Guide
Half the water arrives in two of the six hours
People judge the tide by how fast it moved when they arrived. It accelerates to a peak at half tide and covers ground three times faster than at the start.
The rule of twelfths
The tide rises one twelfth in the first hour, then two, three, three, two, and one. The middle two hours carry half the range.
Why it matters
A sandbank that seemed to be filling slowly at hour one can be cut off in twenty minutes at hour three. That is how people are stranded.
Planning
Work backwards from high water rather than forwards from low. Being on the wrong side of an hour is what causes problems.
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