poop or raised quarter deck is also required if the length is less than . The letter L prefixes the load line marks to indicate a timber load line. Except for the timber winter North Atlantic freeboard, the other freeboards are less than the standard freeboards. This allows these ships to carry additional timber as deck cargo, but with the facility to jettison this cargo. The letters on the timber load line marks have the following meanings: – timber tropical fresh water – timber fresh water – timber tropical seawater – timber summer seawater – timber winter seawater – timber winter North Atlantic The summer timber load line is arrived at from the appropriate tables in the load line rules. The winter timber load line is one thirty-sixth of the summer timber load draft below the summer timber load line. The tropical timber load line is one forty-eighth of the summer timber load draft above the summer timber load line. The timber fresh and the tropical timber fresh load lines are calculated in a similar way to the freshwater and tropical freshwater load lines, except that the displacement used in the formula is that of the vessel at her summer timber load draft. If this cannot be ascertained, then these marks will be one forty-eighth of the timber summer draft above the timber summer and timber tropical marks, respectively. The timber winter North Atlantic load line is at the same level as the winter North Atlantic load line. Subdivision load line marks Passenger ships having spaces which are adapted for the accommodation of passengers and the carriage of cargo alternatively may have one or more additional load line marks corresponding to the subdivision drafts approved for the alternative conditions. These marks show P1 for the principal passenger condition, and P2, P3, etc., for the alternative conditions; however, in no case is any subdivision load line mark placed above the deepest load line in saltwater. See also Ballast tank Coffin ship (insurance) Plimsoll shoe Sailing ballast SS London – sank in 1866 and stimulated Parliament to introduce the Plimsoll line Stability conditions (watercraft) The Onedin Line (episode "Danger Level") Waterline length References External links Historical background to the adoption of the Load Lines convention More historical background Online water density calculator IACS Unified Requirement L: Subdivision, Stability and Load Lines Buoyancy Ship measurements