Twelfths Rule

MARITIME – TIDE & TIDAL TREAM CALCULATOR Twelfths Rule A precise tool.
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What is the Twelfths Rule & How does it work?

Tides follow a semi‑diurnal cycle, rising from low water to high water in roughly six hours and then falling back in the next six hours. The rule of twelfths provides a simple way to estimate the water level at any point during this six‑hour interval without complex harmonic analysis.

The six‑hour interval is divided into twelve equal parts. The proportion of the total tidal range that is covered in each successive hour follows the pattern 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. By adding these fractions cumulatively, you can determine how far the tide has progressed at any given time.

Using the known high‑tide and low‑tide heights, the rule of twelfths lets mariners quickly estimate the current tide height for navigation, anchoring, or planning coastal activities.

h = H – (H – L) times frac{sum_{i=1}^{k} f_i + (t – k),f_{k+1}}{12}
h = estimated tide height, H = high‑tide height, L = low‑tide height, t = elapsed hours since high tide (0‑6), f_i = twelfths for hour i (1,2,3,3,2,1)
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Frequently Asked Questions
What is the Rule of Twelfths?
The Rule of Twelfths is a method to estimate water levels during a six-hour tidal cycle, dividing it into twelve equal parts and following a specific pattern of rising and falling.
How does the Rule of Twelfths work?
The six-hour interval is divided into twelve equal parts. The water level rises by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of the total tidal range in each successive hour.
What is the purpose of the Rule of Twelfths?
The Rule of Twelfths helps mariners estimate water levels during a six-hour tidal cycle without complex harmonic analysis.
How many hours does the Rule of Twelfths cover?
The Rule of Twelfths covers a six-hour interval from low water to high water or vice versa.
Can I use the Rule of Twelfths for any tidal cycle?
The Rule of Twelfths is specifically designed for semi-diurnal tides, which rise and fall in roughly six hours. It may not be accurate for diurnal or mixed tides.
What are the benefits of using the Rule of Twelfths?
It provides a simple and quick way to estimate water levels, aiding navigation safety without requiring complex calculations.
How do I apply the Rule of Twelfths to my journey?
Divide the six-hour tidal cycle into twelve equal parts. Add the fractions (1/12, 2/12, 3/12, 3/12, 2/12, 1/12) to the previous water level to estimate the current water level at any point during the cycle.

Results are for informational purposes only and do not constitute professional advice.