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This is cool!  The coast guard doesn't teach simple effective things like
the rule of twelths.  I am currently a great lakes sailor (lake superior)
with little more than one or two inches of water level change either way,
but I am soon bound for Newport, RI and would greatly appreciate further
discussion of methods of East Coast tides/currents prediction.  Also, as a
coast guard quartermaster speciallizing in aids to navigation positioning I
would be intereseted in comments (on or off list) relating to buoys, fixed
aids, GPS, DGPS, LORAN -- what's good, what's bad--- etc.

Brent Ferrantelli, QM1, USCG
CGC SUNDEW (WLB-404)
DULUTH,MN
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>From- Lu Abel 
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>Subject: Re: tidal heights calculations...
>Date: Thu, Jul 8, 1999, 3:19 PM
>

> At 08:38 AM 7/8/99 +0200, Russel Sher wrote:
>>A friends of mine recently did a nav. theory course in which she said the
>>'rule of twelfths' is no longer taught for tidal calculations. Instead,
>>percentages are used. Has anybody heard of this? - Would that mean 10%, 15%,
>>25%, 25%, 25% 10% instead of 1,2, 3, 3, 2, 1 (in twelfths)?
>
> What course did your friend take?
>
> I've taught Advanced Piloting (which covers tidal calculations) for the
> Power Squadron for the past 15 years and I always teach the Rule of
> Twelfths.  It's so simple and handy it's almost a sin not to teach it --
> for the 20 years I lived and sailed in New England I found it extremely
> useful.  (I'm not 100% certain it's actually in the current USPS text and
> I've loaned out my copy so I can't check).
>
> For those not familiar with the Rule of Twelfths:
>
> If I draw a graph of the height of water vs time for waters (like New
> England) where there are two highs and two lows per day of approximately
> equal height,  it looks like a sine wave with just over six hours between
> high and low points.  The Rule of Twelfths gives a very accurate way of
> approximating that sine wave without actually using sines and cosines.  It
> says going from the time low water to high (or high to low) the tide will
> have risen (fallen)
>
> After one hour          by one twelfth of its range
> After two hours         by an additional two twelfths
> After three hours       by an additional three twelfths
> After four hours        by an additional three twelfths
> After five hours                by an additional two twelfths
> After six hours         by the last one twelfth
>
> This 1,2,3,3,2,1 pattern is trivial to remember.
>
> How accurate is it?  Here is a comparison of the percent of tidal rise over
> a six hour period* as predicted by a sine wave and by the Rule of Twelfths.
>  Only in the first and last hours is it off, and then by only 2%!
>
>
> hours       sinusoidal   twelfths
> after low       prediction  prediction
>               (percent of range)
>     1           6.70       8.33
>     2          25.00      25.00
>     3          50.00      50.00
>     4          75.00      75.00
>     5          93.30      91.67
>     6         100.00     100.00
>
> Sounds like Russ's friend was taught a 10,15,25,25,15,10 percent rule.  It
> too will produce correct results except for the first and last hour (where
> it will be even less accurate than the Rule of Twelfths).  I guess if one
> has been sufficiently "metricated" that one can no longer divide by 12 this
> might be better but to me the pattern is a lot harder to remember than
> 1,2,3,3,2,1.
>
> Lu Abel
>
> *  Yes, the tidal period is closer to 6h 15m, but that's only another
> percent or two of error in this prediction.  Close enough when one realizes
> how old most of bottom surveys are on our charts!
>

   

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