Draft (hull)
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The draft or draught of a ship is a determined depth of the vessel below the waterline, measured vertically to its hull's lowest—its propellers, or keel, or other reference point.[1] Draft varies according to the loaded condition of the ship. A deeper draft means the ship will have greater vertical depth below the waterline. Draft is used in under keel clearance calculations, where the draft is calculated with the available dept of water (from Electronic navigational charts) to ensure the ship can navigate safely, without grounding. Navigators can determine their draught by calculation or by visual observation (of the ship's painted loadlines).[2]
Introduction to specific terminology
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A ship's draft/draught is the "depth of the vessel below the
Draft determines the minimum depth of water a ship or boat can safely navigate in relation to the under keel clearance available.[2] The more heavily a vessel is loaded, the deeper it sinks into the water, and the greater its draft (also referred to as its displacement).[2] 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).[citation needed] 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.[citation needed]
The difference between the forward and aft drafts of a ship is termed its trim.
Ship draft measurements
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- The draft aft (stern) is measured at the perpendicular of the stern.[2]
- The draft forward (bow) is measured at the perpendicular of the bow.[2]
- The mean draft is typically calculated 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 numerical value (given in the ship's drawings or stability manual))[3][2] An alternative visual approximation is that given by reading the draught at the waterline, at or very near to amidships[2]
- 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.[4] In such a case it may be referred to as being down-by-the-head.[citation needed]
In commercial ship operations, the ship will usually quote the mean draft as the vessel's draft.[citation needed] However, in navigational situations, the maximum draft, usually the aft draft, will be known on the bridge and will be shared with the pilot.[citation needed]
Variations
The draft of a ship can be affected by multiple factors, besides the variations caused by changes in displacement:
- Variation by trim[5]
- Variation by list[5]
- Variations in water density due to temperature and salinity
- Variation as a result of a ship moving in shallow waters, or squat[6]
- Variation due to movable appendages, such as centreboards, daggerboards, drop keels, leeboards, and retractable rudders
- Projection of non-retractable rudders, propellers or thrusters below the hull
When measured to the lowest projecting portion of the vessel, it is called the "draft, extreme"; when measured at the bow, it is called "draft, forward"; and when measured at the stern, the "draft, aft"; the average of the draft, forward, and the draft, aft is the "draft, mean", and the mean draft when in full load condition is the "draft load".
Draft marks
These are markings and numbers located on both sides of a vessel, as close as possible to the bow and stern
These hull markings constitute a "banded" scale,[
An internal draft gauge or draft indicator is used on larger ships. It consists of a
Implications
Large ships
Larger ships need to keep the propeller immersed when they are light (without
The draft of a ship can be increased by longitudinal motion in shallow water, a hydrodynamic effect known as
Large ships experience a draft increase to heel effect where the ship's beam angles on one side during an alteration of course (sometimes known as turning effect).[8]
Waterways
Draft is a significant factor limiting navigable waterways, especially for large vessels. This includes many shallow coastal waters and reefs, but also some major shipping lanes, therefore restriction on the maximum draft (the draft limit, a distance from the seabed or riverbed to the water level) is sometimes established (in particular, all ports set up draft limits).
Canals are not the only draft-limited shipping lanes. A
Pleasure boats
A small draft allows
Submarines
A term called
See also
References
- ^ a b c d e Piozet, Charles and the Div. of Personnel Supervision with assistance of the Bur. of Ships (March 2023) [June 1942]. "Nomenclature of Naval Vessels: Glossary of Shipbuilding Terms" (2nd ed.). Retrieved 26 March 2024.
When measured to the lowest projecting portion of the vessel, it is called the "draft, extreme"; when measured at the bow, it is called "draft, forward"; and when measured at the stern, the "draft, aft"; the average of the draft, forward, and the draft, aft is the "draft, mean", and the mean draft when in full load condition is the "draft load". ... TRIM. The arithmetic sum of the drafts forward and aft above and below the mean water-line. The angle of trim is the angle between the plane of flotation and the mean water-line plane. A vessel "trims by the head" or "trims by the stern" when the vessel inclines forward or aft so that her plane of flotation is not coincident with her mean water-line plane.
- ^ ISBN 978-1-905331-60-4.
- ^ "Draft Surveys: Methodology, Calculations, and common errors". 19 October 2020.
- ^ Derrett, D.R. (2006). "12". Ship Stability for Masters and Mates. Sixth: Elsevier. pp. 143–144.
- ^ ISBN 978-1-85609-667-6.
- ^ ISBN 978-1-85609-667-6.
- OCLC 1039382382.
- ISBN 978-1-85609-667-6.
Further reading
- Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Prress. ISBN 0-87033-549-9.
- Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87033-056-X.
- Barrass, C. B. (2009). Ship Squat and Interaction. Edinburgh: ISBN 978-1-905331-60-4.