Aluminum hull issues in small boats

[oleman]

I have been looking at some aluminum boats built as early as the 60's. These are small boats that have been trailered and never stored in water for any length of time. Many have never been in salt water!
Superficially the hulls tend to look fine.
Are there any common failures of aluminum joints I should be aware of.
Most of the boats use rivited construction and a nomially from some 12 to 14 gauge alloy with cast joint reinforcements.
One I like the looks of was a 17' Lone Star from 1965 no visable corrosion.
A classic outboard boat for reconstruction.
Another type of Glen-L project.

[Fifty Plus]

Many times I have seen older riveted alum. boats crack along and near the rivets. I don't think that the crack can be repaired. A lot of the older boats that look fine leak from the rivets. This is repairable with epoxy, but you would still face the cracks long term.

[mcmbuilder]

50+ is very accurate with his reply. Most all aluminum boats joined by riveting will eventually leak arond the rivets, with cracking usually the main culprit. Even with the cracking the rivets have a tendency to become loose and leaks will occur around the shafts. Unless you plan on spending a lot of time expoxying around the rivets I would find another type of boat to restore.

[Kevin Morin]

oleman, Fifty Plus, mcmbuilder,

Hello, I'm a newbie here, but I saw this topic so I'll add a few remarks about riveted production skiffs. I've repaired these skiffs for a few years, and learned a little about them as I did.

I think their rivets 'loosen' in two ways, one is elongation the second is wallowing of the rivet hole in the parent metal. These lapped&riveted joints are most often sealed with a strip of rubber that might harden and crack, depending on its exposure to sun light.

If the elastomer strip is still flexible and not cracked fully across; then the rivets can almost always be tightened at least once, sometimes two times.

Tightening is done with rivet gun (an air hammer for the purpose) and a buck or heavy block as an anvil. As the air hammer hits the head of the rivet the shank collapses slightly allowing the peened ends to come slightly closer together. This will tighten the joint from leaks, stop the flex at that point and work to reduce continued wallowing of the rivet hole in the parent metal.

If the seams of the hull have 'worked' they may become brittle and crack along the rivet line. A boat in this condition has reached its design life and should be recycled.

Most often seam failure of that type is caused by one of two conditions. The first condition is overloading the boat when it is in a seaway. Even freshwater lakes can have enough waves to create this result. When these skiffs are loaded and moving in a quartering sea the hull is in torsion so the seams are twisted. At first the movement is resisted by the hull but if regularly exposed to those forces the parent metal in the rivet hole sides is eventually elongated. The other factor in torsional (over) loading these skiffs is the rivet elongation where the initial rivet peening is slightly stretched and loosened.

The last consideration of this torsional load is similar to the rivet elongation- the rubber strips are worked in compression and get thinned in one area resulting in leaks. This also allows the rivets to move end-to-end and helps increase elongation.

Bucking the rivets tighter will help reduce all these conditions.

The second cause of wear in these boats is not from the water but the highway. When a boat of this type is strapped with a 2ton nylon strap, ratcheting tie-down to a trailer point loads on the hull are greater by far than designed. Compounded by highway speed bumps where the momentum imparted to the boat is dozens of times the uniform waterborne loads- these boats can last for years and be destroyed by trailering.

This type of flexure is most often seen as leaks in the rivets holding transverse members to the bottom. A set of rollers on a trailer instead of long plank 'bunks' seems to contribute to this hull failure most.

Both of these hull flexing conditions can result in a strain hardening of the metal along the seams. This eventually makes the line of rivets into a harder alloy which is then flexed to failure by the causes continuing.

The rivets can also be drilled out removed and replaced with less aged rivets and of a slightly oversized shank. Such a repair might allow the restorer to include new punched rubber in that seam- further extending the life of the skiff. With all the holes lined up and the parts cut to fit, rebuilding would 'doable' but it will take some long hours.

Hope this helps you in making a judgment about buying a used production skiff. They can be repaired within limits but their is also a cutoff point depending on their use, where they may not be worth anything except as a planter.

Cheers,

[Fifty Plus]

Kevin

I like the planter idea. As for the rest of it...way more than I needed to know. I still like the planter idea.
I like wood boats.

Welcome newbie

[oleman]

Well I now own a vintage riveted aluminum boat.
The only immediate problem is the marine plywood, wooden floor replaced 16 years ago is totally rotted and has to be replaced.
So much for how well wood stands stands up in a boat.
At least aluminum has a recycle value, you have to pay to have old wooden or fiberglas boats hauled away.
This was a boat motor and trailer package. The 1978 70 HP Evinrude is worth as much as the whole project cost so construction of the boat is not an issue but I intend to make the most of it.
Appreciate all the input.
As a point, how do they keep old airplanes flying? All are riveted and all come under lot's of stress.

[Kevin Morin]

oleman,
all the aircraft made of riveted sheet metal are bottles and they don't stress their surfaces even in the most violent turbulence or aerobatics.

Open skiffs are more of pan or trough- much different from a 'bottle' and their sheer isn't sufficiently rigid without a full deck. So their behavior is radically different when loaded. Aircraft 'bottles' aren't torsionally flexed while skiff 'pans' are when correctly loaded in a quartering sea.

Airplanes are inspected, detected and maintained; have Federali oversight and mechanics who have registered credentials. Boats are used and put on the trailer to haul them home, not cared much about or for and we all work on them at whatever level of skill and training we have.

Another good comparison example is an egg shell. When they are intact they're incredibly stiff and rigid for their wt and if not punctured - hard to break. On the other hand that same shell with half of it missing is like 'walking on egg shells'- easily broken and fragile-right?

Air "forces" are overall and not point loaded and water is tens times as dense. Trailer rollers are more dense yet and airplanes never see that type of hull skin loading in regular use.

If the small riveted skiffs had a riveted and ribbed deck structure with only seating openings (like old mahogany runabouts) they'd "never" wear out; they'd be more like bottles similar to airplanes instead of troughs that they are.

Airplanes are re-riveted periodically - not all but some - and its a time consuming process- especially lots of the 1930-40 vintage beauties that you'd see in an air show.

When you go back with the wooden sole in your skiff just encapsulate the new ply with epoxy and paint and it will probably last another 16 years.

Cheers,

[oleman]

Thanks for the input.
You obviously know alot about construction issues!!
I was thinking of replacing the plywood with alum sheet thick enough to withstand being walked on. There are crossmembers every 12" so it would not take much.
Think I would gain anything other than the 150# weight saving and the extra duribility?
You have any sealing techniques if there are seams leaking other than replacing/tightening the rivets?

[James]

I was thinking about building an aluminum boat a cat hull something like squat pontoons but alot bigger, maybe 30 - 40 foot. can i not just weld the joints with no rivits? what size? 1/8 or less, grade, temper or not ,? what do ya guys think.

[Kevin Morin]

James,

the rule of thumb for riveting aluminum (versus welding) is that thin material on hulls is not welded very often- so riveting is the accepted method of thin skin hulls; flying and floating.

THIN is <0.080", in my terminology, but many very well respected marine designers/engineers say they limit welding in their designs to 0.100" (12 gauge roughly) or even 0.125" (1/8" or 10 gauge) and thicker.

I respect these designers and my remark about welding a little thinner material is because I'm a welder and have done the work. They are designers so they're specifying work for others whose skills they can't always assess so they tend to follow a conservative approach to hull joints by specifying 1/8" minimum thicknesses and welding as the joining method.

In a case of a pontoon boat of the size you've mentioned -I'd advise thicker material and welding.

If the boat will be in salt water- ever- I'd recommend sheet material be 5086 or 5083 alloy and 6061 T-6 for the extrusions' alloy. If the boat won't be in salt water the 5052 alloy is fine, in fact many builders use this in salt water regularly. I happen to prefer the increased stiffness, strength and corrosion resistance but then my boat owners have been willing to pay the very steep price difference for the 5086 alloy over 5052 alloy sheet aluminum.

If the hull will be like a pontoon boat, those major builders attain great strength by rolling the tubes for the pontoons then welding them together into long cylinders. Rolling sheet aluminum of the alloys mentioned above actually increases the metal's tensile strength and the more or less self-supporting cylinder helps eliminate hull longitudinals and reduces hull weld seams.

If the shape is more like a catamaran combined with a scow or john boat where the two hulls drop below a common bridging center section(?) then the hull would be designed and built to a more common method of sheet metal panels intersecting a chines.

A commercial builder in Mr. Vernon, Washington (State), Bill Munson has a fine design for a commercial landing craft of this size with a similar hull configuration. I do not believe they sell plans- just boats; but the design maybe something you could look at for ideas and confirmation of your thinking.

Speed and load are so important that they have to be described before you could guess at the material scantlings. If the boat will travel at displacement speeds <8 knots and only carry people and not a commercial load, then a hull this size would normally be plated 3/16" on the bottoms and 0.160", or 1/8" on the topsides depending on the panel sizes. [Many designers will add a little to their scantlings to make their hull stiffness calculations more conservative and therefore more safe structurally.]

If the boat were to travel above 8 knots and carry a dynamic load of more than a few tons you'd move the bottom to 1/4" and the topsides to 3/16". Decks for foot traffic can be 1/8" if adequately framed, but if there will be vehicles the framing will become much tighter and the thickness will be 3/16" min.

If a 40 boat is to haul a commercial load at planing speeds it would be common to have 1/4 to 5/16" bottom and the same on decks and topsides with transverse frames of 1/4" to 3/8" depending on their depth.

although we're not standing side by side at your drawing table looking at your sketches together where I'd have a better idea of your ideas; I hope these remarks help you to begin to make preliminary design decisions.

Cheers,

[James]

Kevin Morin,
Thanks and i'd like you to look at this site, http://www.cadanoche.com/gallery/main.p ... temId=1765 I'm wanting to build some thing like this but bigger, with something like a camper on it. a small house boat, I want to build it out of aluminum.

[Kevin Morin]

James,

this boat would build in welded aluminum but the work of forming this type of hull (set) is a non-trivial exercise.

The design critical parts are the joints at the sheer of the amas to the cross beams- this is the critical juncture of these craft and if you haven't built in aluminum before I'd recommend that you consider buying a set of plans from someone with one of these in their design catalog.

Both amas can be folded relatively simply with only a small change in overall shape from flat sheet- that would not be a particularly hard job but if it were the learning hull in aluminum then you have your work cut out.

The little cabin here could be framed in aluminum extrusions easily enough and so could the other designs of more conventional house boats, or as some call them shanty boats.

The mid deck is just an aluminum trailer frame and not too much more than well cut extrusions or sheet braked beams. The real challenge is the ama hull to cross beam joint.

Please don't take my remarks as discouraging, just point out where you'll need to do the most learning. Designing and welding aluminum that is in a torsional loaded joint is very much the area to bring in the pro's to help.

Another way to solve this problem (concentrated stresses) would be to deck all the way between the hulls and over them to some common outer sheer. This would maximize the accommodation spaces, make the hull to deck joint long and distribute the loads of hull torsion over much more material and would help make the boat much (100's of times) stronger.

I do hope you realize the plural thousands of hours implied by this hull in welded aluminum? Looks like a neat project, does Glen have any plans here for houseboats along these general lines? If there are plans here it would be much better to buy some and modify, or pay to have them modified than to take a blank page and a pencil as a beginning.

If you've built before or designed welded aluminum before then I'm not trying to minimize that experience, just making sure we're talking reality.

cheers,

[carlos bairo]

hello i am need help to build my windows from aluminium exstrution ,the situation is that i like to build them on round corners any advice whw to do the bend will be grate ,. my estimation cost to buy them ready make side & windshield tough glass $$AUD3.250 .will by out of my budget . chers CARLOS BAIRO