Marine Grade Plywood - Why do you insist (or suggest) everyone use it?

Marine grade - and the other common gradings - goes back to WW2 or shortly after. There had to be a grade of laminated timber of absolute consistency and weight for the construction of the Mosquito fighter bomber, and in developing that it was realised that such a material was probably suited to other structures requiring strength and lightness, such as performance boats. (The PT and MTB boats where built of two layers of 1" mahogany planks laid diagonally with glued canvas between, not plywood as commonly supposed) It didn't take long for the performance boat builders of the '50s to use quality ply stressed skin construction for race advantage. The home builder market boomed in the 50's and 60's, and uniform timber selection became very desirable in this market. Boat builders of that time as today wanted to purchase a consistent material, and several published standards exist. The problem today is that much crap ply gets stamped as if it is the good stuff, rendering the marks pretty much worthless. None of the standards are enforceable as such - although you could claim false advertising - but that seems to not make any difference to the shonks as there is plenty of fake stuff out there.

The main reason for the marine ply standard is no internal voids, and consistent veneer thickness. The glues are the same as WBP. Particular timber types are specified for Marine grade ply that have good life in a wet environment, face veneers are often exotic timbers for dress value such as mature mahogany. A crude but effective accelerated ageing test for any ply chosen for boatbuilding is to boil a sample for some time and leave to stand overnight - it should stand up to a lot before failure. Aircraft grade ply such as the birch types has no particular rot resistance, aircraft not normally being expected to get wet all the time, the Marine grades should always be built of timber types resistant to rot, but epoxy does help with all types.

Here is a link to a robust discussion on just this subject :- http://backyardboatbuilding.org.uk/forums/showthre... You could reduce those pages to a fairly succinct statement that the labelling is less important than the quality of the supplier themselves, and that Marine ply (the real stuff, not just the stuff with a stamp on it) is lighter than much equivalent WBP ply.

People have built very good boats using no more than WBP and there isn't any argument that these materials build a perfectly serviceable craft capable of all it's designer hoped - and with the modern Epoxies just as good a life span. But it is very gratifying to build a boat using the best Marine ply and have it strong, and the lightest weight possible.

When building anything there is always these choices to be made - do you go for the top shelf and best of everything? Or do you economise for cheaper yet quite adequate materials that may not have the best of appearance but will do the job even so? Often it is a pure question of budget - $240 per sheet compared to $70 is a compelling argument.....

Most designs will specify a minimum grade of materials - substitution is fine as long as the substitute is at least as strong as the original material the designer intended. Mostly the only difference in practice is weight - with a racing dinghy this is very critical, a cruiser much less so. The lesser grades of ply with voids and low quality internal veneers are not consistent in strength and weight, I'm sure you have built enough boats now John to have experienced this, and like most of us rely on a good supplier as well as the good old mark one eyeball when going over your stock as it arrives. :)

Marine Grade Plywood

Marine plywood had been around for 40 years. Most plywood these days has water resistant glue, so the only difference now is the "voids". In normal practice there are voids that are under the outer sheets of plywood. The better the grade of plywood the fewer voids. Marine plywood should have none. Clearly many boat manufacturers use something other than marine grade plywood. There are some specifications as to density of the wood and also strength, but it takes a very knowledgeable plywood man to really know the specifications.

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In GA you simply need to find a lumberyard and get marine grade plywood. I'd suggest against Fir, as it's a pain in the butt to sand (the grain is harder than the clear part) and tends to splinter. The very best of the best is Okume or Meranti plywood, made from an African wood, but any void-free marine grade ply is fine. I've redone a few boats- the thing is, the plywood is just the core- the strength comes from the fiberglass composite sandwich that you make...that being said, if you don't want to redo your work in 3 years, you need marine grade ply. Personally, I like to use 2 sheets of 3/4 ply glued and screwed together, fitted roughly in place, then glass the bejaysus out of it with triaxial cloth and woven roven cloth.

Marine-grade plywood is made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grade may contain wood or synthetic patches. Panels are sanded on both faces or Medium Density Overlay (MDO) or High Density Overlay (HDO). The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a "premium" panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness. Sheets up to 5x12-feet are also available. Available grades are A-A, A-B, B-B (face-back), MDO and HDO.

Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.

The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 Construction And Industrial Plywood.

Marine plywood ‪Manufacturers‬ in ‪‎Karnataka‬, will be plywood made with waterproof paste AND ensured to not have any voids in it. It's utilized as a part of boatbuilding or in applications where it will be in contact with water prove ‪Manufacturers‬ in ‪‎Karnataka‬, ‪Bangalore‬, ‪India‬.Marine plywood is more expensive than regular plywood.

Marine grade plywood uses water resistant glues and has treatments that reduce any organic growth like lichens etc and has fire retarding chemicals applied.It is not necessarily the quality of the wood but more the quality of the glue used to stick the ply layers together. Any construction that is likely to undergo severe changes in temperature and humidity ,bathrooms ,kitchens ,areas open to external weather will all benefit from the superior grade of marine ply.

Although marine ply is more expensive like anything the better the material used the longer the end product lasts.

No voids in the plywood and the glue that would withstand a marine environment. You had to boil the product and the glue had to stand up to the water and the heat. Otherwise it's just plywood and was called that.

as i understand the difference between marine grade and normal plywood, the glue.

the glue used in marine plywood is better formulated for use in a wet enviroment.

i still always recommend sealing it anyways....

in the days before marine plywood, i dont believe much plywood was used in boating at all. i think that was still the days of solid lumber.