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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), 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.

Contents

Drafts of a ship

Draft mark at the bow
Metric bow scale
English system in Roman numeration of the bow scale. Notation in Spanish pulgada=inch, pies = feet
graphical representation of the dimensions used to describe a ship. Dimension "d" is the draft.
  • The draft aft (stern) is measured in the perpendicular of the stern.
  • The draft forward (bow (ship)) 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.
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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), 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 ballasts 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 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 with a draft of 1.20 m for a length of 8.90 m.

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

See also

References

  • Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0-87033-549-9. 
  • Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87038-056-X. 

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