replacing below deck fuel tank
barrett60,
I'd like to suggest that you can make a very informed decision about your old tank regarding rebuilt or replacement.
Many production boats' tanks are not installed as well as we might like, but that is not the only potential for leaks. The first leak source for older welded aluminum tanks is internal acid/water corrosion of the bottom low point. When fuel water separates out into free water it puddles on the bottom of the tank and becomes acidic as it looses its oxygen and that will create a corrosive cell INSIDE the tank.
This is the cause of 90% of internal corrosion- water bottoms left in the tank and de-aerating. Quite unfortunately, in its wisdom the USCG/DOT have regulations that say there will be no openings below the top of gas tanks so.... the nature drain points available to protect aircraft tanks from this normally occurring corrosion are absent in marine tanks. I know that the reg is there to 'help' us but it may have been the reason you and thousands of other boaters didn't "get the water out"?
The second most common reason I've seen for tank failure is external corrosion due to two different causes but having exactly the same results.
Some builders, inadvisedly in my opinion, use urea based foams in the bilges of their metal boats. They foam in place their gasoline tanks and that can result in poultice corrosion as the formaldehyde base foam breaks down into formic acid which is then held against the tank [poultice] externally corroding holes in the tanks' walls.
A related external event is from the metal itself. There is a bright shiny 'mill scale' put onto aluminum sheet when its rolled to enhance release, appearance and saleability. That mill scale is left on finished aluminum products only by the least informed or most cost-cutting builders, perhaps by some in ignorance? Salt or acidic water plus mill scale and the underlying aluminum is a battery, so; the second most common cause of external corrosion I've seen is galvanic corrosion by the mill scale staying wetted in foam poultice. You might as well paint the tank with lead paint!
Fortunately, with today's tool market you can make a very informed choice about rebuilding/repairing/refurbishing the original tank versus replacing it. The tool I'm referring to is the video bore scope, our shop recently got one for a few hundred dollars [Milwaukee brand I think] and got a color, closeup, exact look at the inside of an old gasoline tank. That tool saved the owner hundreds as the tank was nearly 80 gallons. We were able to pin point the two areas of internal corrosion, cut them out, re-weld patches into the wall, air test the welds and send the tank out "as good as new".
If you repair or replace the tank I'd suggest it not be bedded in foam, that it be acid etched with phosphoric acid, inside and out, that it be coated outside with Alodyne and a Americoat 235 epoxy primer and then re-installed on mechanical beds and mount similar to an inboard engine. Any less planning and work is not really seaworthy, even though many builders would consider this level of effort overkill.
If the corrosion is internal and wide spread, patching will cost more than building, if the corrosion is external and widely spread it will be pretty obvious. In either case anyone with a TIG torch and some aluminum experience can give a cost to repair once the problem if completely defined. Then; you can intelligently weigh the options/costs, something very hard to do without a good look inside the tank.
Cheers,
Kevin Morin
