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Draft (hull)
 
 
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The draft (or draught) of a ship's hull
Hull (watercraft)

A hull is the watertight body of a ship or boat. It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking....
 is the vertical distance between the waterline
Waterline

The waterline is an imaginary line marking the level at which a ship or boat floats in the water. To an observer on the ship the water appears to rise or fall against the Hull ....
 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, the construction is dated from this event, with only the ship's Ship_naming_and_launching considered more significant in its creati...
), 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.






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Encyclopedia


The draft (or draught) of a ship's hull
Hull (watercraft)

A hull is the watertight body of a ship or boat. It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking....
 is the vertical distance between the waterline
Waterline

The waterline is an imaginary line marking the level at which a ship or boat floats in the water. To an observer on the ship the water appears to rise or fall against the Hull ....
 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, the construction is dated from this event, with only the ship's Ship_naming_and_launching considered more significant in its creati...
), 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. 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.

Drafts of a ship

  • The draft aft (stern
    Stern

    The stern is the rear or aft part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter to the taffrail....
    ) is measured in the perpendicular of the stern.
  • The draft forward (bow (ship)
    Bow (ship)

    The bow is a List of nautical terms 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....
    ) 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 change
  • Allowance of fresh water draft variation by passage from fresh to sea water or vice versa
  • Heat variation in navigating shallow waters


The 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.

The implications of draft


Large ships

Larger ships try to maintain an average water draft when they are light (without cargo
Cargo

Cargo refers to goods or produce transported, generally for Commerce gain, by Cargo ship, Cargo airline, Train#Freight trains, van or truck. In modern times, containers are used in most intermodal freight transport long-haul cargo transport....
), 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....
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 action of the stern settling deeper when power is applied).

Pleasure boats

A small draft allows pleasure boats to navigate through the 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, the centre of gravity is lower (ballast over the keel of the boat). For example: The ballasts placed very low in the keel in a boat such as a dragon boat
Dragon boat

A dragon boat or "dragonboat" is a very long and narrow, canoe style human-powered transport boat now used in the team paddling team sport of dragon boat racing which originated in China over 2000 years ago....
 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 hull s, or Vaka s, joined by some structure, the most basic being a frame, formed of Aka s....
 can mitigate the problem by retrieving good stability in a small draft, but the width of the boat increases.

See also

  • Hull (watercraft)
    Hull (watercraft)

    A hull is the watertight body of a ship or boat. It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking....
  • Naval Architecture
    Naval architecture

    Naval architecture is an engineering discipline dealing with the design, construction and repair of marine vehicles.Naval architecture involves basic and applied research, design, development, design evaluation and calculations during all stages of the life of a marine vehicle....
  • Ship building
  • Waterline
    Waterline

    The waterline is an imaginary line marking the level at which a ship or boat floats in the water. To an observer on the ship the water appears to rise or fall against the Hull ....