Draft (hull)

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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.

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

A ship's draft/draught is the "depth of the vessel below the

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 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

Metric bow scale
Imperial system in Roman numeration of the bow scale

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:

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

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

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,[

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.[7]

Implications

Large ships

Larger ships need to keep the propeller immersed when they are light (without

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.[6] This in effect causes a ship to 'vertically sink 'down' leading to a reduction in under keel clearance.[6]

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).

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

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

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

A term called

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

References

  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. ^ .
  3. ^ "Draft Surveys: Methodology, Calculations, and common errors". 19 October 2020.
  4. ^ Derrett, D.R. (2006). "12". Ship Stability for Masters and Mates. Sixth: Elsevier. pp. 143–144.
  5. ^ .
  6. ^ .
  7. .
  8. .

Further reading

External links