Draft (hull)
Overview
 
The draft of a ship's hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

 is the vertical distance between the waterline
Waterline
The term "waterline" generally refers to the line where the hull of a ship meets the water surface. It is also the name of a special marking, also known as the national Load Line or Plimsoll Line, to be positioned amidships, that indicates the draft of the ship and the legal limit to which a ship...

 and the bottom of the hull (keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

), with the thickness of the hull included; in the case of not being included the draft outline would be obtained. Draft determines the minimum depth of water a ship or boat can safely navigate.
The draft can also be used to determine the weight of the cargo on board by calculating the total displacement of water and then using Archimedes' principle
Archimedes' principle
Archimedes' principle relates buoyancy to displacement. It is named after its discoverer, Archimedes of Syracuse.-Principle:Archimedes' treatise On floating bodies, proposition 5, states that...

.
Encyclopedia
The draft of a ship's hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

 is the vertical distance between the waterline
Waterline
The term "waterline" generally refers to the line where the hull of a ship meets the water surface. It is also the name of a special marking, also known as the national Load Line or Plimsoll Line, to be positioned amidships, that indicates the draft of the ship and the legal limit to which a ship...

 and the bottom of the hull (keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

), with the thickness of the hull included; in the case of not being included the draft outline would be obtained. Draft determines the minimum depth of water a ship or boat can safely navigate.
The draft can also be used to determine the weight of the cargo on board by calculating the total displacement of water and then using Archimedes' principle
Archimedes' principle
Archimedes' principle relates buoyancy to displacement. It is named after its discoverer, Archimedes of Syracuse.-Principle:Archimedes' treatise On floating bodies, proposition 5, states that...

. A table made by the shipyard shows the water displacement for each draft. The density of the water (salt or fresh) and the content of the ship's bunkers has to be taken into account. The closely related term "trim" is defined as the difference between the forward and after drafts.

Drafts of a ship

  • The draft aft (stern
    Stern
    The stern is the rear or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite of the bow, the foremost part of a ship. Originally, the term only referred to the aft port section...

    ) is measured in the perpendicular of the stern.
  • The draft forward (bow
    Bow (ship)
    The bow is a nautical term that refers to the forward part of the hull of a ship or boat, the point that is most forward when the vessel is underway. Both of the adjectives fore and forward mean towards the bow...

    ) 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 F with respect to the average perpendicular.

Variations of the draft

The draft of a ship can be affected by multiple factors, not considering the rise and fall of the ship by displacement:
  • Draft variation by list
  • Draft variation by water level
    Sea level
    Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

     change
  • Allowance of fresh water draft variation by passage from fresh to sea water or vice versa
  • Heat variation in navigating shallow waters

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 English units or metric units. If the English 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 and each mark is one decimeter high.

Large ships

Larger ships try to maintain an average water draft when they are light (without cargo
Cargo
Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...

), in order to make a better sea crossing and reduce the effects of the wind (high centre of velic force). In order to achieve this they use sailing ballast
Sailing ballast
Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...

s to stabilize the ship, following the unloading of cargo.

The water draft of a large ship has little direct link with its stability, the latter depends solely on the respective positions of the metacenter of the hull and the centre 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 a strong or on the contrary, a weak stability, depending upon the manner by which the ship is loaded (height of the centre of gravity).

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

Draft is a significant factor limiting navigable waterways, especially for large vessels. Of course this includes many shallow coastal waters and reefs, but also some major shipping lanes. Panamax
Panamax
Panamax and New Panamax are popular terms for the size limits for ships traveling through the Panama Canal. Formally, the limits and requirements are published by the Panama Canal Authority titled "Vessel Requirements"...

 class ships—the largest ships able to transit the Panama Canal
Panama Canal
The Panama Canal is a ship canal in Panama that joins the Atlantic Ocean and the Pacific Ocean and is a key conduit for international maritime trade. Built from 1904 to 1914, the canal has seen annual traffic rise from about 1,000 ships early on to 14,702 vessels measuring a total of 309.6...

—do have a draft limit (and an "air draft" limit for passing under bridges) but are usually limited by beam
Beam (nautical)
The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...

, or sometimes length overall, for fitting into lock
Lock (water transport)
A lock is a device for raising and lowering boats between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water level can be varied; whereas in a caisson lock, a boat lift, or on a canal inclined plane, it is...

s. However in the much wider Suez Canal
Suez Canal
The Suez Canal , also known by the nickname "The Highway to India", is an artificial sea-level waterway in Egypt, connecting the Mediterranean Sea and the Red Sea. Opened in November 1869 after 10 years of construction work, it allows water transportation between Europe and Asia without navigation...

, the limiting factor for Suezmax
Suezmax
Suezmax is a naval architecture term for the largest ship measurements capable of transiting the Suez Canal, and is almost exclusively used in reference to tankers. Since the canal has no locks, the only serious limiting factors are draft , and height due to the Suez Canal Bridge...

 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
Malaccamax
Malaccamax is a naval architecture term for the largest size of ship capable of fitting through the -deep Strait of Malacca. Because the Sunda Strait is even shallower at minimum depth, a post-Malaccamax ship would need to use even longer alternate routes such as:*Lombok Strait, Makassar Strait,...

 ship has the deepest draft able to transit the very busy but relatively shallow Strait of Malacca
Strait of Malacca
The Strait of Malacca is a narrow, stretch of water between the Malay Peninsula and the Indonesian island of Sumatra. It is named after the Malacca Sultanate that ruled over the archipelago between 1414 to 1511.-Extent:...

. There are only a few ships of this size.

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 ensures a good level of stability in strong wind, as the centre 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
Dragon boat
A dragon boat is a human-powered watercraft traditionally made, in the Pearl River delta region of southern China - Guangdong Province, of teak wood to various designs and sizes. In other parts of China different woods are used to build these traditional watercraft...

 with a draft of 1.20 m for a length of 8.90 m.

A boat like a catamaran
Catamaran
A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

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

Submarines

For submarines, which can submerge to different depths at sea, a term called keel depth
Keel depth
Keel depth is the depth of water from the water surface to the keel of a submarine, its deepest part. The keel establishes a commonly defined reference point to measure to....

is used, 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 bottom, 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.
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