Technically Speaking—How Water Ballast Works  
By; Ramblin’ Rod Brandon

First Published in Clipper Snips the magazine of the Trailer/Sailors Association Fall 2007 Pg 10

I remember as a young child, my father posing the question, “What weighs more, a pound of feathers or a pound of lead?” Of course the answer is, that they both weigh exactly the same. This is a critical premise to understanding how water ballast works.  

Some people believe that because water has neutral buoyancy in water, that water ballast would have to be lifted above the waterline, to have any effect on righting moment. This isn’t true. To understand why, one needs to know how righting moment is generated. 

Every vessel has a centre of gravity (CG) and a centre of buoyancy (CB). The centre of gravity is the axis on which the boat will tend to roll when heeled. The centre of buoyancy is the relative volumetric centre of the water displaced by the vessel. 

As the boat heels, CG remains stationary, whereas CB shifts laterally. The lateral distance between CG and CB is proportional to the righting moment GZ. The greater this distance, the stiffer the boat will be. 

Dense ballast such as lead has the benefit of lowering CG which helps extend GZ. Since water ballast must be carried higher in the vessel, raising the CG, more weight in water ballast is required to create the same GZ as lead.

Note how in Illustration 4, the water in the ballast tank is not raised above the waterline outside of the vessel, and yet contributes significantly to righting moment. Without the water ballast, the CG would be much higher still, resulting in less GZ. This is why a water ballasted boat is so much more “tipsy” (technical term totally unrelated to beer or rum ballast), before the water ballast is loaded. 

Water ballast must be raised above the water line to be effective? Myth busted!