Fiberglass or plastic over aluminum ? - (insane thought of the day)

I was just thinking....
From a boat building perspective...

The biggest reason to not use aluminum I can see, is that welding is tricky, and loses strength. On a personal level, for a 1-off boat, that is a major issue. At a commercial level, I can see the same issue; quality control, plus the fact of the engineering behind proper construction.

But I got to thinking, what if a boat hull was build all aluminum, but everything was bolted or riveted, and then coated with fiberglass, truck bed liner, or any imaginable coating product to block water?
It would be stronger than a welded aluminum boat, Repairs would not require an aluminum welder, plus there wouldn't be the common issues with aluminum reacting with a dissimilar metal.

One downside I see is the weight of the added material over top, but I don't think it would be a deal breaker.
Another downside, is that not every hull shape would be attainable assembling with rivets or bolts.
For a simpler boat design though, such as a trawler or back yard design, maybe this could work good?

This might sound crazy, but I'm even imagining a 1/4" thick or so layer of silicone caulking type stuff apllied evenly and neatly over the aluminum. I know I'm getting crazy now.

 

The sea is full of aluminum welded boats. You only need to work with a little more care and with the right tools. In the field of large yachts, the aluminum is associated with high quality. Welding is not difficult, it is different, and does not lose strength, the contrary, the material provided is of better quality than that of the base plate.
Air planes are riveted but probably is due to the small thickness of the plates.
The riveted construction, in the thicknesses required on ships, is extremely complicated. The joining plates and reinforcements, require very special techniques.

 

I'd like to hear more about getting an aluminum boat back to full strength after a weld causes the aluminum to lose temper (and 2/3 of its strength).

 

Any material burned loses much of its strength.
Do not see why aluminum can burn when welding bad, and steel does not.

 

The problem is avoided either by careful engineering or by choosing the "O" (annealed) temper for the base material. The latter one basically mean that both HAZ and the base material will have the same temper and similar mechanical characteristics. That choice has its good and bad sides, the overall result being - lower yield strength but higher ductility relative to H or T-grade tempered alloys.

I'm waiting Ad Hoc to jump into this discussion, he knows his stuff when it comes to aluminum.

 

I have heard of gluing aluminum joints in aircraft and bikes. It was done specifically to avoid the temper loss and thermal strain of welding. If the glue had sufficient strength for simple lap joints it might be useful in boats.

Fiberglass or plastic 'over' aluminum would not work because of different coefficients of thermal expansion. The coating would need to have greater adhesion to the aluminum than strength (thin like paint).

 

Hydroforming could be done with a cement or other somewhat cheap material.
http://en.wikipedia.org/wiki/Hydroforming

 

Epoxy glued aluminium has a higher shear strength than epoxy glued wood according to one of my books. But rivets would be a step backwards. The industry progressed from riveted hulls to welded.

 

The HAZ is not going to be the same strength as the parent material for any of the commonly used 5000 series heat treated aluminium alloys. Approx 60% of the parent material strength is the figure that comes to mind.

As for the original idea, I have the same objection as someone else has already posted - better watch the coefficients of expansion and the bonding of dissimilar materials because it's odds-on it'll delaminate.

In short I wouldn't do it and I wouldn't buy something done like this. Well, maybe a dinghy.

PDW

 

I expressed myself incorrectly. What I meant was that the material provided is of better quality than the material being welded. I have no doubt that the core material, affected by heat, loses strength. The 60% ​​seems much but I have no data on this.

 

Coatings won't add any significant strength unless they match the stiffness of the base material and have a good shear strength bond equivalent to the base material. Putting polyester or vinylester based GRP over metal is problematic in immersed areas and commonly blisters early on with acidic fluid.

Weld strength loss with welding isn't such an issue with the low strength annealed magnesium alloys and it's common to use annealed 5083-O for hull plate and floors and the higher strength 6000 series for extrusions which do lose a large amount of their strength in welding. So rather than adding a laminate you simply use thicker plate. You can also join your stiffeners at the point of contraflexure.

There are very good guides on alloy boat building, start by downloading some of the class rules.

 

Airplanes are riveted because weight-saving requirements impose the use of high-strength alloys. It could be typically a 7xxx alloy with up to 700 MPa yield strength - roughly 3-4 times the value of a typical marine-grade aluminum.
Unfortunately, they have three characteristics which make them unsuitable for welding:
1) they contain zinc;
2) form high stress concentrations in the HAZ, which leads to hot-cracking;
3) their strength is drammatically reduced in the annealed condition (which would be the HAZ of the welds), dropping down to 150 MPa, which would cancel out any advantage of using these alloys.
For these reasons, riveting is used in airplane construction instead of welding, allowing the maximum and reliable use of the available alloy strength.
Cheers

 

I take it that an oven big enough to heat a boat to 1000F for heat treatment is completely impractical?

Friction stir welding sounds interesting for aluminum.

Lotus (car manufacturer) is using glue for aluminum although AFAIK airplanes are still using rivets.

 

OK, I value all the input from all you guys so far, and a few of the points such as expansion rates, I can see.
I didn't mean or intent that the surface layer would provide structural strength, it would just seal out water, and act as I guess a bit sacrificial in the sense that it could eventually get worn off and re-done over the aluminum years later.

"Rivets would be a step backwards"
I can see how you would think so, maybe to someone how is perfect at welding, and already owns every aluminum welding and tool ever made.
Consider that with bolts or rivets, someone could build a boat with a saw, and a drill, even if they can't weld. And they don't have to worry about heat distortion, hidden weld flaws, cracking, etc.
Maybe adhesives WITH riveting would work even better, I don't know. I also don't know if I'd trust joints like that that are not done professionally.

Using thicker plate to have more strength after welding adds more weight.

As much as those pop rivets seem to be hated around here, I've used them lots for other things, and they're pretty strong. Maybe not as strong as solid aluminum slugs, so just use more of them? There is a huge advantage to being able to work you're self and without going deaf. If joining 2 1/4" aluminum plate sections, I have to imagine if you backed the joint with 1/4" strap, you probably wouldn't need washers on the backs of the rivets either, but I don't know for sure. 2 Rows of offset rivets into the backing plating, and it seems like it would be a strong joint to me.

Cover the whole hull with truck bed liner, or another product that would work, and the thing would be almost worry free.
If any rivets ever did get loose, just drill them out and re-do. Re apply bed liner over effected area.

 

Yes, I think airplanes use glue too, but from an amateur stand point, I'd trust the average persons rivets, but I wouldn't trust glue unless it was built by professionals.

Thats just what I'm trying to say; Is riveting and a 2nd sealing layer "THE BEST POSSIBLE IDEA", no, it might not be quite as strong as welding or fancy glues and adhesives. On the other hand, if you get 300 people to build a boat, 100 out of riveted aluminum with a coating, 100 with welding, and 100 with adhesives, I bet the 100 riveted ones are the safest.