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Working out the tide, the rule of twelfths and drying out
Working out the tide by the rule of twelfths spreads the tidal range over six hours in the
proportion 1-2-3-3-2-1.
Calculator
In the first hour after high water the water falls by one twelfth of the range, in the second by two, in the third and fourth by three each, then by two and one again. That is an approximation, and it breaks down in estuaries and narrow channels; the official tide tables remain the authority.
Drying out is normal in the Wadden Sea and on the Atlantic coast, and it is not a claim, as long as it is planned and the boat is built for it. A keelboat that settles onto hard ground unintentionally, by contrast, suffers damage to its keel, rudder and hull.
The difference lies in the event: planned drying out counts as normal use of the boat, while taking the ground unintentionally counts as a grounding, and a grounding is usually covered by comprehensive hull cover.
Frequently asked questions
Is drying out insured?
Planned drying out counts as normal use of the boat, not a claim.
An unintentional grounding is usually covered by comprehensive hull cover.
How accurate is the rule of twelfths?
Usable as an approximation.
In estuaries and narrow channels it breaks down, and the tide tables apply instead.
Worked out for your type of boat
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