SHIP DIMENSIONS AND TONNAGE
A ship’s length is measured in different ways depending on scope of measurement. Terms used for technical or registry purposes include registered length, tonnage length, floodable length or length by Class rules. The most common used length measurements are length overall, length between perpendiculars, and length on load waterline.
LOA - Length overall : Distance measured parallel to waterline between the extreme forward end of the bow and extreme aft end of the stern.
LPP - Length between perpendiculars : Distance measured from the forward surface of the stem, or main bow perpendicular member, to the after surface of the sternpost, or main stern perpendicular member. This can give an idea of ship carrying capacity as it exclude volumes at the ends which are commonly used for other scopes (ballast, steering, stores, etc.)
LWL – Length on water line : Length of ship measured at water line
B - Breadth - Greatest distance between the two sides of the ship at the greatest width.
D - Depth - Distance measured from under the deck to keel bottom amidship.
T - Draught - Distance between the keel and the waterline
- Sheer - Height of the deck at the side above the height of deck at side amidship
- Camber - Height of the deck at center line above height of deck at side
Tonnage
Displacement is the weight of the water that a ship displaces when it is floating considering the ship’s fuel tanks are full and all stores are aboard. Displacement is the actual weight of the ship, since a floating body displaces its own weight in water. Other terms used for this maximum weight can be loaded displacement, full load displacement and designated displacement. Displacement is a measurement of weight, do not confuse with similarly named measurements of capacity, net tonnage, gross tonnage, or deadweight tonnage.
The density of fresh water sea water is different, for calculations being considered 1000kg/cbm for fresh water, respectively 1025kg/cbm for sea water. When a 100-ton ship is passing from sea to river it will slightly lower. That is because ship is displacing same 100 tons of water but has to displace a greater volume of fresh water to amount to 100 tons. Displacement measurements include light displacement, standard displacement, and normal displacement.
The traditional method for determining a ship’s actual displacement is by use of draft marks.A merchant vessel has six sets of draft marks: forward, midship, and astern on both the port and starboard sides. These drafts can allow the determination of a ship’s displacement to an accuracy of 0.5%.The draft readings are entered into the ship’s hydrostatic tables, giving a displacement.
Full load displacement
Full load displacement is the displacement of a vessel when floating at maximum draft. For warships full load condition means full ammunition and stores, with available fuel capacity used.
Light displacement
Light displacement is the weight of the ship excluding cargo, fuel, ballast, stores, passengers, crew, but with water in boilers to steaming level.
Standard displacement
Standard displacement is the displacement of the ship complete, fully equipped ready for sea, including all equipment, outfit, provisions and fresh water for crew, miscellaneous stores, but without fuel or reserve boiler feed water on board.
Normal displacement
This rare term has been used to mean the ship’s displacement with all outfit, and two-thirds supply of stores, ammunition, etc., on board.”
Shortly: Displacement = the actual total weight
Lightship or Lightweight measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.
Deadweight tonnage (DWT) is a measure of the weight a ship is safely carrying. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew. This term is used to specify a ship’s maximum permissible deadweight, the DWT when the ship is fully loaded so Plimsoll line is at the point of submersion.
Shortly: dead weight = fuel, water ballast , fresh water , cargo, passenger and stores.
Displacement = DWT + Light weight