlayment ply and did a nice job and it delaminated before he even got to sail it, very sad. Please spend the money for at least BS-6566 as Par said.

Steve.

 

Encapsulation only works if the film thickness is sufficient (10 mils is a recognized minimum). Some species, such as Douglas fir absolutely need some form of reinforcement (cloth, etc.) or the internal stresses in the wood will check, split or other wise just piss you off. I've found woods with coarse grain require less reinforcement than woods with tight and/or tannin or oil filled grain. The "problem species" do need reinforcement, but most species can live without it, under a clear finish. Okoume and meranti typically don't, though okoume could benefit from a sheathing, just because it's pretty delicate. Douglas fir surely needs a sheathing and this should be substantial if you want it to stay put.

The APA has recently changed it's grading system (again, the ********) and now you need to specifically look for sheets marked "Exterior". There are sheets marked "Exposure 1 Exterior", but these aren't the same and will delaminate.

If you'd like FlatsBuilder, send me an email (click on my icon) and I can put you in touch with a buddy and he can hook you up with the plywood you need.

 

10 mils ? I know you cant mean 10 mm, thats about 3/8" inch, and you dont even get that thick with 6oz fg cloth.

What is the depth you where meaning ?

 

Many times paint film thickness is quoted in Mils..a mil is one thousanth of an inch

Film thickness can also be quoted in Microns..one thousandth of a mm.

hence 25 micron equals one mill.

Epoxy vapour barrier is commonly quoted as 10 mil for 3 coats.

 

Exterior plywood uses the same glue as marine. However, the quality of the veneers is much lower. It also has less veneer count and more voids. A BS-6566 plywood will be flat and bend into a more fair curve too. That makes it much easier to work with.

 

OK. In the old world we called it a Thou - I see on wiki its only the US that call it a mil - for some strange reason

 

For some reason , even the painters over here in the shipyard talk "mil" thickness and refer to the thickness gauge as a mil gauge when surveying and logging off paint work. I seldom hear micron, only from the big guys in suites up in the office. . Perhaps its just accepted slang.