twistedtree
Guru
The bladder idea seems to have faded, but let me just add that with a bladder you would lose the baffling, and the baffling is important to keeping the fuel from sloshing around and impacting stability. It's that whole free surface of liquids thing.
Now, here's another idea. Use the steel tank as a mold and lay up an FRP tank against it from the inside. It's what FF suggested a while ago, but don't build it as a liner or a repair - build it as a completely intact and self supporting tank. Then it doesn't matter whether it sticks to the metal outer tank or not. Now you have a boat with FRP tanks which I think it the ideal material. You would need to pay special attention to the materials because I gather there are special requirements for holding fuel, but some research should reveal all that. You could even get clever and pre-cut the steel tank such that once the new tank is laid up, you can pull out the old one in pieces.
Anyway, it's just a thought.
Now, here's another idea. Use the steel tank as a mold and lay up an FRP tank against it from the inside. It's what FF suggested a while ago, but don't build it as a liner or a repair - build it as a completely intact and self supporting tank. Then it doesn't matter whether it sticks to the metal outer tank or not. Now you have a boat with FRP tanks which I think it the ideal material. You would need to pay special attention to the materials because I gather there are special requirements for holding fuel, but some research should reveal all that. You could even get clever and pre-cut the steel tank such that once the new tank is laid up, you can pull out the old one in pieces.
Anyway, it's just a thought.