Im replacing the 70 gallon tank in my 1977 Sea Ray 240 Fly Bridge. The tank has a 1/4 hole near the front on the side. Ive already pulled the tank out. Im probably going to have a new alumimun one made. Question is do I foam the new tank into place or just put rubber between the bottom and the hull and just attach it to the side stringers?

My OE tank hangs by the stringers and the space underneath is not foamed. I don't see the purpose for foam if rigidly mounted with brackets and foam is notorious for holding in water.

Yeah...the foam just gets wet eventually and STAYS wet...forever.

Paint the tank in coal tar epoxy paint and fiberglass some thin runners into the hull under the tank so air or water can flow around it or put neoprene under the tank the bolt the tank into the stringers. I've heard that rubber will cause corrosion next to a aluminum tank so don't use rubber but if the tank is coated I don't know if it matters.

My old Chaparral developed a fuel leak. Cut floor and dug out all the old foam (PITA). Found where a loose screw was left under the tank and over abt 10 years, it finally wore a hole thru alum tank. Eventually found someone that would weld it after extensive flushing/drying procedures. He also used a nitrogen purge while welding. Put it back in using expanding foam. That stuff is tricky. Put in too much and it will move stuff it shouldn't when it expands. Mounting the tank can be tricky. That's a lot of weight to keep secure when the going gets rough. You got my sympathy either way.

Thats how I did mine too. No foam

I even dont foam in plastic tanks.