Dick Everitt explains how you can get a little lift from the tide
If our destination is directly to windward we’ll have to tack towards it.
The diagram to the right shows we have a 7-knot wind on the nose and a 2-knot tide running at right angles to us, and later on in the passage the tide will change and run in the opposite direction.
This is all theoretical, but it will demonstrate the principle of what is usually called ‘lee bowing’.
With the wind dead ahead, we can start on either port or starboard tack.
If we start on starboard (A) we’ll have the tidal current on our lee bow and it will push us back towards the rhumb line – the shortest straight line to our destination.
But if we go on port (B), the tide’s on our windward bow and will push us away from the rhumb line.
What isn’t quite so obvious is the effect of the tide on the wind direction that we’ll feel onboard.
The vector triangles in the diagram show how a 7-knot wind when combined with a 2-knot tide at right angles will create a ‘resultant’ or ‘tide-induced wind’ of 7.3 knots (the red arrow).
But the important thing is it’s a much better angle, so we’d get a bit of a lift if we started out on starboard. And in our ideal theoretical world we’d be in a much better position when the ‘tide-induced wind’ angle changes with the tide.
But on course (B) we’d be headed when the ‘tide-induced wind’ changes direction.
Of course, in the real world it’s not so simple. The apparent wind direction can alter, so it then makes even more sense to keep close to the rhumb line to take advantage of any wind shifts, and the ‘tide-induced wind’ effect drops off at slack water as the tide turns.
But the principle of keeping the tide on the lee bow is worth remembering when planning a passage. It might not seem to gain you much, but the alternative of being pushed off in the other
direction can be quite dramatic.
This Nav in a Nutshell was published in the May 2012 issue of PBO. For more useful archive articles explore the PBO copy shop.
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