Ballast is weight placed low in ships to lower their centre of gravity, which increases stability (more technically, to provide a righting moment to resist any heeling moment on the hull). Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel filling with water and/or 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 the cargo is loaded. If a cargo vessel (such as a tanker, bulk carrier or container ship) wishes to travel empty or partially empty to collect 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

ballast tank A ballast tank is a Compartment (ship), 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 co ...

s with sea water. Serious problems may arise when ballast water is discharged, as water-borne organisms can create havoc when deposited in new environments.

Uses

Ballast takes many forms.

Small sailing vessels

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 In geography and seamanship, windward () and leeward () are directions relative to the wind. Windward is ''upwind'' from the point of reference, i.e., towards the direction from which the wind is coming; leeward is ''downwind'' from the point ...

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

lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...

age 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.

Larger sailing vessels

On larger modern vessels, the

keel The keel is the bottom-most longitudinal structural element of a watercraft, important for stability. On some sailboats, it may have a fluid dynamics, hydrodynamic and counterbalancing purpose as well. The keel laying, laying of the keel is often ...

is made of or filled with a high

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

material, such as

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, or

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

. 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 mechanism to lift the massive

foil Foil may refer to: Materials * Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine * Metal leaf, a very thin sheet of decorative metal * Aluminium foil, a type of wrapping for food * Tin foil, metal foil ma ...

. 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.

Canting keel

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.

Water ballast

Sailing vessels

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 hullfresh 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

draft Draft, the draft, or draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a v ...

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 In geography and seamanship, windward () and leeward () are directions relative to the wind. Windward is ''upwind'' from the point of reference, i.e., towards the direction from which the wind is coming; leeward is ''downwind'' from the point o ...

ballast tank and fill the

tank as the boat tacks, and the quantity of ballast can be varied to keep the boat at the optimum angle of heel. 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.

Powered vessels

Tugboat Boss discharging ballast water before departure

On empty cargo vessels water is added to

s to increase propeller immersion, to improve steering, and to control trim and draft.

Environmental impacts and regulations

References

This article incorporates text from a

public domain The public domain (PD) consists of all the creative work to which no Exclusive exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly Waiver, waived, or may be inapplicable. Because no one holds ...

Congressional Research Service The Congressional Research Service (CRS) is a public policy research institute of the United States Congress. Operating within the Library of Congress, it works primarily and directly for members of Congress and their committees and staff on a ...

report: Copeland, Claudia
"Cruise Ship Pollution: Background, Laws and Regulations, and Key Issues"
(Order Code RL32450).

(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. {{Authority control Sailing Watercraft components