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Tidal Curve example questions

Forum - Coastal Navigation - Tidal Curve example questions
Module Content - Sea Talks2
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Student
Conan Hom
Lexington, MA, USA
Crew Level 0
Status: Sailor
Qualified days: 8
Tidal Curve example questions
Posted 2016, Nov 09 12:36
I am confused on the tidal curve example (the first animated one)... it asks the range of time during the day when depth of water will be above 2m. However it uses the same High low tide slope for after the high tide at 8:52 (7:52) as well as before it. However the low tide before 8:52 which is actually at 2:52 (or 1:52 I can't remember since the sea talks page doesn't allow me to keep the module open at the same time) is 1.6m instead of the 1.0 m after the 8:52 (7:52) high tide. This would seem significant and indiate you'd need to do a different line. Furthermore it would also appear that the curve would not accommodate the prior low tide (it's out of the time range of the curve which only has a 5 hour range to either side whereas the low tide is more than 6 hours from the high tide. How does one adjust for these issues or am I overthinking this?
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Wilfred Darr
Calgary, Canada
Skipper Level II
Status: Big Time Sailor
Qualified days: 28
Reply 2017, Nov 22 01:25
Interesting! I didn't notice that first time through. I would think you are correct, that you should not use the same line for both tides, but theres no reason you couldn't use a 1.6-3.3 line to find when the tide goes above 2m, and a second line 1-3.3 to find when it falls back bellow 2m. Ultimately, even a tide table is only an estimation, since weather can change the actual depth of the water quite significantly (severe lows and highs: hurricane storm surges could drop the water level 20 feet before adding 30 feet to the high tide in some areas). And this curve is only an estimation of the tide table (an estimation of an estimation as it were). A straight sign curve being an estimation of this (estimation of an estimation of an estimation), and rule of 12s being least desirable as it is an estimation of an estimation of an estimation of an estimation. But it's better than what Giovanni Caboto had, and a heck of a lot better than sailing around with nothing ;-)
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