Gluing canvas & wood, folding dinghy

[Trojan Dog]

I found a nifty article reprint for building a "Handy Andy" folding dinghy. No designer credited, but I'm guessing Atkins? The fold areas where the ply abuts use a 2" wide canvas strip glued to the ply, then with a 1/4" thick strip of wood on each side over the canvas for reinforcement. My question is this:
The specs call for waterproof canvas. My instincts are that the waterproofing will interfere in a big way with the glue bond. So, I'm thinking, maybe I should use untreated canvas, glue it all up and then waterproof the canvas that is actually exposed. Any thoughts on that? In either case, what sort of glue might be used? I've used a lot of boatbuilder's epoxy and for some reason I'm not wanting to use more. Also, has anyone ever built/used one of these boats?

[Cuyahoga Chuck]

If these plans are old it may be to your benefit to look into a more modern fabric. Cotton canvas has a nice old timey feel but isn't to good at warding off the microbes that eat cotton. Take a look at some more modern fabrics like nylon, polyester.
Charlie

[outofthenorm]

I've not seen this particular dingy before but I have seen other folding ones using canvas as hinges. Unfortunately, the canvas always fails. It seems to me that something like Sunbrella would be a better choice. It's amazingly durable and it can be glued. Where did the plans come from Dog? Sounds like an interesting device. - Norm

[Todd Bradshaw]

Although Sunbrella has superb durability when it comes to UV exposure, it's not very good at abrasion resistance - including folding it, where the fibers are rubbing against each other.

I'd pretty much ignore "backpack-type" nylons - pack cloth, cordura, ballistics, etc. Though they are often labeled "waterproof" they aren't generally talking about "underwater-proof" and most will leak. They also aren't designed for long-term U.V. exposure.

I would suggest the following options:

In cotton canvas, I would use "Sunforger" brand (it's about 10 oz.) and glue it down with Ambroid Cement. I don't think the waterproofing will give you a problem, but haven't tried it. Though more prone to mold than synthetics (Sunforger does contain a biocide to help) cotton is good at resisting abrasion and not bad for resisting U.V. A Sunforger tent will last longer in the sun than one made from most regular nylon or polyester fabrics.

In Marine canvas, I would use "Top Gun" which is a coated polyester (somewhat similar to Sunbrella (11 oz.) but better for abrasive situations and similar in resisting U.V.). It's fibers won't swell shut, like cotton will, but it has a pretty hefty coating. I would probably use Goodyear "Pliobond" or Ambroid cement for glue. This type of specialized synthetic canvas will be less prone to mold, shrinking, etc. than cotton.

Another possibility is vinyl-coated nylon. This is not like backpack nylon, it's the rubbery stuff that's used for truck tarps and big event tents and shelters - flexible, totally waterproof, tough and lasts well in the sun. It generally weighs 12-24 oz. so it's going to take more abrasion than the others. The vinyl cements like Vinabond will stick it to itself well, but probably not to plywood. Roughing up the surface and using a heavy-duty contact cement like Pliobond would likely work better. This type of fabric may be the best all round bet that's still pretty reasonable, price-wise.

All these fabrics are available from places like sailrite.com and some may be available from a local tent and awning company. Pliobond is available at hardware stores, as are other good, heavy-duty contact cements. Ambroid Cement can be found by doing a little web surfing.

For premium construction. I would go with strips of Hypalon fabric (the stuff used for river rafts) It's actually a nylon or polyester core fabric coated heavily on both sides and impregnated with Hypalon rubber. It will probably outlast the others two or three to one. It's heavier, but still pretty flexible. It may even be more durable left exposed than a plywood strip placed over it for protection would be. You use special contact adhesives to glue it to itself or to wood. The best of these cements are catalyzed versions like Bostick or Clifton make. They will stick to, and seal, joints on bare plywood (like inflatable sportboat transoms) which last for decades without letting go. Shore Cement is pretty decent too, for a one-part glue, but likely not quite as secure for fabric-to-wood bonding. Hypalon fabric isn't cheap, but it sure is tough. You can get it in several forms from Northwest River Supplies (nrsweb.com - go to repair stuff). If you use Hypalon, spring for good plywood, too - because the boat should last a long time.

The glues mentioned generally also work over epoxy-coated ply, but I probably wouldn't apply any of these adhesives over painted or varnished plywood without testing first. Most have pretty strong solvents (usually Toluene-based) which may eat the finish, rather than bond to it.

[outofthenorm]

Todd, what would you think about using a section of fire hose as a hinge?

[Todd Bradshaw]

I don't know what exactly it's made from, but it's surely got to be tough and waterproof if it will flex enough to do the job. If it has rubber showing on the inside it's probably neoprene, which most of the contact cements and rubber raft glues will stick to. If it has that coarsly woven fabric inside the hose as well as outside, it might be hard to get a really good bond. I've never seen the inside of a fire hose, so I just don't know.