Sailing ballast
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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. Too much heel may result in the boat capsizing.
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[edit] Uses
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 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.
[edit] High density ballast
There are disadvantages to using high density ballast. The first is the increased mass of the boat. A heavier craft accelerates slower than a lighter craft, and is generally less responsive to steering input. A heavier boat is also more difficult to put on a trailer and tow behind an automobile. 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 incapable of sailing in very shallow water, and more difficult to place on a trailer. While prohibited by most class racing rules, some cutting edge boats use a bulb of ballast on a long, thin keel that can cant 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 straight.
[edit] 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 seems illogical that placing water in the hull (which is, after all, the same density as the water outside the hull) would add any stability, in fact, adding water ballast below the vertical center of gravity increases stability. When the boat heels, it must then lift the ballast clear of the water, at which point it is obvious that it does provide righting moment. The advantage of water ballast is that it can be dumped out by having a valve at the bottom of the ballast chamber, reducing the weight of the boat for trailering, and then added back in by opening up the valves and letting the water flow in after the boat is launched. 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. The 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 velocity 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.
[edit] Ship Ballast
On cargo vessels water ballast is added to the ballast tanks when empty to increase propeller immersion, to improve steering, and to control trim and draft.
One disadvantage of water ballast in cargo vessels (which use ballast to maintain correct trim when not fully laden) is the dispersal of marine organisms, picked up with the water in one area and dumped with the ballast in another area. These alien organisms can then go on to cause significant damage in their new habitat and once established it is almost impossible to eliminate it. Many smaller sailing vessels use water ballast to save weight and allow easy trailering, and the ballast is loaded at launching and dumped when the boat is re-trailered, usually at the same location.
In the United States the National Invasive Species Act of 1996 is meant to control the invasion of aquatic nuisance species.
