To calculate tides, you can use simplified methods like the Rule of Twelfths, which provides an estimate of the tide height between high and low tide times. The method outlined in the NauticEd Sailing Blog reference details a step-by-step process for this calculation.
Here are the steps based on the provided reference:
- Determine the Tide Range: First, you need to find the range between the adjacent high tide and low tide. This is the difference in height between the peak of the high tide and the trough of the low tide in a specific area.
- Calculate Fractional Range Values: Divide the calculated tide range by 12. Then, find the values corresponding to 1/12, 3/12, and 6/12 of this range. These fractions represent the approximate cumulative change in tide height during the first, second, and third hours of the tidal cycle, respectively (relative to the low or high tide time).
- 1/12 of the range
- 3/12 of the range
- 6/12 of the range
- Set up a Time vs. Height Table: Create a table listing time intervals, focusing on times 3 hours after and 3 hours before the relevant high or low tide time. This helps organize the calculation for specific points in the tidal cycle.
- Example Table Structure (relative to a Low Tide at time 'T'):
| Time Interval | Calculation | Estimated Height |
| :------------ | :---------- | :--------------- |
| T - 3 hours | | |
| T - 2 hours | | |
| T - 1 hour | | |
| T (Low Tide) | Base Height | [Low Tide Height] |
| T + 1 hour | Base Height + 1/12 Range | |
| T + 2 hours | Base Height + 3/12 Range | |
| T + 3 hours | Base Height + 6/12 Range | |
- Example Table Structure (relative to a Low Tide at time 'T'):
- Apply Fractional Range Values: Add the calculated 1/12, 3/12, and 6/12 of the range to or subtract them from the height of the relevant tide (low or high tide).
- If calculating the height after a low tide (rising tide), you add the cumulative fractional range values (1/12, 3/12, 6/12) to the low tide height at approximately 1, 2, and 3 hours after the low tide.
- If calculating the height after a high tide (falling tide), you subtract the cumulative fractional range values (1/12, 3/12, 6/12) from the high tide height at approximately 1, 2, and 3 hours after the high tide.
- The same principle applies when calculating the height before a high or low tide.
- Calculate the Tide Height: Based on the values added or subtracted in step 4, you can calculate the estimated tide height at the specific time intervals listed in your table.
This method, rooted in the Rule of Twelfths, assumes that the tide rises or falls at an uneven rate during the approximately six hours between low and high tide. The rate is slowest in the first and last hours and fastest in the middle hours. Specifically, the change in height is roughly proportional to 1:2:3:3:2:1 over the six hours of a tidal phase, summing to 12 "twelfths," hence the name. The reference specifically lists the cumulative changes after 1, 2, and 3 hours as 1/12, 3/12 (1/12 + 2/12), and 6/12 (3/12 + 3/12) of the total range.
It's important to remember that this is a simplified estimation and actual tide heights can be influenced by other factors not included in this basic calculation.
