Ship ballast
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If a cargo vessel (such as a tanker, bulk carrier or container ship) wishes to travel empty or partially empty to collect a cargo, it must travel "in ballast". This keeps the vessel in trim and keeps the propeller and rudder submerged. Typically, being "in ballast" will mean flooding the ballast tanks with sea water. Serious problems arise when the ballast water is discharged, as water-borne organisms may create havoc when deposited in new environments.
Uses
Ballast takes many forms. The simplest form of ballast used in small
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.
High-density ballast
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
Water ballast
A common type of ballast for small boats that avoids many of the problems of high-density ballast is water ballast. While it may seem 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, the water serves to displace air from the bottom of the hull; 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
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.
Environmental impacts and regulations
See also
- Ballast
- Environmental issues with shipping
References
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).
External links
- Globallast partnership (IMO)
- Profile - Ballast Water – National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for ballast water.