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  #1  
Old 03-24-2011
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Tidal Charts, Tidal Graphs.

I've never had to use one of these in anger but they popped up in a navigation course I've been doing.

Do people use them all that often ?

What do use as a template for the graph ? (for my course examples I made up the grid myself but I wondered if there was a commonly used template or do people simply use graph paper ? )
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Old 03-24-2011
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I do use form to calculate tides and currents. Here are two of mine.

http://www.swiftsuresailing.com/documents/tide.pdf

Intermediate tides use tables 5 and 5A of the tables

http://www.swiftsuresailing.com/documents/current.pdf

Used in British Columbia

Do you you the rule of 12ths?
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Old 03-24-2011
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Here are some used by a colleague

http://www.swiftsuresailing.com/docu...20COMBINED.pdf

http://www.swiftsuresailing.com/docu...eworksheet.pdf
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Old 03-24-2011
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Here is an example of table 5 (it used to be table 6)

http://www3.telus.net/jackdale/navle...Table_5(6).pdf


Step One – Note the Venue
Fill in the date, locality and time for which we require the information.
DATE _______October 1, 2003_________________
LOCALITY___Montague Harbour______________
TIME_________0700________________________

Step Two – Note the Duration and Interval
From the previous data, we see that we have a duration of 0327 to 1141 or 8 hour and 14 minutes. The nearest time to 0700 is 0327 or 3 hours and 33 minutes.
DURATION of rise and fall ______8 h 14 m______________
INTERVAL of time from tabulated level ____3 h 33m_______

Step Three – Calculate the Range of the Tide
The height of the tide at 0327 is 2.2 feet and at 1141 is 11.4 feet. Therefore, the range is 9.2 feet.
RANGE of rise and fall ______9.2 feet__________

Step Four – Determine the correction
Using Table 6, locate the nearest duration to 8h 14m, which is 8h 10m, and follow the row across the table until reaching the nearest time interval to 3h 33m, which is 3h 34m. Note that is in column 8. Using Table 6A follow column 8 down until it intersects with the closest range to 9.2 feet, 9 feet. This number, 3.6 feet, is the correction. Since we are using the nearest height, which is a lower water, we will add 3.6 feet to 2.2 feet. The answer, 5.8 feet, is the prediction of the height of the tide at 0700.
CORRECTION to be tabulated ___3.6 feet______________
HEIGHT OF TIDE ______5.8 feet_____________________
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Old 03-25-2011
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Jack, Thanks for that. Those forms are terrific and yes I do use the rule of 12ths in calculating.

My question though was more to do with tidal graphs. I should have posted an example.



Do you use these ?
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Old 03-25-2011
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I gather that the tidal graph is used for intermediate times. I use Table 6 which is now Table 5 in the our Tides and Current Tables.

There is a similar one for estimating currents.

BTW - You posted the Aztec site as an image. I got to it through my email. Good site.
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Old 03-25-2011
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I use the Rule of 12ths routinely and have never found a need for anything else. But then that probably just means I'm missing something.

What do you get from these charts? Is it of practical value or is just to gain a better understanding? How often and when do you use them?
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The rule of 12ths is good when there is a 6 hour tide.

We often get shorter or longer periods, which why I use the table in the Tide and Current Tables.
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Quote:
Originally Posted by jackdale View Post
The rule of 12ths is good when there is a 6 hour tide.

We often get shorter or longer periods, which why I use the table in the Tide and Current Tables.
It is certainly simplified by 6 hour tides. But they are rarely 6 hours, they always range by 15 or 20 minutes where we are. What sort of variation are you guys accustomed to?

Our tidal range is always around 3 metres. Sometimes 2.9 sometimes 3.3. I do a mental calculation of say, 2 hours into the flood means 3/12ths which is 1/4 of 3000mm which is 750mm of tide. If the range was 3200 the depth would change to 800mm of tide. To me that is not a critical change and would have zip effect on what the reason was for calculating the tide. And if the timing of the tides is 15 minutes out, the variation in tidal movement is even less profound.

So just as a matter of interest, how accurate do the boffins believe the tidal predictions need to be? And is it about water depth or tidal flow? If I am out on my calculations by a few minutes, the water depth will change by the thickness of my hull. I am certainly not going into water where that sort of change will scare me.

I suppose in areas of strong tidal streams it is more important.
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A quick survey of my tide table show a range of 4 to 7 hours.

Our tides are mixed semi-diurnal. The range can be as slight as .1 meters and as much as 4 meters.

We use separate tide and current tables. The turn (slack) seldom coincides with high tide. The worst current we deal with is 16.5 knots.
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