Sheeting with aluminum

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Brian
Posts: 246
Joined: Mon Feb 18, 2013 10:06 pm
Location: Hawaii

Sheeting with aluminum

Post by Brian »

Having built a Monte Carlo, I'm now probably going to build a stretched Gentry (19') as a bit of a hot rod. I have been rethinking the whole process and came up wondering if a boat like this could be sheeted with aluminum, rather than the usual cold-molded process. To make matters even sillier, I'm considering framing it in wood, per plans, and glue and screw the aluminum to the wood. Stop laughing.

There are, of course, problems with aluminum/wood contact, as I learned just in time when sliding the tanks into the Monte Carlo. I have been unable to find anything on the internet about wood types that do not react with aluminum. If there are none that would be suitable, then there needs to be a moisture barrier at the frame intersections. The frames would have wider faces, so the seam between panels would have plenty to screw into on each edge.

Unknown is the thickness and type of plate to use, and whether it could be cold-bent onto the frames. Many modern high performance cars are glued together, so modern epoxy and screws can do a lot. it would be a bit like airplane construction, but with wood framing. Just as a guess, I'm thinking 3/16" aluminum would be fairly bendable in large sheets to the sheer. Certainly most of the chine to keel would be easy, but the bow would require some "creative encouragement". Obviously, no welding. The seams would be filled with epoxy and the hull would be painted, not Mahogany.

"WHY?", you may ask. Well, I'm not looking forward to the herringbone ply glue & screw again, I'd like to save weight, and it just seems like an interesting concept. Of course, I also do not want it to come apart at speed. Anyone out there as crazy as I am?

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Roberta
Posts: 7199
Joined: Tue Jul 31, 2007 5:23 pm
Location: East Troy, Wisconsin

Re: Sheeting with aluminum

Post by Roberta »

The Gentry and other boats designed for cold molded planking have compound curves that are convex and concave. This is why the planking is applied in narrow strips. This would be difficult to do with large AL sheets. You would be better off using a design made for plywood sheet planking.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

Kevin Morin
Posts: 773
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Sheeting with aluminum

Post by Kevin Morin »

Brian, there are several metal skin to wood frame matters that will make your project difficult to engineer and build. As Roberta notes, the first issue, regardless of the scantlings of either material will be the issue of developed surfaces. Sheet metal/plate will not cold form to compound curves. To obtain a compound curve in sheet metal you have to stretch that stock with a 'shrinker/stretcher' or an English Wheel.

Next, as you've already noted gluing sheet metal to wood frames, even with screws to assist conformity to the frames will be a real challenge as the two materials have vastly different coefficients of expansion (and contraction) so adhesion will be an ongoing problem.

Further, airplanes that are made with glued parts are usually so thin a part that it's hard to imagine using that on a hull - even a stretch formed canoe! So while 3/16" might be a very reasonable thickness hull for a welded aluminium hull? it will be very difficult to cold form even in a softer 5052 alloy.

Wood and aluminium can be bedded together and the wood can be epoxy encapsulated to seal but.... there remains the crevice corrosion problem of keeping all joints dry. If water can form a thin film by wicking into a joint via capillary action at a joint face- then a corrosion cell can form- and this includes metal to metal joints that are inadequately sealed. So wide frame faces likely needed for two rows of fasteners at the butt seams in either transverse or long wise present a real challenge to sheathing a wood framed hull with aluminium sheet.

I've never tried this building method so can't speak from experience but I've built in welded aluminium, so I do fully appreciate Roberta's remarks about developed surfaces being a major hurdle to the idea of building metal sheathed wooden frame boat that has compound curved surfaces.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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