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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 takes many forms. The simplest form of ballast used in small day sailers is so-called "live ballast", or the weight of the crew. By sitting on the windward side of the hull, the heeling moment must lift the weight of the crew. On more advanced racing boats, a wire harness called a trapeze is used to allow the crew to hang completely over the side of the hull without falling out; this provides much larger amounts of righting moment due to the larger leverage of the crew's weight, but can be dangerous if the wind suddenly dies, as the sudden loss of heeling moment can dump the crew in the water. On larger modern vessels, the keel is made of or filled with a high density material, such as concrete, iron, or lead. By placing the weight as low as possible (often in a large bulb at the bottom of the keel) the maximum righting moment can be extracted from the given mass. Traditional forms of ballast carried inside the hull were stones or sand.
There are disadvantages to using high-density ballast. The first is the increased mass of the boat; a heavier boat sits lower in the water, increasing drag when it moves, and is generally less responsive to steering. A heavier boat is also more difficult to put on a trailer and tow behind an automobile. Secondly, since the ballast needs to be as low as possible, it is often placed into a centerboard or retracting keel, requiring a heavy-duty crank to lift the massive foil. The simplest solution is to use a fixed ballasted keel, but that makes the boat nearly incapable of sailing in very shallow water, and more difficult to handle when out of the water. While prohibited by most class racing rules, some cutting-edge boats use a bulb of ballast on a long, thin keel that can tilt from side to side to create a canting keel. This lets the ballast be placed on the windward side, providing a far greater righting moment with a lower angle of heel. Tilting the keel, however, greatly reduces its lift, so canting keels are usually combined with a retractable centerboard or daggerboard that is deployed when the keel is tilted, and retracted (to reduce drag) when the keel is returned to the vertical. Some canting keels are designed so that when fully extended to either side they have an angle of attack of about 5° allowing the hydrofoil effect of the blade to lift the boat up and reduce wetted surface area for an increase in boat speed.
A common type of ballast for small boats that avoids many of the problems of high-density ballast is water ballast. While it seems counter-intuitive that placing water in the hull (which is, after all, close to the same density as the water outside the hull –fresh vs salt water) would add any stability, adding water ballast below the vertical center of gravity increases stability. The water ballast does not need to be lifted above the waterline to affect stability, as any material having greater bulk density than air will have an effect on the centre of gravity. It is the relationship between centre of gravity and centre of buoyancy that dictates the righting moment.
The advantage of water ballast is that the tanks can be emptied, reducing draft or the weight of the boat (e.g. for transport on ground) and water added back in (in small boats, simply by opening up the valves and letting the water flow in) after the boat is launched or cargo unloaded. Pumps can also be used to empty the leeward ballast tank and fill the windward tank as the boat tacks, and the quantity of ballast can be varied to keep the boat at the optimum angle of heel. On empty cargo vessels water is added to ballast tanks to increase propeller immersion, to improve steering, and to control trim and draft.
A disadvantage of water ballast is that water is not very dense and therefore the tanks required take up more space than other forms of ballast. Some manufacturers offer flexible ballast bags that are mounted outboard of the hull on both sides, and pumps that use the boat's speed through the water for power. When under way, the pump can be used to fill the windward side, while the lee side is allowed to drain. This system, while not very attractive, does allow significant gains in righting force with no modifications to the hull.
A trick commonly used on boats with water ballast is to link port and starboard tanks with a valved pipe. When preparing to tack, the valve is opened, and water in the windward tank, which is higher, is allowed to flow to the lee side, and the sheet is let off to keep the boat from heeling too far. Once as much water as possible has been transferred to the lee side, the boat is brought about and the sail sheeted in, lifting the newly full windward tank. A simple hand pump can then be used to move any remaining water from the lee to the windward tank.
Cruise ships, large tankers, and bulk cargo carriers use a tremendous amount of ballast water, which is often taken on in the coastal waters in one region after ships discharge wastewater or unload cargo, and discharged at the next port of call, wherever more cargo is loaded. Ballast water discharge typically contains a variety of biological materials, including plants, animals, viruses, and other microorganisms. These materials often include non-native, nuisance, exotic species that can cause extensive ecological and economic damage to aquatic ecosystems. Ballast water discharges are believed to be the leading source of invasive species in U.S. marine waters, thus posing public health and environmental risks, as well as significant economic cost to industries such as water and power utilities, commercial and recreational fisheries, agriculture, and tourism. [1] A recent study suggests that if no action is taken on ballast water management, species invasion can propagate to any port in the world via global shipping network with an average of two intermediate stops. [2]
Meanwhile, studies suggest that the economic cost just from introduction of pest mollusks (zebra mussels, the Asian clam, and others) to U.S. aquatic ecosystems is more than $6 billion per year. [3]
In case of a bulk cargo ship, there is another environmental effect of ballast water. After unloading the payload a bulk carrier cannot simply return to the starting point, but it must load ballast to get the propeller submerged below water surface. The weight of the ballast increases fuel consumption compared to a hypothetical situation that the ship did not need ballast. [4]
A June 2011 National Research Council (United States) study provided advice on the process of setting regulatory limits. The study found that determining the exact number of organisms that could be expected to launch a new population is complex. It suggested an initial step of establishing a benchmark for the concentrations of organisms in ballast water below current levels, and then using models to analyze experimental and field-based data to help inform future decisions about ballast water discharge standards. [5]
To minimize the spread of invasive species in U.S. waterways, the Environmental Protection Agency and the U.S. Coast Guard regulate the concentration of living organisms discharged in the ballast water of ships. [6] [7]
A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying goods or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished from boats, based on size, shape, load capacity, and purpose. In the Age of Sail a "ship" was a sailing vessel defined by its sail plan of at least three square rigged masts and a full bowsprit.
Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the water, on ice (iceboat) or on land over a chosen course, which is often part of a larger plan of navigation.
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. As the laying down of the keel is the initial step in the construction of a ship, in British and American shipbuilding traditions the construction is dated from this event.
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 stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.
A centreboard or centerboard (US) is a retractable keel which pivots out of a slot in the hull of a sailboat, known as a centreboard trunk (UK) or centerboard case (US). The retractability allows the centreboard to be raised to operate in shallow waters, to move the centre of lateral resistance, to reduce drag when the full area of the centreboard is not needed, or when removing the boat from the water, as when trailering. A centreboard which consists of solely a pivoting metal plate is called a centerplate. A daggerboard is similar but slides vertically rather than pivoting.
A monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.
Capsizing or keeling over occurs when a boat or ship is turned on its side or it is upside down in the water. The act of reversing a capsized vessel is called righting.
A bulk carrier,bulker is a merchant ship specially designed to transport unpackaged bulk cargo, such as grains, coal, ore, steel coils and cement, in its cargo holds. Since the first specialized bulk carrier was built in 1852, economic forces have led to continued development of these ships, resulting in increased size and sophistication. Today's bulk carriers are specially designed to maximize capacity, safety, efficiency, and durability.
A leeboard is a form of pivoting keel used by a sailboat in lieu of a fixed keel. Typically mounted in pairs on each side of a hull, leeboards function much like a centreboard, allowing shallow draft craft to ply waters fixed keel boats cannot. Only one, however, the leeward, is used at a time, as it does not get lifted from the water when the boat heels under the force of the wind.
A dry bulk cargo barge is a barge designed to carry freight such as coal, finished steel or its ingredients, grain, sand or gravel, or similar materials. Barges are usually constructed of steel. They have an outer hull, an internal void that is fitted with heavy struts and cross braces or scantlings, and an internal cargo box. The outer hull of a barge can come in one of two configurations. A rake barge has a curved bow to provide less resistance when being pushed and is usually placed at the head of the tow. A box barge is usually placed in the center and rear of the tow and can hold more cargo.
The winged keel is a sailboat keel layout first fitted on the 12-metre class yacht Australia II, 1983 America's Cup winner.
A canting keel is a form of sailing ballast, suspended from a rigid canting strut beneath the boat, which can be swung to windward of a boat under sail, in order to counteract the heeling force of the sail. The canting keel must be able to pivot to either port or starboard, depending on the current tack.
A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide stability for a vessel, to reduce buoyancy, as in a submarine, 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. It also makes it easy for the crew to reduce a vessel's draft when they enter shallower water, by temporarily pumping out ballast. Airships use ballast tanks for similar reasons.
Twin keels or bilge keels are two keels that emerge at an angle from the hull of a sailboat, at or near the bilge. The angle allows the boat to have a shallower draft while still allowing for minimum leeway while sailing. The placement of the twin keels also allows the boat to stand upright when out of the water without additional support, as opposed to a single-keeled boat that would fall over if water levels dropped. Twin-keeled boats are typically used in coastal areas that experience extreme changes in tide. When the tide is low, the boat will sit on her keels and remain stable and upright. This configuration is especially useful for sailors in Britain and might in the future be applied in the parts of the Atlantic North America that are extremely tidal such as the Fundy waters that are shared by Maine, New Brunswick, and Nova Scotia.
The draft or draught of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel). Draft determines the minimum depth of water a ship or boat can safely navigate.
Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward in order to counteract the effect.
Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the metacenters of vessels, and on how these interact.
An LNG carrier is a tank ship designed for transporting liquefied natural gas (LNG).
Ballast water discharges by ships can have a negative impact on the marine environment. The discharge of ballast water and sediments by ships is governed globally under the Ballast Water Management Convention, since its entry into force in September 2017. It is also controlled through national regulations, which may be separate from the Convention, such as in the United States.
Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship, submarine, or other floating structure that holds water is called a ballast tank. Water should move in and out from the ballast tank to balance the ship. In a vessel that travels on the water, the ballast will remain below the water level, to counteract the effects of weight above the water level. The ballast may be redistributed in the vessel or disposed of altogether to change its effects on the movement of the vessel.
 | Wikisource has the text of the 1911 Encyclopædia Britannica article " Ballast ". |
This article incorporates text from a public domain Congressional Research Service report: Copeland, Claudia. "Cruise Ship Pollution: Background, Laws and Regulations, and Key Issues" (Order Code RL32450). Congressional Research Service (Updated February 6, 2008).
|journal= (help)The unloaded ship must take ballast water in tanks and in the cargo holds to get the propeller submerged below water surface. This later on causes costs for cleaning and in spite of that, the ship offers a greater resistance than before and will probably need more bunker than for her first destination.
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