 As in the free surface tanks, as the ship begins to roll the fluid flows from wing tank to wing tank causing a time varying roll moment to the ship and with careful design this roll moment is of correct phasing to reduce the roll motion of the ship.They do not restrict fore and aft passage as space above and below the water-crossover duct is available for other purposes.This was another concept introduced by Frahm and used in several ships in the early 1900s.