Anti-rolling tank

An anti-rolling tank is a roll-damping device that uses water moving between port and starboard reservoirs to counter a ship’s roll. The passive U-tube type, patented by Hermann Frahm, sizes the water column’s natural period to the ship’s roll period so the slug of water lags the roll by about 90 degrees and opposes it. Active versions use pumps or controlled air pressure across the tank tops to force the transfer and stay effective across a wider frequency band. Unlike bilge keels or fin stabilizers, a tank works at zero speed, which suits cruise ships, FPSOs, and naval vessels. The trade-off is the free-surface effect, which lowers metacentric height GM.