Fuel tanks

[tuantom]

This is interesting. Ike, you obviously have quite a bit of experience in this arena. However, I'm not sure if I follow the downfalls to sealing up an aluminum tank. You mentioned paint won't work; but is that because the paint won't form a good bond to the aluminum oxide? What about a layer of epoxy and cloth? This seems as if it would isolate the tank from any water.

I pulled the original, foamed in (wet), 60 gal tank from a '67 gasoline powered boat. It did have a general layer of oxide on it and some pitting in several spots - resulting in a pinhole on the bottom. My plan is to clean then fill the pinhole and other pitting with JB Weld, epoxy a layer of 7oz cloth around the entire tank, and throw it back into new foam. I was able to look inside the tank through the sending unit opening with a mirror - aside from the rock in there, it looks brand new! Are there different grades of aluminum used?

I realize this is not representitive of all aluminum tanks; but 39 years and counting for my foamed in tank makes me think there may also be a quality of materials issue contributing to the premature failure of the tanks you're referring to. Up to 10% ethanol has been the gas here in Chicago for many years now with seemingly no ill effects to the aluminum.

Just trying to throw a different angle at this,

-Tom

[Ike]

As I have said before it is best not to encase an aluminum fuel tank. If left to it's own natural oxide it will last longer as long as it is kept dry. If you want to glass it in again you would have to first remove the oxide using some kind of solvent so that you achieve a good bond. Acetone would probably work well. Anything that damages the bond is going to allow moisture to accumulate.

Wow! You had remarkable luck with the tank on the 1967 boat. since the average life is 10 years, 39 years is astounding. Somebody did something right.

The common aluminum grades used are in the 5000 series, usually 5052, 5053 or 5086.

I would get rid of the foam. Foam won't bond well to fiberglass so you may have problems with water collecting between the foam and the glass. Glass is not totally impervious to water. Water will migrate in tiny amounts through glass although epoxy is better at resisting this than polyester resins.

[Ike]

I realize it's been a long time since this was discussed but I thought this latest bit of information was very relevant, especially since Tuantom asked; why not?

I recently had a discussion with Kevin Morin of the Metal Boat Society, on their forum. The subject was attaching insulating foam to aluminum. I brought up the subject of foaming in aluminum tanks. Essentially what he said is that aluminum has to be etched. Here is a quote from Kevin:

Next I'll take a shot at the foam issues on aluminum. If the original material is not etched it may have the original mill scale finish which keeps most materials from sticking well. You can tell if the mill stampings are visible and the material is shiny in the field it wasn't etched or buffed off. The minimum step for foam is to etch- although with lots of framing the foam will stay put by mechanical stiffness if there are angles and T's on the surfaces.

If you etch you'd wash with phosphoric acid solution and rinse and dry. This will leave a surface that will take paint (primers) or foam.

Please recall that aluminum's main physical characteristic that makes it commercially viable is the self healing oxide it forms with air in just seconds. This few mill oxide film doesn't offer much adhesion to paint or coatings. The best 'grip' on this surface is to mechanically rough the surface COUPLED with a chemical treatment that leaves a "less slick" surface underneath.

You don't NEED to paint before foaming and you don't NEED to etch with acid but both are better practices. You can foam over an allodyne conversion coating but priming that 2-3 mill coating with primer means that the underlying material will not be subject to a poultice corrosion cell if there were water trapped behind the foam. Vertical surfaces do tend to drain except for foamed panels, but if you use urethane spray on aluminum in the bilge areas without surface protection (full paint system) you will seriously risk the deterioration of the urethane foam in to formic acidic and THAT WILL EAT THE HULL: completely through.

I'm completely supportive of the idea that tanks shouldn't be "bedded" in foam. But the mass of the tanks, essentially free floating in relatively low density foam, is the exclusive agent of the eventual mechanical failure in that case. The tanks should have been mounted correctly with welded or bolted doublers and engine type mounting rails.

Also the surface bond is harder to maintain where there is a nearly infinite mass being entrapped by foam so the surface shear expression of the momentum of the tank underway tears any bond that may have existed.

Almost all the "foamed in" tanks I've seen weren't even etched! The below decks centerline foamed tank is usually not etched so the sheet is covered in mill scale and the foam won't adhere. Then the mechanical weakness of skipping an engine bearer type rail mount for tanks' installations adds to the insanity, and finally the decks are not air tested instead the builder has relied on the foam to displace any eventual bilge water!

That pretty much sums it up why aluminum tanks should not be foamed in place. The foam simply will not bond properly to the tank unless the surface is prepared first. I haven't ever seen anyone do this. They just foam the tank in and hope for the best.

[Pericles]

Hello Ike,

Just a thought. Have you looked at rigid and flexible diesel tanks in the Vetus catalogue http://www.vetus.com. They seem to have all the RCD points covered. Having read all the posts on this subject I think I will now view all petrol engined boats on sale in the UK with the greatest suspicion. The idea of sitting on gallons of high explosive stored in less than safe circumstances is appalling. What is your opinion about the Boat Safety Scheme? http://www.boatsafetyscheme.com/down...uide_chap2.pdf

Regards,

Pericles

[Ike]

Diesel is a good choice. No boom, and fires on diesel boats are much rarer than on gasoline powered boats. But they do hapen so it's best to follow the ABYC, ISO or RCD rules for diesel powered boats. I looked at their tanks. They are good. They flexible tanks are OK on a diesel boat but I would be very cautious about using one for gas.

I looked at the Boating Safety Scheme. It's very good. I haven't read it in detail but it appears based on ISO standards for fuel systems, which are basically the same in ABYC, SAE, NFPA and so on.

[Pericles]

Ike,

Thanks very much for your observations. Via Guillermo who posts here, I made contact with the UK distributor for JetPac. http://www.fleetwatermarine.com/Flee...propulsion.htm

Three units have been sold in Scotland and I shall use the 200 hp Diesel unit, because 5 gallons US per hour at 3400 rpm will deliver 23 knots on my 30 footer, plus freeing up vital interior space.

The Vetus rigid diesel tanks have internal baffles, which surprised me, so that's another plus. My plan is to build a compartment adjacent to the transom in epoxy/marine ply so that the tank(s) is a snug fit. Then foam the spaces left (but not the top) to prevent movement and screw down the lid to close the compartment, except for inspections. The compartment should be air and water tight. The fuel filling point and all hoses and connections will comply with the BSS requirements and any spillages directed overboard. What do you think?

Pericles