The Hull Markings midship on large shipping vessels are known as International Load Lines or Plimsoll Lines (after Samuel Plimsoll – British member of Parliament 1876).
The Plimsoll line is shown in the diagram as the white circle with a line through it marked ‘L’ and ‘R’ (Llyod’s Register of Marine Shipping). Notice how this line matches the delineation between the hull colors and the ‘waterline’.
It is common knowledge that as more weight is added to a vessel, it will float lower in the water. A ship is considered fully loaded when the horizontal line that runs through the circle (Plimsoll Line) is at the water line. If the Plimsoll Line falls below the surface of the water, the vessel is over loaded.
Vessels experience different buoyancies in different types of water. Dense water is more buoyant. Salt water is more dense/buoyant than Fresh water. Cold water is more dense/buoyant than warm water.
Plimsoll lines were based on water buoyancy in English salt water at Summertime.
The ‘S’ line on the hull markings stands for Summer waters in temperate. The ‘T’ refers to salt water in Tropical climates and the ‘W’ means salt water in Winter in temperate climates and finally, ‘WNA’ refers to salt water in the Winter North Atlantic.
The ‘F’ refers to the vessel full load limit in Fresh waters in temperate climates and the ‘TF’ defines the full load limit in tropical fresh water climates.
You would think that the load full limit in cold salt waters (WNA) would be higher than the ‘T’ (Tropical salt water) load full limit since cold salt water is more buoyant. The ship should hold more cargo – right?
The problem is that a vessel fully loaded in Anchorage, Alaska will sink in the water as it travels south and the water becomes warmer and less buoyant. By the time the same ship arrives in Panama, the fully loaded vessel in Anchorage should now sit in the water with a water line running along the ‘T’ line.
Plimsoll lines take into account not only where the ship is being loaded but where the ship is going.
The hull markings near the bow relate to other maters. The symbol that looks like the number 3 is not a number but marking used to indicate that the bow has a bulbous bow. The symbol represents the bow from deck to water with a bulbous bow.
The circle with an “X” indicates that the vessel has a bow thruster. This is important to know for recreational boaters, especially sailors as sailboats are not a maneuverable. Always pay attention when passing a large vessel that is docked. It is amazing how adept and quickly a large vessel can maneuver. You may think you are the ‘stand on’ vessel because you are a sailboat but you must ‘give way’ to larger vessels under power where they are restricted in their ability to maneuver due to water depth or size of channel.