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Postby Greg » Wed Jan 10, 2007 7:15 am

Tides: Highs, Lows, Neaps, Springs,etc.
Tides are caused by the gravitational pull of the sun and the moon on the earths oceans, and also the centriugal force which results from the earth's annual circurlar trek around the sun.
Although the moon is much smaller then the sun, it is much closer to the earth, and its effect on tides is about twice as great as that of the sun.
When the moon is half, the gravitational forces are at right angles and tend to cancel each other, and tidal fluctuation is not as great. These tides are called Neap Tides.
When the moon is full the moon's gravitation pull supplements the centrifugal force (result of the earth's annual orbit around the sun) and tides are again higher and low tides are lower than during neap tides.
When the moon is new or full the tides are described as Spring Tides. NOTE: this has nothing to do with the season of the year, even though the term is the same. Some persons call these tides moon tides. This is an incorrect and rather confusing way to refer to them because all tides- neaps as well as springs- are caused, in great part, by the moon.

Determining water depth at times between low and high tides:
It should be clear that at half way between low and high tides the water level will be half way between its high and low levels. At other times, if some degreee of precision is sought, it is not desirable to use a simple linear progression because water movement is not uniform. Instead, water levels tend to hover at both low tides and high tide, and change in level occurs most rapidly at half tide. The following rule of twelfths can be used to determine water levels fairly accurately.

low tide: from tide table, EXAMPLE 4'
1 hour after low add 1/12 (+3") 4'3"
2 '' '' '' '' 3/12 (+9") 4' 9"
3 '' " " " 6/12 (+18") 5'6"
4 " " " " 9/12 (+ 27") 6'3"
5 " " " " 11/12 (+33") 6'9"
6 " " " " 12/12 (+36") 7'

Example assumes 4' depth at MLW with low water at datum and high water 3' above datum.

copied from the Cruizing Guide to Abaco,
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Postby knyfe » Wed Jan 10, 2007 1:43 pm

3rd seem to get going here if I look an the cam. Another hour and we might get a ride ... SFO is already over 20knt ...

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