Yep, not necessarily a good idea to have an ali tank pressing against anything that inhibits ventilation or in other words no air. It sets up a perfect situation for whats referred to as "crevice corrosion". The oxide that forms on the outside of aluminium exposed to air protects the aluminium from further corrosion. I agree dont use foam.I had a similar problem and all it took was one corner of the tank touching some sloppy glasswork in the well. Also imo ,fit as large a tank , if you can, as you had before or you are devaluating your tub , should you decide to sell one day. Marineline made up mine.

Thanks for that. I'll contact them for a quote asap

 Apparently, Andy (sea-kem) makes great fuel tanks!

 Had the same problem, had to use a knife to cut out the foam to pull the tank out which had turned to swiss cheese on the bottom. Pain in the bum to slowly chop out, but a job that had to be done to get to the cause of the leak in my case.

Thanks for the replies.

Have cut through all the foam on the sides and the front now. Next, have to see if a fork lift, or block and tackle maybe can break the bond of foam to the bottom of the tank. Will post more when this is done

 I had a leaking fuel tank once Ericl and couldnt find the leak so i drained all the fuel, blocked all hoses off, pick up, return and breather hoses leaving fuel cap on and sprayed compressed air SLOWLY with my air compressor down one of the hoses into the tank.. Squirted soapy water around, found the problem which turned out to be the seal underneith the fuel float/sender.

Only danger is over pressuring the fuel tank hense i said slowly. But you dont need much pressure to find the leak. Might save you pulling it out if its a simple fix as this is what they will do anyway when you do get it out.

 Reading THT, it seems that it is common practice for US builders to fit tanks this way, ie, just sitting them in expanding foam?  Very easy solution in the short term, just wedge it in place and sit it on some blocks, then squirt the foam in. Long term, looking for trouble as you can be creating permanently wet areas which will then give in to crevice corrosion, as Savarna says above. Not as much of a problem for an SS/plastic  tank, but I wouldn't be happy with it in either case, if only because of the difficulty of removal, as above. 

As the last poster said, I'd be looking for the leak with compressed air. You only need a couple of PSI above atmosphere to find a leak--leave your fuel filler cap on, and introduce the air via a breather . The pressure may expand the rubber filler neck a bit, you can use that as a gauge of pressure. I regularly  used compressed air to empty the tank on my previous boat to both clean it out and check the level, as the fuel flow sender wasn't quite accurate, and i did it twice a year to empty and recalibrate. If that amount of pressure is enough to flow fuel up and out at a good rate, it'll certainly be enough to find a gravity leak. 

Hi

Thanks again for the new replies.

So, we now have the tank out. A big thanks to Doug (wangler on this forum) I was ready to give up on 3 occasions, but Doug forced me to continue and in the end, insanity prevailed and we got the bugger out. If I never see expanded foam again, it will still be too soon

On inspection, there are numerous areas of corrosion (white gooey bits) where there are pits. There is still quite a lot of foam stuck to the bottom of the tank, much of which was petrol logged and also water logged and I suspect there will be lots more white gooey bits there (although I dont plan to bother scraping that off). I think there is probably more than one leak so not planning to even try repair

I now have a new tank on order. It will be smaller, around 300 litres as I dont intend to have it running under the inaccesible area under the deck where the deck cover does not reach. It will be bolted to bearers on the bottom and also secured to cleats on the top - NO FOAM !. This will also allow any water that gets in to the bilges, forward and rear to flow out much more easily than before - a 37mm pvc pipe imbedded in the foam which didn't take much to block. Will post again when we do the install

To clarify the pic, bottom is the front and it shows the area where the toilet plumbing goes and the top shows where the tank carries on for about another 600mm under the deck

Cheers - Eric

Not sure where the picture went, I tried dragging and dropping and the pic is there in the preview mode. Any suggestions as to how I can get the new picture to show ?