North Atlantic For the purposes of loadline marks, freshwater is considered to have a density of and typical seawater . Freshwater marks make allowance for the fact that the ship will float deeper in freshwater than saltwater. A ship loaded to her fresh water mark in fresh water will float at her summer mark once she has passed into seawater at the same displacement. Similarly, if loaded to her tropical freshwater mark she will float at her tropical seawater mark once she passes into seawater. The summer load line is the primary load line and it is from this mark that all other marks are derived. The position of the summer load line is calculated from the load line rules and depends on many factors such as length of ship, type of ship, type and number of superstructures, amount of sheer, and bow height. The horizontal line through the circle of the Plimsoll mark is at the same level as the summer load line. The winter load line is one forty-eighth of the summer load draft below the summer load line. The tropical load line is one forty-eighth of the summer load draft above the summer load line. The fresh water load line is an amount equal to millimetres above the summer load line where Δ is the displacement in tonnes at the summer load draft and T is the tonnes per centimetre immersion at that draft. In any case where Δ cannot be ascertained, the freshwater load line is at the same level as the tropical load line. The position of the tropical fresh load line relative to the tropical load line is found in the same way as the freshwater load line is to the summer load line. The winter North Atlantic load line is used by vessels not exceeding in length when in certain areas of the North Atlantic Ocean during the winter period. When assigned it is below the winter mark. Timber load line marks Certain vessels are assigned timber freeboards, but before these can be assigned, certain additional conditions have to be met. One of these conditions is that the vessel must have a forecastle of at least 0.07 the length of the vessel and of not less than standard height, which is for a vessel or less in length and for a vessel or more in length with intermediate heights for intermediate lengths. A