4 The inability of the discernible navigation test to completely solve the inherent difficulty in assessing and predicting the navigation of vessels in or at the entrances of narrow channels:
Collision avoidance is reliant on the ability of vessels to foretell the movements of other vessels, in order to take the correct measures. As we have previously seen, these predictions can be based on different factors. Of course, the frequent and continuous observation over time of the manoeuvres of a vessel is a primary source of information. This was a major point of focus for the UK supreme court in The Alexandra I. But the location of navigation is equally important. Again, The Alexandra I decision touched on this factor when determining whether a change of course made in order to keep-to-starboard within a narrow channel would be incompatible with the duty of a stand-on vessel to maintain course and speed. The answer was in the negative. Due to the location of the stand-on vessel within a narrow channel, such course alterations are to be expected and they are not in violation of rule 17(a)(i). The applicable regulations are also a major source of information for predicting the navigation of other vessels. In a narrow channel, the keep-to-starboard requirement applies, and alterations made towards achieving that goal are to be expected. All of these factors play a role in predicting the intentions of nearby vessels. The problem arises when two or more of these sources can lead you to different conclusions.
This chapter argues that the risk of reaching conflicting, but equally plausible, predictions in regard to the navigation of an observed vessel may be higher in or near areas such as narrow channels, where vessels are expected to be in close-quarters. These uncertain scenarios are what we dubbed earlier equivocal encounters.
The Heranger(1) The Heranger, [1938] 62 Ll.L.Rep. 204.illustrates reasonably well a situation where the expectation of one vessel, based on the locus of navigation and regulations applicable to that area, do not conform with the discernible navigation or even the actual intentions of another vessel. It therefore deserves some attention (infra, 4.1). The pervasiveness of the problem with equivocal encounters in cases involving narrow channels casts some doubt over the practicality of the UK supreme court’s approach to the assessment and prediction of other vessels’ navigation (infra, 4.2).