Tricks of the tide

Here’s a puzzle for you. In the photo above, what is that straight line on the water, ahead of the anchored yacht?

The picture was taken at the upper end of Loch Linnhe, just outside the narrows that leads into the wild, mountain-girt Loch Eil. We had left the Caledonian Canal at the Corpach sea lock and moved across to the southern shore for the night, anchoring over a gravelly shoal. The tide was flooding quite strongly and London Apprentice lay with her bows pointing towards the sea.

LOCAL TRICKS

Only twice in my life have I seen that straight line on the surface displayed so clearly. On the earlier occasion I was in a dinghy and could float over the line to examine it closely. At that time, I had already been boating for more than 40 years, so I suspect that there are many people who have never witnessed the effect or would not recognise it. Which is why I am writing this article: as small boat sailors, we have inherited the technology and some of the skills that were developed by seamen over thousands of years, but there may be gaps in our knowledge.

In the days of working sail, when a typical barge crew of man and boy would often have guided their craft up a complex, narrow channel in fluky winds, taking advantage of each quirk of the current, the skipper would have been keenly aware of every patch of ripples, judging whether they indicated a helpful flow, a shallow patch or a swirl that could push his bows into the bank. And the boy would have learnt quickly — or else.

Nowadays, if we can’t be bothered to learn, auxiliary engines save us much of the hassle, and yet flowing water still affects our cruising and racing. If we understand its behaviour, then we can use it to our advantage, and avoid misjudgments that would get us into trouble.

There are a wide variety of phenomena that occur in flowing water and when different types of water meet one another. Many of these are well known to