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Vastbinder 06-28-2005 07:43 AM

Marine Grade Plywood?
So, if I''m building a plywood sailboat and sealing the whole thing with dynel and resin,...

What are the differences/advantages to marine grade plywood over normal plywood? Marine grade plywood pretty darn expensive stuff... is it really necessary?

RichardElliott 06-28-2005 08:01 AM

Marine Grade Plywood?
Adhesive is the same as exterior grade. Therefore no difference in water resistance. The major difference is absence of core gaps in marine grade which could be important structurally.

Jeff_H 06-28-2005 08:39 AM

Marine Grade Plywood?
I guess that the answer depends on how long you want the boat to last and whether you ever intend to hit something.

Unlike 10 years ago when exterior plywood used the same gluing schedule as marine plywood, today, only some exterior grade plywoods use the same adhesive as marine grade plywood. Most exterior grade plywoods have gone to less expensive glue formulations and less stringent application schedules.

More significantly marine grade plywood has fewer voids and patches than exterior grade materials. Marine grade plywood often uses better grade (more rot resistent) flitches and marine grade hardwood plywoods are often flat cut rather than rotary cut so are less likely to form checks and ''fish eyes''.

If you were building up multiple layers of cloth and epoxy on both the interior and exterior of the plywood, were sure that no fasteners were going to pierce your moisture proof membranes and you were never going to hit anything that might crush a void, then exterior grade plywood would be adequate. But in the real world, if you are building a boat that you would like to keep for a period of time, then you probably should go with marine grade materials.

I still think that given your long term goals and your ideas about using a double hull, you would be way far ahead of the game to plank the boat with a rigid foam core and the glass over that core. In the big picture you will end up with a stronger, cheaper, more durable and lighter structure.


Silmaril 06-28-2005 09:40 AM

Marine Grade Plywood?
For any marine structural hull component, the wise builder will settle for nothing less than Lloyds registered/approved BS 1088 Okoume plywood. It is stable, has met stringent manufacturing standards, and has an excellent strength to weight ratio. Both the wood used, along with the glues, are rot and mildew resistant, water, steam, and boiling water resistant. It curves easily to form a nice hull shape if handled correctly. I replaced a number of stringers and a couple of bulkheads with it on my sloop and was quite pleased with how easy it was to work. My sheets were made in Greece, and carried the Lloyd''s of London certificate of inspection. 3/4" 4'' X 8'' (or the metric equivilant) ran about $220/ea. delivered.

sneuman 06-28-2005 04:01 PM

Marine Grade Plywood?
Marine grade is an absolute must. Jeff H, others right. The lack of voids is even more important than the adhesive. I have experience with (interior) bulkheads that were replaced with "normal" plywood. Not worth going into here, but the result in (admittedly humid) Hong Kong over a few years was unspeakable. Those bulkheads have now been replaced with what I hope to be a bullet-proof marine-grade sheathed in epoxy.

Vastbinder 06-29-2005 10:14 AM

Marine Grade Plywood?
I appreciate the advice. $220 a sheet is WAY beyond my budget.

I''m thinking that I shall be making a smaller (20-25) ft daysailer which will be taken out of the water and stored in shelter after each use... and that I''m going to have to be practical and go with normal, exterior grade plywood. I shall have to look into the details of foam core construction as my knowledge level is too low to be useful on that one.

Jeff_H 06-29-2005 01:10 PM

Marine Grade Plywood?
First of all Marine Grade Fir A-B plywood is only about 15% higher in cost than Exterior Grade A-B Fir plywood. It is worth every bit of that 15% increas in price. The other part of this is that if you can''t afford the 15% I can''t imagine how you are going to be able to afford to build a whole boat. (You can price this stuff at Harbor Sales who carrries good quality materials at reasonable prices. )

The hull of a boat is roughly 20-25% of the overall cost of building a boat. A reasonable grade of 3/4" A-B fir exterior plywood for your purposes would be close to $100 per sheet. A small beamy boat like you are proposing will use 12-15 sheets (more or less depending on how beamy and how thick you decide to plank). That is roughly $1200-$1500 just for hull planking. Add in another $1,000 or so for glass, resin and fastenings. If that is 25% of the boat you are talking about spending something like $10,000 before you are done.

There are a lot of very good boats out there for a lot less than $10,000.


dman 06-29-2005 06:37 PM

Marine Grade Plywood?
Vastbinder you are all over the map here and on previous posts.You have decided on just about every hull shape and construction there is,but you can only have 1 on 1 boat.There has been alot of discussion on materials and that information is dead on.The most important piece of information that you heard is Jeff-H "there are a lot of very good boats out there for a lot less than $10,000" Go and scribe this quote on your bedroom door so you will see it most every day.If you are worried about the price of plywood you better look at the bedroom door.EX.30 footPearson,diesel,a1 sails,good selection,roller furling.......Needed about 10 hours of deck work and a little fiberglass reinforcement (chainplates,rudder tube)6500 dollars and I still have it because I just do not want to sell it.My advice Go buy a boat and go sailing.You build boats when,You have too much money,too much time,and when you have too much money you get someone else to build it,and when you have too much time you go sailing.If you have a passion for building boats get a job at a boatyard,you will get paid real money and you will not be broke at payday.I know this sounds funny but don`t laugh.I am starting to sound negative on every post!

Vastbinder 06-30-2005 06:00 AM

Marine Grade Plywood?
No worries! ;-)

Of course, I''m all over the map on design questions. I don''t plan on starting construction for several months so I''m looking at what styles are out there and what general info I can pick up on their performance characteristics.

Right now, I like the cape cod catboat style but have not tracked down specifics on it yet.

One final note... For me, the construction process is almost as important as the boat itself. I REALLY like making things.

I also appreciate your advice about working at a boatyard and such. As a matter of fact, I did work at the Todd shipyard in Galveston Texas for awhile back in ''84. Not the same thing, but I have spent some time crawling around the insides of a large number of ships.

Like I said, I consider all the replies I get to be useful, even if the information is negative. I also know what you mean about the financial considerations of buying a small used sailboat... but in my case, I''m likely to get more joy out of building the boat than actually sailing it. (though I plan to do that too!)

Jeff_H 06-30-2005 08:36 AM

Marine Grade Plywood?
Dman, In fairness to Vastbinder,he seems to be sorting through this process in a very directed way. While I do not agree with many of his conclusions, I do think that he taking advise, weighing it and either accepting it or coming up with a basis for rejecting it. My reaction like yours makes sense when viewed with a preference toward getting out on the water quickly in a boatthat sails well. Vastbinder seems to have a different set of goals which very much values the design and construction process.

In my experience, he is not alone when he says that he enjoys the idea of building a boat as much as he does sailing it. I have helped a number of people through boat building projects who knew less about sailing than Vastbinder. I have taught some of these people how to rig and sail the boats they built. I don''t think that what he is saying is inherently wrong even if it is a different thought process than you or I might chose for ourselves.

Vastbinder, I was wondering whether you are thinking about actually designing this boat yourself or selecting an existing design to build?


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