This paper is based on work done originally for the Chevron Shipping Company, whose agreement to its publication in its present form is gratefully acknowledged.

Ever since the International Regulations for Prevention of Collision at Sea (IRPCS) were introduced as a control system for collision avoidance, there have been disputes and arguments about exactly what they mean, and how they should have been applied in particular collision incidents. The principle of deciding by international agreement which ships should be given the duty of keeping clear of other ships generally works well but, as traffic volumes increase, so do situations in which the IRPCS alone do not provide a clear indication of what is expected of the mariner. Various solutions have been addressed to this problem, ranging from proposed radical revisions of the whole principle of collision avoidance, through periodic thoughtful adjustments to the details of the IRPCS, to informal additions and requirements imposed by individual shipowners on their fleets. The residual problem with any solution (except perhaps the first mentioned) is that there will always be exceptions where the rules are unclear or ambiguous. Adding adjustments may reduce these exceptions but, unless all uncertainties can be removed, the adjustments may create a new and even more complex set of exceptions, which are yet more difficult to interpret.