Eyeball navigation

Brian Parish, former head of home waters charting at UKHO, reveals how you cannot always believe your own eyes

As a producer of paper and electronic charts for many years I was careful to impress on all users of hydrographic information, in whatever form, that it is merely an aid to safe navigation, just like radar, sonar and GPS. No navigator should be shy of looking out of the window to confirm what is happening to his or her vessel.

Last summer I had a wonderful awakening to the shortcomings of this advice when crossing the English Channel in my own boat. My experience showed that even your eyes, the longest serving of all aids to navigation, can deceive you.

I was crossing the shipping lanes, heading north-west at about 5 knots, close hauled on starboard tack in 18 knots of north-easterly wind, and at right angles to the flow of traffic. I could see four large vessels on my port beam, coming my way in line astern at generous intervals. Using my hand bearing compass I took a bearing on all four and repeated this several times over a five-minute period.

My observations showed that the first would pass ahead of me and that the fourth was likely to pass astern. The second and third however appeared to be on a collision course with me.

‘How can this be?’ I thought. ‘They appear to be a good distance apart; if I’m on course to collide with one I must surely miss the other’. I continued to monitor the situation, happy to let the first vessel pass ahead and reluctant to alter course until I was sure the fourth would pass astern of me.

As my position closed with the second and third vessels they both remained on a constant bearing, still apparently in line astern with a generous gap. Could my compass be wrong? What I was observing from the helm and what my compass was telling me appeared to be at odds.

There was of course only one sensible thing to do. As soon as I was sure that the fourth vessel and any others appearing behind her would pass astern of me I changed course by bearing away through about 60° to point my boat astern of the third vessel.

As the second and third vessels passed ahead of me I kept my bow pointed astern of the third vessel, gradually bringing my boat back onto her original course.

What I was able to observe was that the second vessel was significantly smaller and slower than the third vessel and was being overtaken by her Had I held my original course all three of us would have been in roughly the same place at the same time, exactly as my compass had predicted.

The moral of the tale is: your eyes, like all the other services, are an aid to safe navigation. Beyond a certain range they do not judge size or distance very well. Perhaps I should have been telling people all those years to apply common sense to what they observe out of the window.