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Concept
Draft (hull)
Summary
The draft or draught of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel). The draught of the vessel is the maximum depth of any part of the vessel, including appendages such as rudders, propellers and drop keels if deployed. Draft determines the minimum depth of water a ship or boat can safely navigate. The related term air draft is the maximum height of any part of the vessel above the water.
The more heavily a vessel is loaded, the deeper it sinks into the water, and the greater its draft. After construction, the shipyard creates a table showing how much water the vessel displaces based on its draft and the density of the water (salt or fresh). The draft can also be used to determine the weight of cargo on board by calculating the total displacement of water, accounting for the content of the ship's bunkers, and using Archimedes' principle.
The closely related term "trim" is defined as the difference between the forward and aft drafts.
The draft aft (stern) is measured at the perpendicular of the stern.
The draft forward (bow) is measured at the perpendicular of the bow.
The mean draft is obtained by calculating from the averaging of the stern and bow drafts, with correction for water level variation and value of the position of forward (F) with respect to the average perpendicular.
The trim of a ship is the difference between the forward and aft drafts relative to the designed waterline. When the aft draft relative to the designed water line (DWL) is greater the vessel is deemed to have a positive trim, or to be trimmed by the stern, and it has a negative trim, or is trimmed by the bow, when the forward draft relative to DWL is the greater. In such a case it may be referred to as being down-by-the-head.
In commercial ship operations, the ship will usually quote the mean draft as the vessel's draft. However in navigational situations, the maximum draft, usually the aft draft, will be known on the bridge and will be shared with the pilot.
Official source
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