Tonnage opening

A tonnage opening is a structural opening, classically the tonnage hatch on a shelter-deck ship, fitted so that the tween-deck space above it is treated as open and excluded from the ship’s measured gross and net tonnage. On 1950s and 1960s shelter-deck cargo ships this hatch kept port dues and tonnage-based fees low, but it left an unprotected opening that could spread fire or flooding. The 1969 Tonnage Convention introduced the tonnage mark to let owners close these openings and restore watertight integrity without the freed space being added back to the ship’s tonnage. Closing the opening converts an open shelter-decker to a closed one with higher measured tonnage.