Although various codes of practice, regulations and international marine organisation (IMO) resolutions set out current stability criteria, these still have major inadequacies. For example, dynamic effects are not considered; instead, a static snapshot of a ship in a particular condition is calculated. Further, the buoyancy calculations often assume that a vessel is in one position in still water. In practice, of course, the stability of a moving ship changes constantly as water moves around the hull.

Although research is being done (primarily with the assistance of computers) into non-linear behaviour of ships and particular requirements for non-ship shaped forms, e. g. , mobile offshore drilling units ("MODU's") and multi-hull vessels, nevertheless a formal assessment of stability is a relatively modern science when compared to the age of shipping.

Although it is an evolving science, nevertheless it can have a very significant impact in legal disputes involving vessels - in particular, there is often a question as to the seaworthiness of a vessel and the checking of crew statements and bills of lading to verify a sailing draught with reference to stability, are often necessary and preliminary steps.

Similarly, in total loss cases, a full damage stability analysis may verify (or otherwise) the loading condition of the vessel which could assist to establish a likely sequence of events.

Further, and self-evidently, stability plays an essential role in most salvage and grounding cases. The major consideration in salvage cases is the danger that the vessel was in at the time she was salved. If progressive flooding is taking place, it is always necessary to assess whether this will stabilise or sink the vessel and how long this process will take.

Recent cases (some of which this firm has been involved in) illustrate the difficulties. The free service effect of liquid caused by a slack tank or a deck containing free water can dramatically reduce the stability of a ship. Ro-Ro ships are particularly vulnerable because of their large open vehicle decks.

This was a major factor in the "Herald of Free Enterprise" case which was lost off Zebrugge in 1987 and the "Estonia" which resulted in the loss of over 800 people in the Baltic in 1995.

Whether or not the fishing vessel, "Margaretha Maria", which sank about 70 miles south of Lands' End in November 1997, had sufficient stability to operate in all reasonable foreseeable conditions (Rule 16 of the 1975 Fishing Vessel Safety Regulations) is an issue that remains to be decided by the Courts once preliminary issues have been resolved by the House of Lords.

The obvious effect on stability is that of shifting cargo;

frequently, ships are unable to recover, without assistance, from a heel caused, for example, by shifting grain, deck timber falling overboard or heavy plant moving inside a hold. Although damage stability calculations are now a basic design requirement for most cargo ships over 80 metres in length, this is not the case for smaller vessels. It is time this important issue was given careful attention before more lives and ships are lost at sea.

By Charles Hattersley, who is a partner at Plymouth and Exeter based law firm, Foot Anstey Sargent (telephone 01752 675000, e-mail: cwh@foot-ansteys.co.uk).

MJ Information No: 17316