If you don’t encapsulate with epoxy no ply will last long.

There is no difference in the number of veneers generally, although WPB may have very thin external veneers and a big thick middle one (not good for boats!). Marine grade plywood should have several plies of identical thickness. Many boats are built of WPB and last just fine.

You need to use your mark 1 eyeball to pick good consistent sheets, and if purchasing from a reputable dealer tell them what you want the timber for – if you find it not to be up to your standard return it, and make it absolutely clear that is your intention right up front before buying so there is no confusion or difficulty, and you will get taken seriously.

Ask on boat builder forums for recommendations for reliable suppliers – these are gold, and once found you will find little difficulty with trust and getting what you want in your area.

Good luck.
References :

I build boats, too, when I’m on shore. I use marine-grade plywood (Okume or Meranti, when I can get it) for visible, exposed spots, where i want to see the grain and where I need extra resistance to water intrusion, like the deadwood of a hull. I use decent exterior plywood for small skiffs, because it’s cheap, lasts, and I encapsulate it in polyester resin and fiberglass.
References :

Daniel,

The bottom (no pun intended) line is that there really is (and is not) a difference – well… not if you know what to look for. . .

I am 62 and over my lifetime, I have built 20 (something) boats, all using wood as a structural base, and then either epoxied or fiber-glassed over. After my first (serious) live a-board size vessel I began asking the same question. . . And here is what I discovered…

In 1924 Chris-Craft began manufacturing "standard" boats (we would call that a "production" boat today). Prior to that, the company had already become the largest boat manufacturer in the world. So, needless to say, lumber mills were eager to get his business. As a result, Chris-Craft was extremely "picky" about the wood he would accept from the mills. As his team would go to the mills and pick only the top quality wood for use in their boats. The lumber mills in turn, began calling (and marking) this wood "Chris-Craft quality". Later, as more production boat manufacturers entered the market, the term "Chris-Craft quality" was changed to "Marine Grade".

In fact however, (as you look into Chris-Craft history) what their "team" was doing, was simply going to the mills, and looking over all the wood (including plywood) and choosing only the very best. Which of course, was the wood with the fewest knots, bends, and gaps.

If willing, and you have the time, you can do the same at your local lumber yard. In fact, if you go to a lumber mill (not lumber yard, and certainly not places like Lowes or Builders Supply) the "mill" staff will tell you that the first runs with the best quality is marked as "Marine Grade" and in fact, fewer and fewer mills even bother anymore.

When buying plywood at your local supplier (Lowes, Home builders, etc.) surely you have noticed that there are a number of different grades. The normal grading system uses the letters A, B, C & D, where A is the best quality, with virtually no blemishes and very well sanded. Grade D typically contains up to the maximum number of blemishes allowed.

The letter grades are in pairs, where one letter refers to the "better" side, and the other letter to the back side. As such, a sheet of A-A plywood will be very well finished on both sides, and A-C will be finished on the face and rough sanded on the back. They also (by the way) use the same exact glue for exterior grade plywood as they do Marine Grade plywood.

I (for 20 years) have been using A-A, A-B, and A-C exterior plywood (not treated, just exterior) on all my live aboard vessels and have never had one problem in structural integrity. However… the key factor here is using "boat epoxy" to totally laminate and encapsulate all your wood.

This newer high-tech "Boat resin and epoxy" is really the key to success with any wooden boat. Once encapsulated – it makes your wood (and therefore your boat) totally impervious to rot, worms, insects, air, and moisture. It indeed prevents decay, splits, and cracks from normal wear and tear as well as weather. (You can read much more on this at the link below.)

So, to answer your question: You need really good exterior grade wood, A-A, A-B, or A-C depending on the finish you want on both sides. In addition to that, you will want to laminate and encapsulate your wood (all of it – plywood, skeg, stem, side battens, seats, etc.) in boat epoxy. Then, you will need to either paint it, or fiberglass it. (Note: if you laminate all your wood with boat epoxy, you do not need to fiberglass it – the wood (or boat) will last just as long. Fiber-glassing a small boat does little good – it however is great for "stiffness" and less on going maintenance to longer, larger vessels that sit in the water months at a time or all year long. For a small boat, I would not bother. (My favorite dinghy ( 12′ ), which I have kept through 3 live a-board size boats now, is not fiber-glassed, and when cleaned up – still is and looks as good as new.)

So, check out the link under "source"… there, you can learn alot about boat epoxy and how to use it. Believe me – I have no idea of what it was "before sliced bread" – but this stuff (for a boat builder) is absolutely the best thing since sliced bread.

Happy & Safe Boating,
John
http://boatwrights.org
References :
Boat building epoxy, boat building methods, (even boat building plans):
http://glen-l.com/

So there isn’t really any such thing as ‘regular’ plywood other than this, nowadays: WBP is the ‘new’ regular. If you can still call this new.

Not to be confused with so-called ‘exterior grade’ or ‘shuttering’ plywood. Which, while still laminated using WBP adhesives, is of very inferior timber quality and grain. It is suitable for sheds or as the name implies, boarding up derelict sites. This certainly can warp and even crack, due to the poor timber quality and often inadequate seasoning. And it is often warped already. But correspondingly, very low priced.

Marine plywood is a higher grade WBP sheet material which conforms to BS 1088, and so must not only meet the very same adhesive test, but in addition, the laminates (exterior and interior) must all be of an equal thickness as well as being of a timber that is specified as ‘very durable’ (can stand being placed in the ground for 25 years before decay).

Marine plywood is extremely expensive by comparison and way over-spec if you want to as you say, "go cheap". It typically starts at about twice the price of regular WBP and from there on the sky’s the limit, depending on what the interior and more importantly, the exterior timber specified might be. A lot of these more exotic and very expensive boards are made to customer order by highly specialised companies such as Bruynzeel. Often for the fit-out on just one vessel.

Normally use marine plywood for particularly vulnerable areas, or where you want a more decorative, brightwork, finish – such as transom and wheelhouse coachroof (for examples of each).

Use regular joinery-standard weather-boilproof (WBP) plywood from a good manufacturer so as to ensure a good quality board. It will have an external veneer of fair to very good quality which can be stained and varnished if desired. (Douglas-fir, okoume, etc).

As for any modern timber boat, whatever finishing system you use, it is best applied after sealing all the timber first with epoxy resin. But not essential.

And be sure to use the same specification for your adhesives: WBP. Not merely ‘waterproof’ – which is another specification for glues – such as those white PVAs (polyvinyl acetates) – used for interior joinery in the cabin.

-|–)
References :

Small boats:

While small stitch and glue boats can be successfully build with inexpensive exterior plywood, there are many advantages to the use of quality marine plywood.

•Marine plywood provides greater protection: a small scratch through the resin or glass, if not repaired, may expose the plywood to water. Inexpensive plywood may not last long in that case.
•Marine plywood strongly increases the resale value of your boat.
•Marine plywood is stronger and has consistent mechanical properties: no voids.
•Marine plywood is much nicer and easier to work with.
•For small boats, quality 4 mm plywood is a good substitute for the 1/4" exterior ply often specified.
•Marine plywood (except for Fir) will not check. That feature may by itself pay for the difference in cost. To get a good finish with Fir or Pine, the hull will require extra fiberglassing and large amounts of fairing compound. This is not necessary with Meranti or Okume.

http://plywood.boatbuildercentral.com/

here is a good web site that discusses the use of different plywoods

http://www.boat-links.com/plyfaq.html

while marine plywood is the best for small boat building, we have sucessfully built small skiffs out of Luan plywood designed for floor underlayment. it is made with waterproof glue and works well as a substitute for marine ply. the skiff that we built in 1995 is still in great shape with no signs of delamination.

hope this helps
References :
old salt

The few.
The proud.
The Marine Plywood.
References :
http://www.marines.com/