Ballasting your vessel correctly is more than simply shifting water between ballast tanks. When well and properly ballasted, your vessel will handle properly in all seas. If your vessel is ballasted poorly, the end result can be devastating. The formula for determining how much ballast to take on without hampering performance, based on the vessel's static condition (not underway), depends on knowing the vessel's metacentric height, the current state of her ballasting and her list when in that static condition, if any.
Measure the amount of ballast water you have on board. Convert the number of gallons to pounds; if you have 100 gal. of fresh water aboard, 1 gal. of fresh water weighs 8.43lbs., so multiply 100 by 8.43.
Multiply the weight of the water by the metacentric height of your boat. If your boat's metacentric height (the distance between the centre of gravity and the metacentre, the point at which a vertical line from the centre of buoyancy during heel intersects a line from the keel through the original centre of buoyancy when the vessel was upright) is 7 feet, multiply 843 by 7.
Multiply the result by 0.1746. The answer, 1030.3 is how much ballast you will need to have aboard.
Subtract 843 from 1030.3. The answer, 187.7, is how many pounds of ballast must be added to correct a 1 degree list. If the list is greater than 1 degree--say 8 degrees-- then multiply the answer by the number of degrees. So, 187.7 multiplied by 8 equals 1,2261kg. of additional water would be needed to correct an 8 degree list.
This formula does not account for live loads (weight not permanently attached to the vessel, like cargo or people) aboard the vessel.
Caution should be exercised during ballasting to ensure that the vessel's hull is not placed under an undue stress because of weight, lest the vessel break or be overballasted. Your vessel's ballasting software, if equipped, provides the necessary information. If your vessel is not equipped with ballasting software, your boat's owner's manual will contain basic loading information. Overballasting can result in a stiff vessel, that handles poorly because of excessive draft; underballasting can result in a tender vessel, which rolls excessively in seas.