Tidal height
Tracey quickly identified the key information: it was Springs, HW Plymouth was 1906 BST and there was very little difference in the tide times and heights at the River Lynher. She approximated that we’d have enough rise of tide by late afternoon. She had good instincts, but when I pressed her about how she’d arrived at the late afternoon timing I felt that her broad brush approach was masking a reluctance to tackle the technical details.
RULE OF TWELFTHS VS TIDAL CURVE
Tracey had used the Rule of Twelfths to work out the tidal height. This rule breaks up the six hours between high and low water and states that for each hour a certain proportion of the overall tidal range is gained or lost. It’s a quick to use if you’re numerically minded: Work out the range by subtracting the low water height from the high water height
5.6 – 0.7 = 4.9m Range Divide the range by 12 to work out your twelfths 4.9/12 = 0.41m
On a flood tide work out the number of twelfths for each hour of tide towards high water, multiply that by 0.4m and add that on to the low water height. The indication is that we’ll have a 4m height of tide before 1700.
However in many areas the way that the tide moves in and out won’t match the rule of twelfths model. This is true in places like the Solent or Poole Harbour, which have complex water movements and lopsided tidal curves. But even in a harbour such as Plymouth, where a tidal calculation is critical, you should use the proper curve.
It was surprising to find there was almost an hour’s difference in calculations: on the tidal curve we predicted we’d have 4m at 1606, rather than just before 1700. If we’d relied on the Rule of Twelfths we’d have had to wait an extra hour to make our approach.
It’s better to undertake pilotage exercises on a rising tide, with a generous tide remaining. If you do go aground, there should be enough rise left to get off again.