NAVIGATION


Bow tide With the tide running against you at about 45° on the bow, we use the ‘two-thirds rule’. This estimates that the tide will act two-thirds along the track and two-thirds across it as shown in Fig 9:5. At first this may seem confusing but let’s assume a bow tide of 3 knots, two-thirds of which is 2 knots. You can then think of your boat as experiencing a 2 knot head tide and a 2 knot cross tide – both at the same time. This would mean that our 12 knot boat would only make 10 knots over the ground and would be carried 2nm downstream over an hour.


9


Fig 9:5


There’s no point in being absolutely accurate. Tidal rates are only a prediction after all, and can be influenced by all sorts of unexpected events. Better by far to have a rough idea how the stream will affect you and then make a judgement as to what corrective action is about right in those circumstances.


Stream rates – ‘rule of twelfths’


This rule doesn’t belong in the same group as the previous ones, but is extremely useful for estimating the rate of rise and fall at any stage of the tide. It assumes that the time between LW and HW (or HW to LW) takes 6 hours, which isn’t strictly accurate but is close enough for our purposes. Once the tide turns, it starts slowly, then gradually increases, finally slowing down again as it reaches the end of its range. The rule then goes on to say that:


■ During hour 1: the tide will rise or fall for one twelfth of its range. ■ During hour 2: the tide will rise or fall two twelfths of its range. ■ During hour 3: the tide will rise or fall three twelfths of its range. ■ During hour 4: the tide will rise or fall three twelfths of its range. ■ During hour 5: the tide will rise or fall two twelfths of its range. ■ During hour 6: the tide will rise or fall for one twelfth of its range.


RYA Seamanship for Sea Anglers 67


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