I think I'm going to wimp out of a plastic tank and get an identical aluminium one built. ("Nooo!" I hear you scream at your computers!)
I was wrong about how long this one lasted. There is a test mark on it from Feb 1990, so I think 25 years is an acceptable lifespan between replacements!
I worry about plastic because 1) I don't trust myself to cut/glass a hole in the floor of the boat, and 2) I worry more about chafing a hole through the plastic tank with cables/crap put under the seat and 3) it's not quite the right size.
There is not much room under the seat and it is used for spare battery/cables/fuel flow gauge/gloves and hats/chocolate bars
The old tank rested straight onto the deck gelcoat (photo album updated), with a flange at fore and aft that was glassed down. Under the tank was a good 5mm of wet sand/sawdust/crud that must have caused the corrosion. photos: RIBnet Forums - steve-ribnet's Album: Tornado 5.5 fuel tank replacement
I am debating sealing all around the perimeter of the new tank with sikaflex or similar, to keep out the crud, but risk trapping water as much as keeping it out...
The other mitigating step I may take is to specify slightly thicker aluminium for the base plate of the new tank, to get another few years out of it
Should I sit the tank on anything other than the deck gelcoat- a sheet of 2mm rubber perhaps? Anything that doesn't hold the water I guess
Old tank just had filler cap (with bleed screw) and one fuel pipe union. New one I will specify with a filler cap, fuel float gauge port, fill and vent lines.
watch this space.