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Draft (hull)

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]

Graphical representation of the waterline of a ship (blue line), absent a lower projecting keel or propeller, with the draft (lower image) indicated as dimension d ; for other dimensions used to describe a ship, see also ship measurements.
Draft markings on the stern of the Cutty Sark, an example of the Imperial system of such markings.

Introduction to specific terminology edit

A ship's draft/draught is the "depth of the vessel below the waterline measured vertically to the lowest part of the hull, propellers, or other reference point".[1] That is, the draft or draught is the maximum depth of any part of the vessel, including appendages such as rudders, propellers and drop keels if deployed.[citation needed] The related term air draft is the maximum height of any part of the vessel above the water.[citation needed]

Draft determines the minimum depth of water a ship or boat can safely navigate.[citation needed] The more heavily a vessel is loaded, the deeper it sinks into the water, and the greater its draft.[citation needed] 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 edit

 
Metric bow scale
 
Imperial system in Roman numeration of the bow scale
  • The draft aft (stern) is measured at the perpendicular of the stern.[citation needed]
  • The draft forward (bow) is measured at the perpendicular of the bow.[citation needed]
  • 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)[clarification needed] with respect to the average perpendicular.[citation needed][2][verification needed]
  • 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.[3] 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 edit

The draft of a ship can be affected by multiple factors, besides the variations caused by changes in displacement:

  • Variation by trim
  • Variation by list
  • Variations in water density due to temperature and salinity
  • Variation as a result of a ship moving in shallow waters, or squat
  • 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 edit

 
Draft marks on a ship's bow
 
Load line mark and draft marks on the side of a ship

These are markings and numbers located on both sides of a vessel, as close as possible to the bow and stern bow and stern, and then also, often amidships.[1] The number and its associated marking indicate the distance from the marking to the bottom lowest fixed reference point of the vessel (e.g., its keel).[1] The numbers and markings were large and clear; for instance, on U.S. naval vessels, the numbers were, historically, as a standard, 6-inches tall, with spacing of 12-inches bottom to bottom, vertically.[1]

These hull markings constitute a "banded" scale,[clarification needed] and may be accompanied by international load line markings.[citation needed] The scale may use Imperial units or metric units; the Imperial system is as stated above (markings 6-inches high, spaced at 12-inch intervals, where the bottom of each marking is the draft in feet); in metric marking, the bottom of each draft mark is the draft in decimeters and each mark is one decimeter high, spaced at intervals of 2 decimeters.[citation needed]

An internal draft gauge or draft indicator is used on larger ships. It consists of a pressure gauge attached to a seacock below the light-load line and calibrated to reflect the draft of the ship.[4]

Implications edit

Large ships edit

Larger ships need to keep the propeller immersed when they are light (without cargo), and may ballast further to reduce windage or for better directional stability or seakeeping, or to distribute load along the hull to reduce hogging and sagging stresses. To achieve this they use sailing ballast distributed among ballast tanks to stabilize the ship, following the unloading of cargo. The draft of a large ship has little direct link with its stability because stability depends mainly on the relative positions of the metacenter of the hull and the center of gravity. However, a "light" ship may have an excessively high stability which can cause uncomfortable rolling of the ship. A fully laden ship (with a large draft) can have either a high or low stability, depending on the height of the center of gravity, which is affected by the distribution of cargo.

The draft of a ship can be increased by longitudinal motion in shallow water, a hydrodynamic effect known as squat, which causes a local pressure reduction under the vessel.

Waterways edit

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). Panamax class ships—the largest ships able to transit the Panama Canal—do have a draft limit (and an "air draft" limit for passing under bridges) but are usually limited by beam, or sometimes length overall, for fitting into locks. However, ships can be longer, wider and higher in the Suez Canal, the limiting factor for Suezmax ships is draft. Some supertankers are able to transit the Suez Canal when unladen or partially laden, but not when fully laden.

Canals are not the only draft-limited shipping lanes. A Malaccamax ship, is the deepest draft able to transit the very busy but relatively shallow Strait of Malacca. The Strait only allows ships to have 0.4 m (1.31 ft) more draft than the Suez Canal. Capesize, Ultra Large Crude Carriers and a few Chinamax carriers, are some of the ships that have too deep a draft when laden, for either the Strait of Malacca or the Suez Canal.

Pleasure boats edit

A small draft allows pleasure boats to navigate through shallower water. This makes it possible for these boats to access smaller ports, to travel along rivers and even to 'beach' the boat. A large draft may increase ultimate stability in, depending on the hull form, as the center of gravity can be lower. A broad beamed boat like a catamaran can provide high initial stability with a small draft, but the width of the boat increases.

Submarines edit

A term called keel depth is used for submarines, which can submerge to different depths at sea, specifying the current distance from the water surface to the bottom of the submarine's keel. It is used in navigation to avoid underwater obstacles and hitting the ocean floor, and as a standard point on the submarine for depth measurements. Submarines usually also have a specified draft used while operating on the surface, for navigating in harbors and at docks.

See also edit

References edit

  1. ^ 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.
  2. ^ "Draft Surveys: Methodology, Calculations, and common errors". 19 October 2020.
  3. ^ Derrett, D.R. (2006). "12". Ship Stability for Masters and Mates. Sixth: Elsevier. pp. 143–144.
  4. ^ René de baron Kerchove (1961). "Draft Gauge". International Maritime Dictionary: An Encyclopedic Dictionary of Useful Maritime Terms and Phrases, Together with Equivalents in French and German (2 ed.). Van Nostrand Reinhold. p. 239. ISBN 978-0-442-02062-0. OCLC 1039382382.

Further reading edit

  • 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.

External links edit

draft, hull, this, article, multiple, issues, please, help, improve, discuss, these, issues, talk, page, learn, when, remove, these, template, messages, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, ci. This article has multiple issues Please help improve it or discuss these issues on the talk page Learn how and when to remove these template messages This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Draft hull news newspapers books scholar JSTOR May 2022 Learn how and when to remove this template message This article possibly contains original research Please improve it by verifying the claims made and adding inline citations Statements consisting only of original research should be removed March 2024 Learn how and when to remove this template message Learn how and when to remove this template message 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 Graphical representation of the waterline of a ship blue line absent a lower projecting keel or propeller with the draft lower image indicated as dimension d for other dimensions used to describe a ship see also ship measurements Draft markings on the stern of the Cutty Sark an example of the Imperial system of such markings Contents 1 Introduction to specific terminology 2 Ship draft measurements 2 1 Variations 3 Draft marks 4 Implications 4 1 Large ships 4 2 Waterways 4 3 Pleasure boats 4 4 Submarines 5 See also 6 References 7 Further reading 8 External linksIntroduction to specific terminology editThis section needs additional citations for verification Please help improve this article by adding citations to reliable sources in this section Unsourced material may be challenged and removed March 2024 Learn how and when to remove this template message A ship s draft draught is the depth of the vessel below the waterline measured vertically to the lowest part of the hull propellers or other reference point 1 That is the draft or draught is the maximum depth of any part of the vessel including appendages such as rudders propellers and drop keels if deployed citation needed The related term air draft is the maximum height of any part of the vessel above the water citation needed Draft determines the minimum depth of water a ship or boat can safely navigate citation needed The more heavily a vessel is loaded the deeper it sinks into the water and the greater its draft citation needed 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 editThis section needs additional citations for verification Please help improve this article by adding citations to reliable sources in this section Unsourced material may be challenged and removed March 2024 Learn how and when to remove this template message nbsp Metric bow scale nbsp Imperial system in Roman numeration of the bow scaleThe draft aft stern is measured at the perpendicular of the stern citation needed The draft forward bow is measured at the perpendicular of the bow citation needed 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 clarification needed with respect to the average perpendicular citation needed 2 verification needed 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 3 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 edit This section does not cite any sources Please help improve this section by adding citations to reliable sources Unsourced material may be challenged and removed March 2024 Learn how and when to remove this template message The draft of a ship can be affected by multiple factors besides the variations caused by changes in displacement Variation by trim Variation by list Variations in water density due to temperature and salinity Variation as a result of a ship moving in shallow waters or squat 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 hullWhen 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 edit nbsp Draft marks on a ship s bow nbsp Load line mark and draft marks on the side of a shipThese are markings and numbers located on both sides of a vessel as close as possible to the bow and stern bow and stern and then also often amidships 1 The number and its associated marking indicate the distance from the marking to the bottom lowest fixed reference point of the vessel e g its keel 1 The numbers and markings were large and clear for instance on U S naval vessels the numbers were historically as a standard 6 inches tall with spacing of 12 inches bottom to bottom vertically 1 These hull markings constitute a banded scale clarification needed and may be accompanied by international load line markings citation needed The scale may use Imperial units or metric units the Imperial system is as stated above markings 6 inches high spaced at 12 inch intervals where the bottom of each marking is the draft in feet in metric marking the bottom of each draft mark is the draft in decimeters and each mark is one decimeter high spaced at intervals of 2 decimeters citation needed An internal draft gauge or draft indicator is used on larger ships It consists of a pressure gauge attached to a seacock below the light load line and calibrated to reflect the draft of the ship 4 Implications editThis section does not cite any sources Please help improve this section by adding citations to reliable sources Unsourced material may be challenged and removed March 2024 Learn how and when to remove this template message Large ships edit Larger ships need to keep the propeller immersed when they are light without cargo and may ballast further to reduce windage or for better directional stability or seakeeping or to distribute load along the hull to reduce hogging and sagging stresses To achieve this they use sailing ballast distributed among ballast tanks to stabilize the ship following the unloading of cargo The draft of a large ship has little direct link with its stability because stability depends mainly on the relative positions of the metacenter of the hull and the center of gravity However a light ship may have an excessively high stability which can cause uncomfortable rolling of the ship A fully laden ship with a large draft can have either a high or low stability depending on the height of the center of gravity which is affected by the distribution of cargo The draft of a ship can be increased by longitudinal motion in shallow water a hydrodynamic effect known as squat which causes a local pressure reduction under the vessel Waterways edit 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 Panamax class ships the largest ships able to transit the Panama Canal do have a draft limit and an air draft limit for passing under bridges but are usually limited by beam or sometimes length overall for fitting into locks However ships can be longer wider and higher in the Suez Canal the limiting factor for Suezmax ships is draft Some supertankers are able to transit the Suez Canal when unladen or partially laden but not when fully laden Canals are not the only draft limited shipping lanes A Malaccamax ship is the deepest draft able to transit the very busy but relatively shallow Strait of Malacca The Strait only allows ships to have 0 4 m 1 31 ft more draft than the Suez Canal Capesize Ultra Large Crude Carriers and a few Chinamax carriers are some of the ships that have too deep a draft when laden for either the Strait of Malacca or the Suez Canal Pleasure boats edit A small draft allows pleasure boats to navigate through shallower water This makes it possible for these boats to access smaller ports to travel along rivers and even to beach the boat A large draft may increase ultimate stability in depending on the hull form as the center of gravity can be lower A broad beamed boat like a catamaran can provide high initial stability with a small draft but the width of the boat increases Submarines edit A term called keel depth is used for submarines which can submerge to different depths at sea specifying the current distance from the water surface to the bottom of the submarine s keel It is used in navigation to avoid underwater obstacles and hitting the ocean floor and as a standard point on the submarine for depth measurements Submarines usually also have a specified draft used while operating on the surface for navigating in harbors and at docks See also editAir draft Hull watercraft Naval architecture WaterlineReferences edit 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 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 Rene de baron Kerchove 1961 Draft Gauge International Maritime Dictionary An Encyclopedic Dictionary of Useful Maritime Terms and Phrases Together with Equivalents in French and German 2 ed Van Nostrand Reinhold p 239 ISBN 978 0 442 02062 0 OCLC 1039382382 Further reading editHayler 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 External links edit nbsp Wikimedia Commons has media related to Draft marks Retrieved from https en wikipedia org w index php title Draft hull amp oldid 1215815059 Waterways, wikipedia, wiki, book, books, library,

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