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. 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 depth of water to ensure the ship can navigate safely, without grounding. Navigators can determine their draught by calculation or by visual observation.
Related terminology
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". 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. The related term air draft is the maximum height of any part of the vessel above the water.Draft determines the minimum depth of water a ship or boat can safely navigate in relation to the under keel clearance available. 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. 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 difference between the forward and aft drafts of a ship is termed its trim.
Ship draft measurements
- The draft aft is measured at the perpendicular of the stern.
- The draft forward is measured at the perpendicular of the bow.
- 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 with respect to the average perpendicular numerical value ) An alternative visual approximation is that given by reading the draught at the waterline, at or very near to amidships.
- 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 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.
Variations
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
Draft marks
These are markings and numbers located on both sides of a vessel, as close as possible to the bow and stern, and then also, often amidships. The number and its associated marking indicate the distance from the marking to the bottom lowest fixed reference point of the vessel. 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.These hull markings constitute a "banded" scale, and may be accompanied by international load line markings. The scale may use Imperial units or metric units; the Imperial system is as stated above ; 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.
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.
Implications
Large ships
Larger ships need to keep the propeller immersed when they are light, 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 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. This in effect causes a ship to 'vertically sink 'down' leading to a reduction in under keel clearance.
Large ships experience a draft increase to heel effect where the ship's beam angles on one side during an alteration of course.
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 is sometimes established. Panamax class ships—the largest ships able to transit the Panama Canal—do have a draft limit 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 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.