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.
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.
The closely related term "trim" is defined as the difference between the forward and aft drafts.
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.
The draft of a ship can be affected by multiple factors, besides the variations caused by changes in displacement:
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 ]
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.
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. 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.
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.
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.
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 ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying goods or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished from boats, based on size, shape, load capacity, and purpose. In the Age of Sail a "ship" was a sailing vessel defined by its sail plan of at least three square rigged masts and a full bowsprit.
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.
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.
This is a glossary of nautical terms; some remain current, while many date from the 17th to 19th centuries. See also Wiktionary's nautical terms, Category:Nautical terms, and Nautical metaphors in English. See the Further reading section for additional words and references.
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 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 any other craft can experience.
The Newport 16 is an American trailerable sailboat that was designed by Bill Lapworth as a daysailer and a pocket cruiser and first built in 1965.
Twin keels or bilge keels are two keels that emerge at an angle from the hull of a sailboat, at or near the bilge. The angle allows the boat to have a shallower draft while still allowing for minimum leeway while sailing. The placement of the twin keels also allows the boat to stand upright when out of the water without additional support, as opposed to a single-keeled boat that would fall over if water levels dropped. Twin-keeled boats are typically used in coastal areas that experience extreme changes in tide. When the tide is low, the boat will sit on her keels and remain stable and upright. This configuration is especially useful for sailors in Britain and might in the future be applied in the parts of the Atlantic North America that are extremely tidal such as the Fundy waters that are shared by Maine, New Brunswick, and Nova Scotia.
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 Capri 26 is an American trailerable sailboat that was designed by Frank W. Butler and Gerry Douglas as a cruiser-racer and first built in 1990.
The Columbia T-23, or Columbia T23, is an American sailboat that was designed by Australian Alan Payne and first built in 1973. The "T" designation indicates that the boat is designed to be trailerable.
The Com-Pac 23 is an American trailerable sailboat that was designed by Clark Mills as a pocket cruiser and first built in 1978. The boat has undergone design changes over time resulting in a series of improved models.
The ETAP 21i is a Belgian trailerable sailboat that was designed by Mortain & Mavrikios as a cruiser and first built in 1997.
The Irwin 25 is an American trailerable sailboat that was designed by Ted Irwin as a cruiser and first built in 1969.
The Pacific Seacraft 25 is an American trailerable sailboat that was designed by Henry Mohrschladt as a cruiser and first built in 1976.
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