Anchor pendant and support buoy sizes are normally expressed in terms of
the reserve buoyancy rather than the physical dimensions. The reserve
buoyancy is equal to the weight of water displaced by the buoy when fully
immersed less the buoy's weight in air. A buoy with 6 Tonnes reserve
buoyancy would be fully immersed if required to support 6 Tonne. In
practice, as the buoy would be fully submerged it would be very difficult to
locate. Therefore standard practice is to use only about half the reserve
buoyancy of the buoy so that approximately half the buoy is showing above
the water.
                                
        Subsurface Buoy                                                      Surface Buoy

The above does not apply to mooring buoys as they need to resist being
pulled under the water when high loads are being applied to the mooring
hawsers. Mooring analysis is generally required to ensure that buoys used in
the mooring of vessels are of the correct size and that the structural steel
within through the buoy is of a sufficient strength.

Navigational buoys are selected on the basis of sea conditions and signal
range requirements. Their buoyancy is required to support the moorings
and give sufficient freeboard and height for the signals (light and day-marks
etc) to be effective. Navigational buoys are also ballasted to aid stability.