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 capsize back to the upright condition again. It is also intended that this exploration of the idea will enhance the understanding of the impact of stability considerations on the design of all small craft, and enable stability to be considered at the earliest stages of the design process.

The International Maritime Organisation (IMO) has set out criteria which have been implemented in national legislation in most countries, but for small craft there are often other bodies (such as the MSA and RYA in the UK) that have developed more relevant standards and codes. The stability behaviour of a vessel at sea is highly complex, and the static stability diagram which plots the righting lever (GZ) against heel angle, can not model such phenomenon as surfing, broaching, and parametric rolling. However it does capture many of the key stability characteristics of a vessel, and can be used as a remarkably powerful analysis tool.

In a design for stability exercise any of these criteria could be used as objectives, in which case the design task would be to achieve a static stability curve that had certain attributes, defined in terms of minimum values for the following: the initial stability (GM), the range of stability, the reserve stability (proportional to the area under the curve), and for the value and heel angle of GZmax (Figure 1).

However, capsize is still considered to be an event to be avoided and that there is no guidance as to the relevant criteria for the stability of the vessel in the inverted state, once capsize has occurred. For a craft to be automatically self righting there must be positive stability at all angles of heel. If this is the objective then the criteria for the inverted state (heel angles greater than 90 degrees) should be that GZ must be greater than zero. However if assumptions are made as to the stability challenges the inverted craft will be subjected to (waves, wind, human intervention), then we could propose other limiting values for this condition also, such as maximum value and angle of occurrence of the negative GZmax and maximum value of reserve stability in the inverted state.

The criteria give us minimum values from the perspective for safety. In a design for stability process, high values of GZ at small angles of heel result in an excessively stiff vessel with sharp