Proper Installation of Aluminum Fuel Tanks

I need to install aluminum fuel tanks on a flat piece of plywood. I know that improper installation will quickly lead to corrosion where the tanks touch any supporting structure. I have sealed the plywood base with resin, but I think I need some strips of some kind between the plywood and the bottom of the tanks to allow air flow and prevent crevice corrosion. I don't want to foam the tanks in as I understand that the foam eventually dries and allows moisture between the foam and the tank, leading to early corrosion. Does anyone have experience with installation of aluminum tanks? If so, any suggestions would be greatly appreciated.

 

Instead of writing up an explanation here is a link to a nice article on how to do it properly http://marinesurvey.com/yacht/fueltank.htm

 

Very nice, this David Pascoe work, and very wrong, IMO. The way to mount tanks is to make them with pieces of angle attached and simply bolt them in place through the angle only, so that there is complete air circulation. When the holes rot out in twenty years, move the bolts over ( try to isolate the stainless bolts and LanoCoat anywhere the stainless touches aluminum). A plywood shelf of any kind is not needed, reduces the ability to inspect and clean, holds water, adds weight, shows a non-professional aspect, and is for what purpose - because aluminum isn't strong enough to support itself? Come on.

 

most small al al boats done this way, good for any other method building

 

Yes. You don't get better than than that.

 

The absolute essentials for this issue cannot be emphasized enough:

Do NOT use stainless hardware unless there are also nylon washers and bushings and an ohm-meter has established that there is no electrical contact. Aluminum screws can be used without insulation.

Do NOT use copper plumbing to the fuel filter.

Connect the tank to ground on one point only, usually near the tank element.

Paint the new, clean tank with something that does not contain anything metallic, like polyurethane or polyester/ epoxy resin, so the condensation will not be in direct contact with the metal.

You can use closed cell polyurethane foam, resin impregnated wood, fiberglass or even pieces of polyethylene hose under the tank or use the hanging construction if the tank is relatively small.

My 100 gallon tank got a few pinholes because seawater entered through the side vent and corroded the bottom from the inside. I had a new bottom welded in and liberally poured PU foam in the compartment before lowering the repaired tank. That was 23 years ago.

 

I experienced 'aluminium-to-SS metallic fusion' on the SS bolts that were used to affix the lateral supports of the gear box (aluminium case) ! They had to be drilled out, and the aluminium case retapped for larger size bolts.

I was told that ordinary steel (or galvanized iron) bolts are problem-free in this respect.

Right or wrong ?

 

Well I hate refering to a bad site, and normally Pasco is pretty good... So Ouch.

Anyway Carioca anytime you have two metal items touching in the presence of an electrolite (salt water is a good one) you will have some gavanic corrosion. The more similar the metals are in alloy the less rapid it will be, but even two pieces of stainless of the same grade will have some, unless they are from the same batch since the alloy percentages will be slightly different. As a practical matter however so long as the two metals are close in galvanic potential the problem is pretty limited.

There is a pretty comprehensive list at http://en.wikipedia.org/wiki/Galvanic_series that covers most of the major metals, and most of the unusual ones too for that matter. But the good rule to follow is that anytime you have dissimilar metals touching you need to take steps to elimitate them from touching to prevent this.

The only reason I could imagine that someone suggested using galvanized bolts is that because zinc is the metal used in this type of plating, and with the exception of magnesium is the metal lowest down on the chart (it degrades first), so a galvanized bolt effectively has a micro zinc encasing it. My preference is to match the fastener to the metal being fastened and use locktite to even further reduce the amount of potential for problems. But the idea of using steel on a boat makes me shudder since even when galvanized they will rust if there is any scrape in the coating down to the bare metal, and since you are using the zinc coating as a sacraficial anode...

 

I live in a place that truly tests differing ideas (Lots of impacts and freeze/thaw cycles). I have seen SO much foam seperate from aluminum tanks that I would never even consider it. Water ALWAYS eventually finds its way into the crevice and rots the tank. There is a plus side - the tank will remove easily, because it will simply be floating in foam mush. One probably gets away with this in temperate climates for, say, twenty...twenty-three years.

 

Wrong, completely wrong. To make it short.


Stumble
Mr Pascoe has several articles (or better statements in them) where he is besides proven facts. Though in general he is not completely wrong, one must read his text with care! It is by no means boaters bible.

 

God no, I never take him as gospel, but he does normally get the high points right. Some of his prejudices are pretty clear, but on purely technical issues he normally comes out alright. To be honest I didn't read the article when I posted the link, and just assumed it would be a decent primer on how to install tanks. Clearly in this instance I was mistaken on that.

 

I concur

 

To epoxy or not

I have seen several suggestions to epoxy the bottom of aluminum fuel tanks before installation and some that warn against. What's the consensus regarding the merit of this step. If I do it, I would first etch the tanks and then apply West 105 epoxy. But, would this reduce the risk of crevice corrosion or make it bigger?

 

well seeing as how you have acknowleged NO posts here, we dont care a toot what you do ciao

 

Mike,

Never having done this I don't have any practical experience with the process, but here are my thoughts on epoxy coating aluminum for protection...

If and only if the job is done right then I think it would work like a charm, since it will act as a sleave to prevent water from ever getting to the metal and causing problems. However anything short of a perfect job will likely lead to pre-mature failure for a number of reasons.

1) Aluminium is self oxidizing in the presence of oxygen, this creates a very hard, almost impermiable layer of Al-oxide that does wonders to prevent corrosion.

2) In order to bond epoxy to Al you have to sand away this layer to bond to fresh metal. So now you have removes the Al's naturam protective coating to substitute your own.

3) If there are any cracks, defects, pinholes or other wats for water to access the bare metal it will most likely be through microscopic holes... making it almost impossible for the water to wash away or dry. This means that crevis corrosin is going to occur, and it will be almost unstopable once it begins.

So for my money I would just install the tanks right in the first place, making sure the tank has room to breath. I could certainly see how an epoxy barrier might be helpful, but again it has to be done perfectly which is outside the ability of most boat yards, let alone a DIY.