Ship ballast

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.

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.

This lets the ballast be placed on the windward side, providing a far greater righting moment with a lower angle of heel.

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.

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 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.

On empty cargo vessels water is added to ballast tanks to increase propeller immersion, to improve steering, and to control trim and draft.