USING TIDE CHARTS
- Rules of 12 & 7
Almanacs and many other nautical publications contain predictions of the times of high and low tides at many major standard ports
. Also listed are differences in times of tides from these ports
for additional secondary ports. To work
with this succinct data we need two extra tools:
of Twelve To interpolate between high and low water
heights we use the Rule
of Twelve. We assume the tidal curve to be a perfect sinusoid with a period of 12 hours. The height changes over the full range in the six hours between HW and LW.
* During first hour after HW the water
drops 1/12th of the full range.
* During the second hour an additional 2/12th.
* During the third hour an additional 3/12th.
* During the fourth hour an additional 3/12th.
* During the fifth hour an additional 2/12th.
* During the sixth hour an additional 1/12th.
Hence, two hours after the HW the water has fallen 3/12 (1/4) of the full range.
Corrolorary - Rule of Thirds: It’s readily apparent that the (2) 6 hour half-cycles
(during which the tide either rises or falls) can each be broken down into thirds.
□ During the first third (2 Hrs) the water level changes 1/4 of full Range.
□ During the middle third (2 Hrs) the water level changes ½ of full Range.
□ During the last third (2 Hrs) the water level changes 1/4 of full Range.
Rule of Seven To interpolate between spring and neap tides. Since the change from spring range to neap range can be assumed linear (instead of sinusoid), each day the range changes with 1/7th of difference between the spring and neap ranges.
Hence, the daily change in range is (spring range - neap range) ÷ 7.
TIDES - Definitions:
Tide: The vertical rise and fall of the surface of a body of water caused primarily by the differences in gravitational attraction of the moon, and to a lesser extent the sun, upon different parts
of the earth when the positions of the moon and sun change with respect to the earth.
Spring Tide: The tidal effect of the sun and the moon acting in concert twice a month, when the sun, earth and moon are all in a straight line (full moon or new moon). The range of tide is larger than average.
Neap Tide: This opposite effect occurs when the moon is at right angles to the earth-sun line (first or last quarter) The range of tide is smaller than average.
Range: The vertical difference between the high and low tide water levels during one tidal cycle.
Tidal Day: 24 hours and 50 minutes. The moon orbits the earth once earth month, and the earth rotates (in the same direction as the moon's orbit) on its axis once every 24 hours.
Tidal Cycle: A successive high and low tide.
Semi-diurnal Tide: The most common tidal pattern, featuring two highs and two lows each day, with minimal variation in the height of successive high or low waters.
Diurnal Tide: Only a single
high and a single
low during each tidal day; successive high and low waters do not vary by a great deal. Gulf of Mexico
, Java Sea and in the Tonkin Gulf.
Mixed Tide: Characterized by wide variation in heights of successive high and low waters, and by longer tide cycles than those of the semidiurnal cycle. U.S. Pacific coast and many Pacific islands.
Chart Datum (or Tidal Reference Planes): These fictitious planes are used as the sounding datum for the tidal heights.
Drying Height: Clearance in meters (or feet) above the chart datum.
: Clearance in meters (or feet) below the chart datum.
: Height of tide + charted depth: the actual depth in meters (or feet).
Height of light: The height of light above the bottom of its structure.
Elevation: The height of the light above the chart datum.
Rule of Twelve: Assuming a tidal curve to be a perfect sinusoid with a period of 12 hours. The height changes over the full range in the six hours between HW and LW with the following fractions during each respective hour: 1/12 2/12 3/12 3/12 2/12 1/12. See above.
Rule of Seven: The change from spring range to neap range can be assumed linear, each day the range changes with 1/7th of difference between the spring and neap ranges.
Hence, the daily change in range = (spring range - neap range) ÷ 7. See above.
E. & O. E.
Further Reading @:
NOAA, Our Restless Tides, Explanation of Astronomical Factors, Tides and Tidal Currents
On-line Tide Charts
2003 Tide Predictions:
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2003 © by Gordon A. May
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