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Draft (hull)

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Draft marks on a ship's bow
Draft marks on a ship's bow
Load line mark and draft marks on the side of a ship
Load line mark and draft marks on the side of a ship

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.[1]

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Hull (watercraft)

Hull (watercraft)

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.

Waterline

Waterline

The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line, that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves that may arise. Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater with a similar lessening effect upon buoyancy.

Keel

Keel

The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. The laying of the keel is often the initial step in the construction of a ship. In the British and American shipbuilding traditions, this event marks the beginning date of a ships construction.

Air draft

Air draft

Air draft 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 the surface, but air draft is expressed as a height, not a depth.

Bunkering

Bunkering

Bunkering is the supplying of fuel for use by ships, including the logistics of loading and distributing the fuel among available shipboard tanks. A person dealing in trade of bunker (fuel) is called a bunker trader.

Archimedes' principle

Archimedes' principle

By Athrv Chandna (SAC) Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes' principle is a law of physics fundamental to fluid mechanics. It was formulated by Archimedes of Syracuse.

Of a ship

Metric bow scale
Metric bow scale
Imperial system in Roman numeration of the bow scale
Imperial system in Roman numeration of the bow scale
Imperial system used on the stern of the Cutty Sark
Imperial system used on the stern of the Cutty Sark
Graphical representation of the dimensions used to describe a ship. Dimension "d" is the draft.
Graphical representation of the dimensions used to describe a ship. Dimension "d" is the draft.
  • The draft aft (stern) is measured at the perpendicular of the stern.
  • The draft forward (bow) is measured at 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.[2]
  • The trim of a ship is the difference between the forward and aft drafts relative to the designed waterline. When the aft draft relative to the designed water line (DWL) is greater the vessel is deemed to have a positive trim, or to be trimmed by the stern, and it has a negative trim, or is trimmed by the bow, when the forward draft relative to DWL is the greater.[3] In such a case it may be 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.

Variations

The draft of a ship can be affected by multiple factors, besides the variations caused by changes in displacement:

  • Variation by trim
  • Variation by list
  • Variations in water density due to temperature and salinity
  • Variation as a result of a ship moving in shallow waters, or squat
  • Variation due to movable appendages, such as centreboards, daggerboards, drop keels, leeboards, and retractable rudders
  • Projection of non-retractable rudders, propellers or thrusters below the hull

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Cutty Sark

Cutty Sark

Cutty Sark is a British clipper ship. Built on the River Leven, Dumbarton, Scotland in 1869 for the Jock Willis Shipping Line, she was one of the last tea clippers to be built and one of the fastest, coming at the end of a long period of design development for this type of vessel, which halted as steamships took over their routes. She was named after the short shirt of the fictional witch in Robert Burns' poem Tam o' Shanter, first published in 1791.

Stern

Stern

The stern is the back 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 the bow, the foremost part of a ship. Originally, the term only referred to the aft port section of the ship, but eventually came to refer to the entire back of a vessel. The stern end of a ship is indicated with a white navigation light at night.

Length between perpendiculars

Length between perpendiculars

Length between perpendiculars is the length of a ship along the summer load line from the forward surface of the stem, or main bow perpendicular member, to the after surface of the sternpost, or main stern perpendicular member. When there is no sternpost, the centerline axis of the rudder stock is used as the aft end of the length between perpendiculars.

Maritime pilot

Maritime pilot

A maritime pilot, marine pilot, harbor pilot, port pilot, ship pilot, or simply pilot, is a mariner who maneuvers ships through dangerous or congested waters, such as harbors or river mouths. Maritime pilots are regarded as skilled professionals in navigation as they are required to know immense details of waterways such as depth, currents, and hazards, as well as displaying expertise in handling ships of all types and size. Obtaining the title 'maritime pilot' requires being an expert ship handler licensed or authorised by a recognised pilotage authority.

Squat effect

Squat effect

The squat effect is the hydrodynamic phenomenon by which a vessel moving through shallow water creates an area of reduced pressure that causes the ship to increase its draft and thereby be closer to the seabed than would otherwise be expected. This phenomenon is caused by the water flow which accelerates as it passes between the hull and the seabed in confined waters, the increase in water velocity causing a resultant reduction in pressure. Squat effect from a combination of vertical sinkage and a change of trim may cause the vessel to dip towards the stern or towards the bow. This is understood to be a function of the Block coefficient of the vessel concerned, finer lined vessels Cb <0.7 squatting by the stern and vessels with a Cb >0.7 squatting by the head or bow.

Draft scale

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.

Implications

Large ships

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.

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, therefore restriction on the maximum draft (the draft limit, a distance from the seabed or riverbed to the water level) is sometimes established (in particular, all ports set up draft limits).[4] 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.

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 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.

Submarines

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.

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Cargo

Cargo

Cargo consists of goods conveyed by water, air, or land. In economics, freight is cargo that is transported at a freight rate for commercial gain. Cargo was originally a shipload but now covers all types of freight, including transport by rail, van, truck, or intermodal container. The term cargo is also used in case of goods in the cold-chain, because the perishable inventory is always in transit towards a final end-use, even when it is held in cold storage or other similar climate-controlled facilities. The term freight is commonly used to describe the movements of flows of goods being transported by any mode of transportation.

Sailing ballast

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 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.

Ballast tank

Ballast tank

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.

Panamax

Panamax

Panamax and New Panamax are terms for the size limits for ships travelling through the Panama Canal. The limits and requirements are published by the Panama Canal Authority (ACP) in a publication titled "Vessel Requirements". These requirements also describe topics like exceptional dry seasonal limits, propulsion, communications, and detailed ship design.

Panama Canal

Panama Canal

The Panama Canal is an artificial 82 km (51 mi) waterway in Panama that connects the Atlantic Ocean with the Pacific Ocean and divides North and South America. The canal cuts across the Isthmus of Panama and is a conduit for maritime trade. One of the largest and most difficult engineering projects ever undertaken, the Panama Canal shortcut greatly reduces the time for ships to travel between the Atlantic and Pacific oceans, enabling them to avoid the lengthy, hazardous Cape Horn route around the southernmost tip of South America via the Drake Passage or Strait of Magellan and the even less popular route through the Arctic Archipelago and the Bering Strait.

Beam (nautical)

Beam (nautical)

The beam of a ship is its width at its widest point. The maximum beam (BMAX) is the distance between planes passing through the outer extremities of the ship, beam of the hull (BH) only includes permanently fixed parts of the hull, and beam at waterline (BWL) is the maximum width where the hull intersects the surface of the water.

Length overall

Length overall

Length overall is the maximum length of a vessel's hull measured parallel to the waterline. This length is important while docking the ship. It is the most commonly used way of expressing the size of a ship, and is also used for calculating the cost of a marina berth.

Suez Canal

Suez Canal

The Suez Canal is an artificial sea-level waterway in Egypt, connecting the Mediterranean Sea to the Red Sea through the Isthmus of Suez and dividing Africa and Asia. The 193.30 km (120.11 mi) long canal is a popular trade route between Europe and Asia.

Malaccamax

Malaccamax

Malaccamax is a naval architecture term for the largest tonnage of ship capable of fitting through the 25-metre-deep (82 ft) Strait of Malacca. Bulk carriers and supertankers have been built to this tonnage, and the term is chosen for very large crude carriers (VLCC). They can transport oil from Arabia to China. A typical Malaccamax tanker can have a maximum length of 333 m (1,093 ft), beam of 60 m (197 ft), draught of 20.5 m (67.3 ft), and tonnage of 300,000 DWT.

Strait of Malacca

Strait of Malacca

The Strait of Malacca is a narrow stretch of water, 500 mi long and from 40 to 155 mi wide, between the Malay Peninsula to the northeast and the Indonesian island of Sumatra to the southwest, connecting the Andaman Sea and the South China Sea. As the main shipping channel between the Indian and Pacific oceans, it is one of the most important shipping lanes in the world. It is named after the Malacca Sultanate that ruled over the strait between 1400 and 1511, the center of administration of which was located in the modern-day state of Malacca, Malaysia.

Capesize

Capesize

Capesize ships are the largest dry cargo ships with ball mark dimension: about 170,000 DWT capacity, 290 m long, 45 m beam (wide), 18m draught. They are too large to transit the Suez Canal or Panama Canal, and so have to pass either Cape Agulhas or Cape Horn to traverse between oceans.

Chinamax

Chinamax

Chinamax is a standard of ship measurements that allow conforming ships to use various harbours when fully laden, the maximum size of such a ship being 24 m (79 ft) draft, 65 m (213 ft) beam and 360 m (1,180 ft) length overall. An example of ships of this size is the Valemax bulk carriers.

Source: "Draft (hull)", Wikipedia, Wikimedia Foundation, (2022, October 19th), https://en.wikipedia.org/wiki/Draft_(hull).

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References
  1. ^ "Glossary of Shipbuilding Terms S-Z". US Navy Naval History and Heritage Command. Archived from the original on 2017-04-29. Retrieved 2015-02-15.
  2. ^ "Draft Surveys: Methodology, Calculations, and common errors". 19 October 2020.
  3. ^ Derrett, Captain D R (2006). "12". Ship Stability for Masters and Mates. Sixth: Elsevier. pp. 143–144.
  4. ^ Arnesen, Mari Jevne; Gjestvang, Magnhild; Wang, Xin; Fagerholt, Kjetil; Thun, Kristian; Rakke, Jørgen G. (January 2017). "A traveling salesman problem with pickups and deliveries, time windows and draft limits: Case study from chemical shipping". Computers & Operations Research. 77: 20–31. doi:10.1016/j.cor.2016.07.017. ISSN 0305-0548.
Bibliography
  • Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Prress. ISBN 0-87033-549-9.
  • Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87033-056-X.
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