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Old 15-01-2011, 09:45   #1
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Tidal Predictions

Hey guys, it there a way to predict a water level at a given time between high and low tides? A friend of mine was telling me about the rule of sixes but I can't remember how to administer it.
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Old 15-01-2011, 09:55   #2
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Don't know about tidal predictions ; -) maybe that's a nautical voodoo I never heard of! But tidal charts give you a pretty good picture of where the water line will be at any give moment...

You need to know your +/- from the base chart for your area. Charts are based on readings at specific spots and the distance from that spot determines the time allowance you need to factor in to figure for where you are going to be.
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Old 15-01-2011, 10:02   #3
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Originally Posted by unbusted67 View Post
Hey guys, it there a way to predict a water level at a given time between high and low tides? A friend of mine was telling me about the rule of sixes but I can't remember how to administer it.
for around here I use this:-

Tide times Lifestyle This Is Jersey

Also available in paperback (new one every year).



that and looking out the window
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Old 15-01-2011, 10:05   #4
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it's not the rule of sixes; it's the rule of twelths.

you can find it here: Rule of twelfths - Wikipedia, the free encyclopedia
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Old 15-01-2011, 10:23   #5
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it's not the rule of sixes; it's the rule of twelths.

you can find it here: Rule of twelfths - Wikipedia, the free encyclopedia
Beat me to it Bash,
unbusted67 note the warning that this a very approximate way to determine height.
Use tide tables and its an easy calculation to determine height for a given time, or find a time for a given height, or you can use this
EasyTide - on-line tidal predictions from the UKHO
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Old 15-01-2011, 10:24   #6
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it's not the rule of sixes; it's the rule of twelths.
you can find it here: Rule of twelfths - Wikipedia, the free encyclopedia
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:

Rule 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 hoursP 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:
Its 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 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.

* Actually, there are about 12 Hours, 25 minutes between high tides, so tidal flow reverses every 6 Hours 12 Minutes.
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Old 16-01-2011, 18:19   #7
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Depends on required accuracy.

Half way is half tide and since the tide curve is quasi sinusoidal (or whatever) then the first and the last hour it has to be less than an equal portion.

b.
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Old 16-01-2011, 19:07   #8
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Old 17-01-2011, 05:28   #9
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Just to say whilst the predictions may be a science - the reality is also affected by High Pressure and weather. 6 inches or 20 Minutes mostly doesn't matter, but useful to be aware of the possibilty, especially when drying stuff is part of the Navigation.
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