Aluminum Fuel Tanks

[Andrew Troup]

Greg from Allied makes a good point about sumps: they do tend to concentrate the attack zone (a bad thing from the point of view of aluminium in particular), if any seawater gets into the fuel.

However, if there's one thing worse than seawater in the tank, it's seawater anywhere near the injector pump.

So I personally think they're a good idea, even in aluminium, provided you drain them OFTEN.

In tanks which are wide in relation to their depth, a sump at both corners on the deepest edge is good, with a takeoff point at or above the top of each, and a drain at the bottom.

(I'm assuming the deepest edge runs athwartships)

The takeoff lines should go to a T selector ball valve (industrial quality, eg Swagelok)

That way, if you're motor sailing on one tack when heeling severely, if you're low on fuel, you can select the lower takeoff. Then would be a good time to drain the sumps, particularly the lower one.

This is all good practice, even for a stainless tank of this shallow nature, which is often the case under the cockpit floor of a small sailing vessel.

If the boat might be left unattended for long periods, AND PARTICULARLY IF THE TANK IS ALUMINIUM, it would be wise for the entire sump to be detachable, so it can be replaced in toto.

This implies machining the mounting flanges and including an O-ring groove in one of them. It may be necessary to machine the mating face on the tank flat after welding, so I'd put the groove in the flange on the sump.

While you're making the original sump, make a spare and keep it on board.

[j-hood]

Gaaah. I got lost in my *own* thread!

So much good information in here. But unfortunately the boat I was trying to purchase with said aluminum tanks was not to be had. :'( At least, not by me...

[klmmicro]

Wow, indeed a huge amount of great information here. I am in the process of purchasing a sailboat that has a single aluminum tank. I knew some of the risks of seam cracking, and the consequent leaking...but now have a better understanding of the why.

[geoleo]

Originally Posted by noelex 77

There are a number of marine aluminium alloys
5083
5086
5251
5254
5383
5454

These all have different strengths and properties, but for internal fuel tanks corrosion resistance is the most important factor.

A lot of the 6 series aluminium alloys are also suitable and much more suitable to extrusions like

6061

In addition there are some alloys that are given trade names like
Sealium

There are a lot more, but also many that are entirely unsuitable. The unsuitable alloys are generally cheaper and more readily available so there is temptation to use them.

Crevice corrosion generally occurs where the tank rests on its supports. A fuel tank will be heavy and must sit on some structures to support the weight (unless its an integral tank in an aluminium boat). If the aluminium under the support is deprived of oxygen (common) and moist ( try to avoid this combination) this is the environment where crevice corrosion occurs.

Dear Noelex,You seem most knowing on this I have a 12.8 Brewer sloop,It has 1-120 gallon Aluminum water tanks. I decided to check the tanks which are under the floorboards but with good access on top. The port tank was leaking . The entire interior of both tanks were encrusted with white discs like mushroom tops of 1/2 to 1 1/2 in diameter and 1/4to 5/8 thick -hundreds of them! So we got down and scraped them off in 2 go arounds -obtaining about 5-8 lbs of the white crap. Under the center of the larger discs was a small(1/4) dent where it had been corroding. Found a 1/8 hole in the port tank and yes it was above a support . Then we put 2 coats of laminating epoxy on entire surface of both tanks. Washed the final coat to remove amine blush----- Then put back together. Filled tanks -OOPS still have a small leak in port tank!!! DARN! Don't know what to do now????More epoxy? Or try that MICROSEAL? If the microseal does not work then wont be able to do more epoxy. The entire area ia reachable but not see-able. Maybe thicken epoxy some this time? Please advise. Thank You

[geoleo]

oops- 2-120gallon tanks of Grade 5254 1/8 thick

[DeepFrz]

Unfortunately I think its time to replace your tanks. Messing around with epoxy coatings will likely only provide temporary relief if at all. Is the epoxy you are using food grade epoxy?

[roverhi]

Owned a smallish IOR boat with minimal aluminum tanks under the quarter berths, fuel to port and water to stbd. Tanks were put in place then two part expanding foam poured in under and around them. Boat was flat bottomed with no bilge sump so any water that got below sloshed up the sides of the hull and around the tanks. Knew the tanks were bad when I bought the boat but didn't know how bad till I pulled them out. Both tanks had nearly totally disappeared where the foam was against them and the hull. Two part expanding foam absorbs water and holds it against a metal tank. Don't ever use the stuff against metal tanks.

[delmarrey]

Originally Posted by georgetheleo

Dear Noelex,You seem most knowing on this I have a 12.8 Brewer sloop,It has 1-120 gallon Aluminum water tanks. I decided to check the tanks which are under the floorboards but with good access on top. The port tank was leaking . The entire interior of both tanks were encrusted with white discs like mushroom tops of 1/2 to 1 1/2 in diameter and 1/4to 5/8 thick -hundreds of them! So we got down and scraped them off in 2 go arounds -obtaining about 5-8 lbs of the white crap. Under the center of the larger discs was a small(1/4) dent where it had been corroding. Found a 1/8 hole in the port tank and yes it was above a support . Then we put 2 coats of laminating epoxy on entire surface of both tanks. Washed the final coat to remove amine blush----- Then put back together. Filled tanks -OOPS still have a small leak in port tank!!! DARN! Don't know what to do now????More epoxy? Or try that MICROSEAL? If the microseal does not work then wont be able to do more epoxy. The entire area ia reachable but not see-able. Maybe thicken epoxy some this time? Please advise. Thank You

Aluminum should not be used for water. It's just asking for trouble. Nothing like the taste of metal in your water. I'd replace the water tanks with plastic.

[noelex 77]

Sorry to hear about your problems. I am a great fan of aluminium as a material to build boats, but using aluminium to construct water tanks in fiberglass or wooden boats is not a great use of the material.

1/8 thick aluminium does not give much leeway for inevitable crevice corrosion that will occur in this application. It has actually done well to last, I assume, about 30 years before giving problems.

The tank can be re welded, but the heat will damage the fibreglass structure and if you are removing the tank you might as well get a new one built or better yet get a polypropylene tank. This material is really a better choice for this application.

It is very hard to get epoxy to stick well to aluminium long term. With any sort of coating the most you can hope for is a temporary fix. The best option is to replace the tank, but unfortunately boat builders, despite knowing a tank like this will have a finite life, do not make any provision for this and it often involves extensive dismantling of the the interior.

The best tempory fix is to cut open the top of the tank and add a bladder tank. These have a poor reputation, but if you can place them in a location with smooth sides like an existing tank they can have a reasonable life and replacement is not difficult. Another option is to build a plywood/epoxy/fiberglass tank inside the existing tank after cutting off the lid. Construction is little like a stitch and glue plywood dinghy, but you need to use special type of epoxy and even then the water will not taste great. As the tank is constructed in situ there is less need to rip out all the interior.

If extensive surgery is needed to remove the tank it sometimes pays to think a bit laterally. Is there another spot where a water tank, or tanks could be fitted without cutting up the interior? This will inevitably cost some storage area but with the top cut off the old water tank sometimes this area can replace the lost storage.

Let us know how you get on.

[yachtcowgirl]

Originally Posted by Andrew Troup

I don't think it's a big surprise that all materials can leak after welding.

What's different about aluminium from most other metals is that it has almost the worst possible combination of factors to produce complete fusion to the root of the weld at every section along a seam with no porosity, inclusions or other defects which may result in leakage (not necessarily evident until it's been in service for some time.)

These difficulties are compounded wherever several welds meet at a corner.

The characteristics which make it difficult include:

an oxide layer whose melting point is so much higher than the parent metal that you'd just about have to vaporise the metal to melt the oxide

a crucial inability of the parent metal to keep a compact weld pool, because the rate of heat conduction is so high, especially in comparison with stainless. The devilish dilemma arises because of a deficiency in 'hot strength': a pool which is even a smidgen too big cannot hold itself in place against the pull of gravity - and melting happens with no colour change, so it's a proper bugger to predict.

One characteristic pushes you in the direction of lots of heat (which would in any case cause unacceptable distortion) and the other pushes you towards flirting with insufficient heat for full fusion to the root.

It's also "hot short", ie very weak during the phase when it's hot but not molten. This makes it extremely prone to cracking as it cools.

It's also a metaphorical 'magnet', in the molten condition, for gases, to the extent that even an inert gas shield is no guarantee against bubbles of porosity in the weld.

These difficulties, while very serious, can be overcome, but virtually none of them apply in the case of stainless.

It has its own difficulties, but these don't make it difficult to weld, provided you understand what parent metals to use, what electrode and filler rod to select, and have sufficient flow of suitable shielding gas.

The actual welding is a delight; the metal flows like water where it's molten, levelling almost like paint, but holds together really solidly in the immediate vicinity.

A good welder with good eyesight and fine motor control can be taught to TIG weld stainless to a high standard in a few hours.

The same person would need to practice for months to get results half as reliable in aluminium TIG welding, for leak-free seams.

Modern synergic pulse MIG can be used in thin material, but it's not a rescue package for a welder who doesn't already have lots of chops.


I'm not saying it can't be done, but you DO have to select a fabricator carefully.

And it's wise to specify a thickness which is on the generous side. By the time you get up to the sorts of thicknesses that hulls are plated from, the difficulties are considerably reduced ... which is just as well, otherwise aluminium boats would not be viable.

And to be extremely vigilant for the possibility of poultice corrosion where it's mounted, as others have pointed out.

Great post, very informative and to the point. We are about to have two possibly three tanks built and will take your advice to heart.