Newbie here with newbie question. Marine plywood or plywood?

Charlie:
You said everything so eliquently.

A+ for effort and a skilled vocabulary.

 

The first time you bend a piece of construction grade over a freshly made form and hear a loud pop, followed by a sudden deformation in the nice smooth curve you were trying to bend, you'll wish you'd used a better plywood. Which had more veneers, better quality with no voids to harbor rot or go pop during a critical bend or stress loading underway (after construction) as an over lapping veneer or other internal defect rears it's ugly head and literally explode, inside the plywood panel.

This isn't to say you can't use construction grade, you can, but with certain risks. The risks are: not as strong a product, not as bendable a product, more prone to rot and internal defect failure under load and finishing difficulties.

A canoe size craft will call for 1/8" to 1/4" plywood, a skiff or row boat will call for 1/4" to 1/2" in the sizes I think you desire. A small skiff of my design has 3/8" bottom and 1/4" topside planks, is 14' long and can handle a 15 HP outboard. This would be typical. A larger engine would require thicker planking and a denser framing schedule. This is also typical.

In thin plywood, it's particularly important to get the most effort from all the veneers available in the panel. This means a higher veneer count in a panel, will substantially increase the strength and workability of the plywood. Internal defects are less critical and absorbed across more glue lines and cross veneers. Look at a piece of 1/4" three veneer lauan from Lowes/Depot. The outer veneers, which offer the only longitudinal support for the panel, are very thin, with a relatively thick center veneer, which has it grain running across the panel rather then length wise. These paper thin outer skins offer little more then a pretty surface to apply a stain to, certainly no strength. Take a small piece of this plywood and stick it a bucket of water for a month, without any protection and see what happens. It's likely it will shed its outer veneers along the edges pretty quickly.

Every once and a while, the big box stores will get a shipment of equal veneer 1/4" lauan exterior. This is pretty good stuff and I keep and eye out for it, as I have a Lowes just 2 miles away to keep tabs on. Learn how to check a piece of plywood for defects, construction flaws, voids, etc., because you can find amazing deals, if you know what you're looking for and what a good panel looks like when you find one. The glue has to be WBP if used for planking, regardless of whatever sheathing you'll apply. Internal components of a boat can be construction grade plywood.

 

Charlie, thanks for the BuckWoodCraft link, unfortunatley deivery to Australia put's the price up a bit . I Guess the price differential is greater here than in the U.S..

PAR, and excelent post, thank you, your eloquent narrative of 'popping' ply really has all the woes of painful experience. That, and the techinical stuff makes me rethink my philosphy.

I want to build a sailing skiff for lakes and quiet waters, nothing too demanding, or too far from shore. One of the first things I read about getting a boat [in AABB I think] said that "...any boat is going to be a compromise..." I think maybe I could use a cheaper ply for the 10mm flat bottom (now that I know how to tell good from bad), and spend a little more on the 6mm that needs bending.

 

An advantage of marine ply in a dry-sailed boat is that you can build the boat without epoxy (except as an adhesive).
The cost of epoxy and cloth saved probably outweigh the extra cost of good ply.
Properly painted and maintained, the boat will last a very long time. Later, you can skin the boat with cloth and epoxy if desired, but you can never upgrade the plywood.
I would choose marine ply with no fiberglass/epoxy over cheap plywood and glass/epoxy. It is lighter but demands more attention to detail. Wood boats can last 100 years without any epoxy at all.

Alan

 

in aus you can get bb hoop pine ply (aa is marine)which if you pick it yourself is equal to the quality of marine ply and at least half the price
sean

 

If you build a boat, then later sell it, marine ply construction will demand a higher price than exteriour.

Tim

 

Thanks for all the replies. I’ve learned a lot reading all the posts.

Originally the question was ply or marine ply. If I can get the marine ply significantly cheaper than the price of 150 per sheet as mentioned, then I’ll go for it.

I said I was thinking of making a kayak, but I was actually thinking of making more of a cross between a kayak and a long rowboat. I want the slimness of a kayak but I want it to be a bit more stable like a rowboat, although I probably wont be taking it out in rough waters. The bay on a nice day is all I’ll be doing. I was thinking of modeling it after the boat in the image below with a couple of changes. I want to build something a little different that will make people say, “cool.”
1/8 plywood seems a little thin for a boat 16 ft long. Would I be better off using ¼, or doubling up 1/8 in the area that people will be occupying?

My skill level is pretty good. I’m self-taught and have completed a bunch of wood, auto, and computer building projects by basically looking at books and the internet. My wife say’s I’m like rain-man when it comes to that stuff.

 

short of having receipts i cant see how you would identify that a boat was built with marine ply as it usually painted inside and glassed outside
sean

 

Quote: Would I be better off using ¼, or doubling up 1/8 in the area that people will be occupying?

I read where some people do use multiple layers but, by doubling up the plywood, thereby having two layers, you will create a void between the layers where water can acumulate and start the rot process. I believe that most would advise against doing so.

I was originally considering glassing over my wood (3/8") hull for strength and durability but, everyone specifically advised against it for the reason mentioned above.

 

Sean,

I'm speaking of a builder eventually selling "his" boat. It would be a matter of being honest with a prospective buyer about the materials used in the boats construction.

Keeping all important reciepts for a project as big as boat construction would be a good idea.

Take care.

Tim

 

That would be like lying about a cars mileage. Besides, isn’t omission of the truth like telling a lie? It’s bad karma if you believe in that sort of thing.

 

Encapsulation is very effective in keeping wood from absorbing water. It works as well for keeping water from getting out of wood. If you completely encapsulate a piece of wood, and then drill a tiny hole in it, and then submerge it, over time it will become saturated. Then it is worse off than a piece of wood that had been left bare. I've seen this happen as a result of a screw hole in a cockpit that filled with water while the owner was away for two years. His tarp had blown off, but he didn't know it.
I bought the boat and had to destroy a custom lazerette and propane locker and rebuild the whole area. It wouldn't have happened if it hadn't been submerged. The teak sole and coamings were unaffected, as if never submerged in water.
The boat had been covered for a year after the cockpit was pumped out. Yet the wood rotted faster than the water could slowly migrate out of one tiny hole.
Well constructed epoxy ply boats have not been around for that long. I can remember a time when nobody I knew of used epoxy to build boats. I see a lot of boats on the used market that are taped or glassed with ployester, and they often are seen to be losing their bonds.
I also see a lot of folks doing repairs and building new using epoxy over inferior woods (I even saw a guy who used waferboard in his cockpit under epoxy, who didn't worry at all since as everyone knows, epoxy is forever!)
Years from now, that ultimate belief in epoxy as a miracle sealant will be seen as what it really is: An excuse to use inferior materials and bad workmanship and get away with it for a while.
Don't get me wrong. In the right hands, epoxy really makes an enormous difference. You can indeed protect wood for a long time. Dry wood is stronger and lighter. But I do see a lot of first boats built with a lop-sided dependance on epoxy to make up for ameteur fits and cheaper materials.
Attention to important aspects of epoxy construction will make the difference.
You CAN use cheaper materials. What you can't do if you are looking for longevity and safety is misunderstand exactly what epoxy does.
Most importantly, epoxy should be used to seperate wood from wood. Water entering a piece of wood should not be allowed to migrate into adjacent pieces. Equally important is maintaing a strict conrtol of the mix ratio. Cured epoxy should be sanded before recoating every time. Care should be taken in drilling holes, especially in places where rain or bilge water could supply a steady flow of water, and absolutely where water could collect (like against a frame that has a clogged limber hole that's right next to a clam screwed on for running a bilge hose---- the clamp's screws weren't both stainless--- one was zinc plated. It corroded away and left a hole. Water got in, and you never realized that that was what rotted the bottom. this wouldn't happen with painted wood. It would dry over time.
Understanding all of the ramifications of epoxy use in sealing wood is at least as important as what woods are used.
Epoxy can work against you later, so any newbie boatbuilder should study the ins and outs of epoxy encapsulating before they start. Using cheaper materials can make sense, but not if they are improperly encapsulated.

Alan

 

Multiple layers of plywood is a well honored method of construction. I'm just finishing up an Ashcroft method, planked with 1/8" ply. I used resorcinol, but could have used epoxy, if I wanted.

Any layering method requires good contact, so you can eliminate voids between the layers. Cold molding, double diagonal, triple diagonal, veneer over strip, Ashcroft, etc. all require this. This is little more then a modern adaptation of tried and true techniques, generations old.

How did we ever build boats without epoxy. Well, felt, tar, varnish, shellac, paint, lead, copper, pine gum, virginal pee and just about everything has been used between the layers, before real waterproof adhesives appeared during WWII.

Photojunky, on a small boat (most any type) weight is a critical issue. Follow a set of plans and try to refrain from making changes to the structure, such as doubling the bottom planking, in the hope things will be stronger. In the vast majority of cases, just the opposite happens and you've increased panel loading, stress risers, reached fastener strength limitations and a host of other, not easily foreseen or understood engineering issues, which may not have existed before you made changes.

If interested in a better understanding of the structural components in a boat, may I suggest you get a copy of "Elements of Boat Strength" by Geer. Or if interested in just the principles involved, "The Nature of Boats" by the same fellow. Neither book will provide enough information to design a boat, but the individual pieces and concepts can be gathered, absorbed and put to use in your next project. If interested in design then there are several good titles on the subject. You'll be very hard pressed to find success, without a reasonable understanding of the convoluted collection of compromises, necessary to develop a design brief into a craft that meets or exceeds expectations.

Northerncat, I can tell that photo is a plywood boat, just by the looks. I can also tell you it's a taped seam construction too. If you look closely along the edge of the sheer, you can see the tape wasn't faired smooth, a pretty easy give away. It's also a conically developed shape, another dead give away.

 

Northerncat
Thanks for the tip, I'll look for 'bb hoop pine ply'

Next dumb newbie question: AA, BB, AC/DC, 'hoop pine', luan - I've seen all sorts of references to types of ply. What do they all mean?

 

Face plies are graded based on the presence of knots and other surface imperfections. "A" means "No defects" (though patching has been done where knots and defects had been, usually shaped like American footballs).
"B" may have some defects, 'C" even more, "D" many defects.
When a board is graded A-B, it means one side is A and one is B. A-A would be used where both sides will be seen when finished. CDX, used for building construction, means C-D grade, exterior application (glue).
Other than that, there are many many other ways of describing plywood--- indicating intended use, species, number of plies, etc..

Alan