Fuel tank damage

[partgypsy]

Hi
I'm looking for opinions or wild-ass guesses as to what would cause a 1" x 8" dent or buckling of a 700 gallon fuel tank.
The crease/dent is at the bottom edge where the side meets the bottom. There is no leak, but the paint has flaked off. The tank does not leak.
I can only think of*three possibilities:
1- the tank had been sent flying in a rough sea and dropped hard (hard to believe) or 2- something has pushed up against the tank and squeezed it. There is no sign of damage to the hull. It's a wooden boat and there would be cracked frames and other noticeable damage if there had been a collision.*Or
3- the boat is somehow settling from above. Once again... no other visible damage.


I'm puzzled

Thanks

[skipperdude]

wooden boats can get pretty soft. When was the last time she was hauled. I have seen hulls give 6 or 8 inches when transfered fron the straps to the stands.
SD

[Keith]

If your vent is blocked, the fuel pumps can draw a vacuum on the tank, and it doesn't take too much to cause buckling. Just an idea. Remove the fill cap and see if there was a vacuum.

[Capt Dan]

I agree with Keith, but wouldn't that mean that the vent would have to be blocked as well as the return line, assuming it is a diesel.

Are there valves on the return lines.* Some are plumbed with valves, some not. Check to see if you have them and if they are shut.