exterior grade plywood? What do I really get for an additional $40 per sheet with marine ?
Wha Ho, Pilgrim;
No voids, which can cause problems with bending for one thing and better glues... (I'm assuming you're talking about thin boatbuilding ply)
A Very Subjective Observation
I'm not a wood expert at all, but I can tell you this. My dad built two boats out of one-quarter-inch (perhaps it was even a bit thinner than that) marine-grade plywood when I was just a tyke, and I have used the "surviving" boat a fair amount over the years (that boat is still in excellent condition; the other one got loaned to a friend after we didn't need it anymore, and he chained it to a tree on the shore of a pond for years, and it eventually succumbed to the elements). The wood on that boat is like nothing I've ever seen. It's incredibly strong, and totally flawless. I ran across scraps of wood remaining from those boat-building projects now and then as I was growing up, and I seem to remember that there were as many plys in that quarter-inch material as you normally see in 3/8 or 1/2 inch plywood of "typical" quality. Just based on what I remember about that wood, I wouldn't consider using anything but marine-grade plywood for boat building. It is waaaaay better than regular exterior grade. At least it seems that way to me, so consider the source.
the non visible
difference is the glue. They use non marine grade plywood on the docks where I winter store my sailboat and the crap just falls apart.
Marine grade plywood uses waterproof glue to bind the layers. Normal plywood does not.
Plywood is graded, (I forget the scale, but the better the grade the smaller and fewer voids. The higher gradss will have no voids.) I have not seen marine plywood with any voids which means I have only seen higher grade marine plywood.
Even more expensive is a plywood with a layer of fine exotic wood as the outside layer. I have used some 3/4 inch plywood with a finest layer of mahogany as the outside layer. I have a sheet of the same type of stuff but cherry in the shed.
More than you want to know but was down your way at the Gulfstream Jet assy plant a few years ago. They use a ply laminant with a veneer of various exotics and the cost per 4x4 sheet was in the thousands. I brought several small scraps home and made drink coasters out of them.
Happy Paddling, and Canoe Building
have at look here
it answers many questions about plywood.
http://www.glenl.com/ (look at the left side of the page, under boatbuilding resources. Marine ply wood has no voids, and specific species. The glue is the same as exterior plywood.
Kind of interesting subject.
I recently designed and built a picnic shelter for one of my daughters using trusses for the roof.
I made the gusset plates out of 3/8" plywood and when I went to buy exterior grade plywood I couldn’t find any with the old rating.
When I questioned the guy at Lowes he said that they don’t rate it exterior any more.
I settled on the one grade they had, since the only plates that will be exposed to the weather are on the two end rfters,
I have had a couple of extra plates sitting out in the weather as a test.
I have one lying flat and one upright so the plys will be directly exposed to rain and so far they have been through a bunch of rain storms, and have had wet snow on them for a few days at a time with out any sign of delamanating or warping.
It should be intesting to see what happens to it over a long period.
The only difference is the grade of wood,The glue is the same.I have been building boats for years with exterior plywood,Its a little more sanding with exterior ply vs marine.You can also get marine fir ply that looks worse than any exterior ply you will find.
Try the boil test
that is the accepted standard for glue water proof-ness. Boil a piece of the plywood, or a glued joint for a few minutes(sorry don’t remember the official time), and see what happens.
the voids have been mentioned.
The ease of working with the wood is another factor. It can be tough to find exterior grade plywood that is easy to build a boat with.
Okoume is beautiful in color and grain, bends evenly, hardly splinters, and is lightweight.
It’s a matter of builder preference. I prefer to build only with Okoume for hulls I plan on using quite a bit.
For a kids pirogue project, I’ll use the stuff from Lowes.
You just have to weigh the pros and cons. Easier said than done, though.
Warning: Using nice plywood is like using high-speed internet. Once you use it once, you’ll never want to go back!
It’s the same glue
The same glue is used on marine and exterior plywood. Voids are the difference. There have been lots of homebuilt boats made from exterior plywood with satisfactory results. It seems the biggest problem comes form checking on boats built without sheathing with fiberglass and epoxy.
Marine wood is always better…
…ask my wife.
My girlfriend tells me the same thing.
Are you dating my wife?..
…Geeze, and they say the husband is the last to find out.
I used Lowe’s exterior hardwood
ply on my practice boat and it had only a few very small voids. I don’t think I am a good enough builder to use the marine stuff - yet.
Few have mentioned the thin multiple
layers in marine plywood. If you need to bend for contours, thin layers work better. If you need strength and stiffness, multiple thin layers work better.
for correcting me on the glue. My info about 40 years old. Now that I boiled down some interior grade plywood, what do I do with the sauce? I remember the punk rockers sniffing glue, but not sure if I should drink it.
I boiled some of the Lowe’s 1/4" for
30 minutes. No problems noted. A piece of the Home Depot is sitting on the back porch, has gotten rained on a few times and is coming apart.
I was curious to find out how much I did not know so I did a search and found a few articals. Here is a link to one that I found. http://www.practical-sailor.com/newspics/charts/908plywood.pdf
Now, about those lists, LOL.