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SELF-RIGHTING BOAT DESIGN 43

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 [5] 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).

GZ

GZmax C

Deck edge
immersion

B

GM Angle of
vanishing
GMsin+1/2BMtan2sin stability

GM GMsin  Dφ

Angle of max. 57.30=1 rad

stability

Range of stablity

Figure 1. Statical stability curve

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[2].

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

Sayı 6, 2016 GiDB|DERGi