# Draft (hull)

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The draft or draught of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel). The draught of the vessel is the maximum depth of any part of the vessel, including appendages such as rudders, propellers and drop keels if deployed. Draft determines the minimum depth of water a ship or boat can safely navigate. The related term air draft is the maximum height of any part of the vessel above the water.

## Contents

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 (salt or fresh). 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.

Trim is the difference between the forward and aft drafts. [1]

## Of a ship

• The draft aft (stern) is measured at the perpendicular of the stern.
• The draft forward (bow) is measured at the perpendicular of the bow.
• 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. [2] [ citation 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.

In commercial ship operations, the ship will usually quote the mean draft as the vessel's draft. However in navigational situations, the maximum draft, usually the aft draft, will be known on the bridge and will be shared with the pilot.

### 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 scale

The drafts are marked on the hull with a "banded" scale, at the bow and stern, and for some ships, also amidships on both sides, where they may be accompanied by international load line markings. The scale may use Imperial units or metric units. If the Imperial system is used, the bottom of each marking is the draft in feet and markings are 6 inches high, spaced at 1 foot intervals. 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

### Large ships

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

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). [5] 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

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 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.

## Related Research Articles

A hull is the watertight body of a ship, boat, or flying boat. The hull may open at the top, or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, derrick, or mast. The line where the hull meets the water surface is called the waterline.

A sailboat or sailing boat is a boat propelled partly or entirely by sails and is smaller than a sailing ship. Distinctions in what constitutes a sailing boat and ship vary by region and maritime culture.

The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.

Boat building is the design and construction of boats and their systems. This includes at a minimum a hull, with propulsion, mechanical, navigation, safety and other systems as a craft requires.

A centreboard or centerboard (US) is a retractable hull appendage which pivots out of a slot in the hull of a sailboat, known as a centreboard trunk (UK) or centerboard case (US). The retractability allows the centreboard to be raised to operate in shallow waters, to move the centre of lateral resistance, to reduce drag when the full area of the centreboard is not needed, or when removing the boat from the water, as when trailering. A centreboard which consists of solely a pivoting metal plate is called a centerplate. A daggerboard is similar but slides vertically rather than pivoting.

This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water. Some remain current, while many date from the 17th to 19th centuries. The word nautical derives from the Latin nauticus, from Greek nautikos, from nautēs: "sailor", from naus: "ship".

Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voyage without cargo, then ballast of little or no value will be loaded to keep the vessel upright. Some or all of this ballast will then be discarded when cargo is loaded.

A vessel's length at the waterline is the length of a ship or boat at the level where it sits in the water. The LWL will be shorter than the length of the boat overall as most boats have bows and stern protrusions that make the LOA greater than the LWL. As a ship becomes more loaded, it will sit lower in the water and its ambient waterline length may change; but the registered L.W.L it is measured from a default load condition.

A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list, to provide a more even load distribution along the hull to reduce structural hogging or sagging stresses, or to increase draft, as in a semi-submersible vessel or platform, or a SWATH, to improve seakeeping. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels, and makes it easy for the crew to reduce a vessel's draft when it enters shallower water, by temporarily pumping out ballast. Airships use ballast tanks mainly to control buoyancy and correct trim.

Ship motions are defined by the six degrees of freedom that a ship, boat, or other watercraft, or indeed any conveyance, can experience.

A whaleback was a type of cargo steamship of unusual design, with a hull that continuously curved above the waterline from vertical to horizontal. When fully loaded, only the rounded portion of the hull could be seen above the waterline. With sides curved in towards the ends, it had a spoon bow and a very convex upper deck. It was formerly used on the Great Lakes of Canada and the United States, notably for carrying grain or ore. The sole surviving ship of the "whaleback" design is the SS Meteor, which is docked in Superior, Wisconsin, as a museum ship.

Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the metacenters of vessels, and on how these interact.

A sailing yacht, is a leisure craft that uses sails as its primary means of propulsion. A yacht may be a sail or power vessel used for pleasure, cruising, or racing. There is no standard definition, so the term applies here to sailing vessels that have a cabin with amenities that accommodate overnight use. To be termed a "yacht", as opposed to a "boat", such a vessel is likely to be at least 33 feet (10 m) in length and have been judged to have good aesthetic qualities. Sailboats that do not accommodate overnight use or are smaller than 30 feet (9.1 m) are not universally called yachts. Sailing yachts in excess of 130 feet (40 m) are generally considered to be superyachts.

Ship measurements consist of a multitude of terms and definitions specifically related to ships and measuring or defining their characteristics.

The term lambo or lamba refer to two types of traditional boats from Indonesia.

The Irwin 41 is an American sailboat that was designed by Ted Irwin as a cruiser and first built in 1982.

The Clipper 23, also called the Clipper Marine 23, is an American trailerable sailboat that was designed by William Crealock and first built in 1976.

This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water. Some remain current, while many date from the 17th to 19th centuries. The word nautical derives from the Latin nauticus, from Greek nautikos, from nautēs: "sailor", from naus: "ship".

The Seafarer 24 is an American trailerable sailboat that was designed by McCurdy & Rhodes as a cruiser and first built in 1974.

The Vivacity 20 is a British trailerable sailboat that was designed by Peter Stevenson and Des C. Pollard as a racer-cruiser and first built in 1963.

## References

1. "Glossary of Shipbuilding Terms S-Z". US Navy Naval History and Heritage Command. Archived from the original on 2017-04-29. Retrieved 2015-02-15.
2. "Draft Surveys: Methodology, Calculations, and common errors". 19 October 2020.
3. Derrett, Captain 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.
5. Arnesen, Mari Jevne; Gjestvang, Magnhild; Wang, Xin; Fagerholt, Kjetil; Thun, Kristian; Rakke, Jørgen G. (January 2017). "A traveling salesman problem with pickups and deliveries, time windows and draft limits: Case study from chemical shipping". Computers & Operations Research. 77: 20–31. doi:10.1016/j.cor.2016.07.017. ISSN   0305-0548.

## Bibliography

• 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.