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The draft or draught of a ship's
hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affin ...
is the vertical distance between the
waterline The waterline is the line where the hull of a ship meets the surface of the water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substa ...
and the bottom of the hull (
keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a Fluid dynamics, hydrodynamic and counterbalancing purpose, as well. As the keel laying, laying down of the keel is the initial step in the c ...

keel
). Draft determines the minimum depth of water a ship or boat can safely navigate. 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 A hazard is an area of a golf course A golf course is the grounds where the sport of golf Golf is a club-and-ball sport Sport pertains to any form of Competition, competitive physical activity or game that aims to use, maintain ...
, and using
Archimedes' principle Archimedes' principle states that the upward buoyant force . Buoyancy (), or upthrust, is an upward force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''p ...
. The closely related term "trim" is defined as the difference between the forward and aft drafts.


Of a ship

* The draft aft (
stern The stern is the back or aft Aft :''For the acronym, see AFT (disambiguation).'' Aft, in naval A navy, naval force, or maritime force is the branch of a Nation's armed forces A military, also known collectively as armed forces, ...

stern
) is measured in the perpendicular of the stern. * The draft forward (
bow Bow often refers to: * Bow and arrow The bow and arrow is a ranged weapon A ranged weapon is any weapon A weapon, arm or armament is any implement or device that can be used with the intent to inflict physical damage or harm. Weapons ar ...
) is measured in 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) with respect to the average perpendicular. * The trim of a ship is the difference between the forward and aft draft. When the aft draft is greater the vessel is deemed to have a positive trim, and it has a negative trim when the forward draft is the greater. In such a case it is often 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 An aircraft pilot or aviator is a person who controls the flight of an aircraft An aircraft is a vehicle or machine that is able to fly Flies are insect Insects or Insecta (from Latin Latin (, or , ) is a classical lan ...
.


Variations

The draft of a ship can be affected by multiple factors, not considering the rise and fall of the ship by displacement: * Variation by trim * Variation by
list A ''list'' is any set of items. List or lists may also refer to: People * List (surname)List or Liste is a European surname. Notable people with the surname include: List * Friedrich List (1789–1846), German economist * Garrett List (194 ...
* Variation by
water level Water level, also known as gauge height or stage, is the elevation of the free surface of a sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which ...

water level
change * Allowance of fresh water draft variation by passage from fresh to sea water or vice versa * Heat variation in navigating shallow waters * Variation as a result of a ship moving in shallow waters, or squat


Draft scale

The drafts are measured with a "banded" scale, from bow and to stern, and for some ships, the average perpendicular measurement is also used. The scale may use traditional
Imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units A system of measurement is a collection of units of measurement A unit of measureme ...
or
metric units Metric units are units Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece ...

metric units
. If the Imperial system is used, the bottom of each marking is the draft in feet and markings are 6 inches high. In metric marking schemes, the bottom of each draft mark is the draft in
decimeters The decimetre (SI symbol dm) or decimeter (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English ort ...
and each mark is one decimeter high.


Implications


Large ships

Larger ships try to maintain an average water draft when they are light (without
cargo In economics, the word cargo refers in particular to goods or produce being conveyed—generally for Commerce, commercial gain—by water, air or land. "Freight" is the money paid to carry cargo. ''Cargo'' was originally a shipload. Cargo ...

cargo
) in order to make a better sea crossing and reduce the effects of the wind (high center of velic force). In order to achieve this they use
sailing ballast 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 boat/ship capsizing. If a sailing vessel should nee ...
s to stabilize the ship, following the unloading of cargo. The water draft of a large ship has little direct link with its stability because stability depends solely on the respective positions of the
metacenter 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 stab ...
of the hull and the center of gravity. It is also true, however, that a "light" ship has quite high stability which can lead to implying too much rolling of the ship (due to memory). A fully laden ship (with a large draft) can have either a strong or weak stability, depending upon the manner by which the ship is loaded (height of the
center of gravity In physics, the center of mass of a distribution of mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a measure (mathematics), measure of the body's ''inertia'', the resistance to acceleration (change ...

center of gravity
). The draft of ships can be increased when the ship is in motion in shallow water, a phenomenon known as squat (nautical term for the hydrodynamic effect of lower pressure pulling the ship down as it moves).


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.
Panamax Panamax and New Panamax (or Neopanamax) are terms for the size limits for ships travelling through the Panama Canal The Panama Canal ( es, Canal de Panamá, link=no) is an artificial waterway in Panama Panama ( , ; es, link= ...
class ships—the largest ships able to transit the
Panama Canal The Panama Canal ( es, Canal de Panamá, link=no) is an artificial waterway in Panama Panama ( , ; es, link=no, Panamá ), officially the Republic of Panama ( es, República de Panamá), is a List of transcontinental countries#Nor ...

Panama Canal
—do have a draft limit (and an "air draft" limit for passing under bridges) but are usually limited by
beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially localized group ...
, or sometimes
length overall __NOTOC__ Length overall (LOA, o/a, o.a. or oa) is the maximum length of a vessel's hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull ( ...
, for fitting into
lock Lock(s) may refer to: Common meanings *, a mechanical device used to secure items of importance *, a device for boats to transit between different levels of water, as in a canal Arts and entertainment *, a 2016 Punjabi film *, a sentient machin ...
s. However, ships can be longer, wider and higher in the
Suez Canal The Suez Canal ( ar, قَنَاةُ ٱلسُّوَيْسِ, ') is an artificial sea-level Mean sea level (MSL) (often shortened to sea level) is an mean, average level of the surface of one or more of Earth's bodies of water from which hei ...
, the limiting factor for
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ships is draft. Some
supertanker An oil tanker, also known as a petroleum tanker, is a ship A ship is a large watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any p ...
s 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 Malaccamax is a naval architecture '' in Tórshavn, Faroe Islands, August 2009 File:Naval Architecture Course of Study.png, alt=Curriculum of Naval Architecture , 256x256px, General Course of Study leading to Naval Architecture degree Naval ar ...
ship, is the deepest draft able to transit the very busy but relatively shallow
Strait of Malacca id, Selat Malaka th, ta, hi, zh, , native_name_lang = , other_name = , image = Strait of malacca.jpg , alt = , caption = The Strait of Malacca connects the Pacific Ocean to ...
. The Strait only allows ships to have more draft than the Suez Canal.
Capesize MV ''Berge Athene'', a capesize bulk carrier of 225,200 DWT, built in 1979. Capesize ships are the largest dry cargo ship 300px, The ''Colombo Express'', one of the largest container ships in the world (when it was built in 2005), owned and o ...
, Ultra Large Crude Carriers and a few
Chinamax Chinamax is a standard of ship A ship is a large watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered ...
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 boat Boating is the leisurely activity of travelling by boat A boat is a watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical ...
s 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'' ensures a good level of stability in strong wind, as the center of gravity is lower (ballast over the keel of the boat). For example: Ballasts placed very low in the keel of a boat such as a
dragon boat A dragon boat is a human-powered watercraft Human-powered watercraft are watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles used in water, including boats, ship A ship is a large watercraft that tra ...

dragon boat
with a draft of 1.20 m for a length of 8.90 m. A boat like a
catamaran A Formula 16 beachable catamaran Powered catamaran passenger ferry at Salem, Massachusetts, United States A catamaran () (informally, a "cat") is a multi-hulled watercraft Watercraft, also known as water vessels or waterborne vessels, a ...

catamaran
can mitigate the problem by retrieving good stability in a small draft, but the width of the boat increases.


Submarines

A term called keel depth is used for
submarines A submarine (or sub) is a watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functional p ...

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

*
Air draft Air draft (or air draught) is the distance from the surface of the water to the highest point on a vessel. This is similar to the " deep draft" of a vessel which is measured from the surface of the water to the deepest part of the hull below th ...
*
Hull (watercraft) A hull is the watertight Waterproofing is the process of making an object or structure waterproof or water-resistant so that it remains relatively unaffected by water or resisting the ingress of water under specified conditions. Such items m ...
*
Naval architecture Naval architecture, or naval engineering, is an engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The di ...
*
Waterline The waterline is the line where the hull of a ship meets the surface of the water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substa ...


References


Bibliography

* *


External links

{{Ship measurements Nautical terminology Navigation Ship measurements