Draught, or draft, is the water depth to which a ships hull is immersed in a given condition of load. It is traditionally measured from the top of the keel (or otherwise lowest fairbody point) to the waterline, and denoted by the capital letter T. As numerous varying draught definitions can be applied to vessels with prominent appendages (such as large skegs or keels), or which operate in numerous modes (such as semisubmersibles), it is important to carefully define reference points and use the "correct" draught term for the context.
Design draught (or design draft) is the anticipated draught of the completed ship, when loaded. The design draught is usually considered as either an operationally restricted draught, or the for the full load departure (as opposed to load line) condition. Early in the design spiral, this may even be an aesthetic choice or a rounded guess. This is because the design draught is dependant upon an accurate and complete weight estimate.
See also Load Line Condition
Loadline draught (or loadline draft) is the maximum permissible draught allowable under a vessels International Load Line Certificate. This draught will be indicated by a Plimsoll mark for all vessels subject to load line conventions. The loadline draught is considered for a vessel at level trim and heel, as the plimsoll mark is afixed to the vessel at midships.
Marks draught is the draught at a standard location along the vessel. Typically, vessels will have fore and aft draught marks (though some may have more) on both the port and starboard side. The marks draught is measured at one of these points, and is unique to that position. With marks draughts from both sides of the vessel, the heel can be determined. With marks draughts from two longitudinal positions, the trim can be determined. The draughts indicated will be to the vessels reference line, which typically is the navigational draught at that location.
For a given condition of load, the corresponding maximum draught (or maximum draft) is measured from the bottom of the keel, centreboard, or otherwise lowest appendage to the depth of the waterline.
For a vessel loaded to the loadline draught (maximum permissible), the navigational draught (or navigational draft) is the distance from the waterline to the lowest projection (appendage, hull, etc.) of the vessel (i.e., the maximum draught at the loadline draught). This represents the minimum water depth the vessel can transit, at level trim, not inclusive of shallow water effects.
Scantling draught (or scantling draft) is the draught at which the ships scantlings have been designed for. When classed, this is the draught for which the vessels compliance is evaluated to and is subject to the regulations of the society. Scantling draughts are generally only provided in publications for vessels whose load lines are different than the design draught; i.e., a large tanker, or a vessel with narrow draught range (such as a naval ship). Classification generally calculate scantling draught as 0.85 x least-moulded-Depth.
For offshore oil and gas platforms, the transit draught (or transit draft) is the draught of the rig while being towed or moving under its' own power. This has particular meaning to semisubmersibles, which are often deballasted to the top of the pontoons while underway.
Drilling draught (or drilling draft) is the draught at which a drilling platform is designed to be maintained while drilling operations are underway. The vessels pontoons are submerged to reduce motions induced by the seaway, leaving enough of a gap between the deck and the sea for typical environmental conditions.
Survival draught (or survival draft) is the draught at which a drilling platform is designed to be maintained during storm conditions. In this operational mode, no drilling is underway and the rig is deballasted to increase the air gap, protecting the platform from waves slamming the underside of the deck.