The "Rule of Twelfths" is a rule-of-thumb that we use frequently to decide if a location is safe to anchor or if a passage is safe to do. It only takes a few minutes and does not need tidal curves or templates. All you need is the tidal range and High Water height.
We use the Rule to decide the best passage north from Dublin to Skerries where we get a surprise in the night.
http://www.saltylass.co.uk/rule-of-twelfths.html
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