Appendix 1 - Hydrostatic conditions of flotation

Summary

For a vessel floating on the surface, the hydrostatic properties of relevance are the conditions of flotation and the stability of the vessel in relation to disturbances from the static flotation condition.

It is assumed that the water surface is calm and any movement of the craft is sufficiently slow for any dynamic effects to be discounted.

Accepting Archimedes principle that a vessel will displace its own weight of water, the initial requirement for flotation is that the intact hull is of sufficient volume to displace its own weight whilst having a reasonable freeboard (height of weather deck above the waterline) and that it floats upright. The volume of watertight hull above the waterline constitutes a Reserve of Buoyancy (ROB). This ROB becomes important when considering the safety of the vessel in damaged conditions when water floods part of the hitherto intact displacement volume below the waterline.

The stability of the vessel concerns the outcome of perturbations from the static flotation condition. Sideways motion (sway), change of heading (yaw) and fore or aft motion (surge) do not change the static conditions of the hull and can be considered as neutral. Whereas roll motion (heel), pitching and vertical motion (heave) result in changes in the distribution of buoyant volume and hence variation in the static equilibrium condition. The question to be answered is whether after a disturbance the vessel returns to its initial equilibrium state.

If a vessel heaves upwards then the displacement volume reduces and the excess of unchanged weight over reduced buoyancy provides a force in a direction restoring the vessel to its original position.