Originally Posted by Knaur Amarsh

Seawater weighs 64 lbs per cubic foot. 35 cubic feet of seawater weighs 2,240 lbs or one LONG tonne. Therefore when calculating the volume of a ship's hull below waterline, you merely divide the cubic footage by 35 to get its displacement tonnage. It is of extreme importance to know the exact cubic footage of each tank, void space and compartment below the water line so correct counterflooding can be done in case of a hole in the hull.