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Old 14-05-2021, 09:37   #1
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harmonics data for current off by 1.5hours for my spot

Hi i am docked in warren RI. Tidal flow is very fast at the dock. I was planing to take off the prop while in water and lined up the slack based on harmonics file and OpenCPN. I live aboard so i will pay closer attention to the actual spot on the map and my slip.

It appears the slack that OpenCPN current prediction showed me was 1.5 hours late. For warren river RI may 13 it showed slack at 11:30AM. Real slack, (i witnessed it) was at 10AM. So needless to say i noticed it early enough to still manage my underwater task but with a higher level of stress and risk than necessary.

Now it made me question how is it that in 15 years of using OpenCPN with harmonics data for tides and currents along east coast, Puerto Rico on USVI, i never ran into trouble elsewhere?
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Old 14-05-2021, 10:01   #2
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Re: harmonics data for current off by 1.5hours for my spot

Did you check that OpenCPN showed your accurate location at that time?


And that any updates were current? (Or that any currents were updated?)


If it was a river, any recent rains upstream?


Additionally there have been solar activity generating geomagnetic storms along with impulses, which may affect GPS data.


Something to consider...
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Old 14-05-2021, 10:13   #3
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Re: harmonics data for current off by 1.5hours for my spot

Well, the official NOAA prediction was:

2021-05-13 08:47 AM flood 1.02
2021-05-13 11:20 AM slack -
2021-05-13 01:19 PM ebb -0.81

Remember, they are predictions. And for rivers in particular they can be a bit tricky. If river flow is high then slack after flood can come early, and slack after ebb can come late as the river flow continues to overrun the tide.
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Old 14-05-2021, 11:14   #4
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Re: harmonics data for current off by 1.5hours for my spot

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Originally Posted by Dsanduril View Post
Well, the official NOAA prediction was:

2021-05-13 08:47 AM flood 1.02
2021-05-13 11:20 AM slack -
2021-05-13 01:19 PM ebb -0.81

Remember, they are predictions. And for rivers in particular they can be a bit tricky. If river flow is high then slack after flood can come early, and slack after ebb can come late as the river flow continues to overrun the tide.
Thank you, never thought of it. But I would think Id run into this more often in the past 15 years. I know i use the same harmonics to dock my sailboat as there is no way to tie up under certain conditions. I will pay closer attention to see if 1.5h offset is river flow induced. I think it must be.
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Old 14-05-2021, 11:44   #5
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Re: harmonics data for current off by 1.5hours for my spot

I hear you there, getting into our slip on a spring tide ebb is a fool's errand. For us, the harmonics are pretty close most of the time, but if I have something critical I start watching the tide a couple of days beforehand to see if it's running early/late.
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Old 15-05-2021, 06:56   #6
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Re: harmonics data for current off by 1.5hours for my spot

phorvati;_ does this picture sound more like it for th currnt at warren?
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Old 15-05-2021, 07:19   #7
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Re: harmonics data for current off by 1.5hours for my spot

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Originally Posted by phorvati View Post
...For warren river RI may 13 it showed slack at 11:30AM. Real slack, (i witnessed it) was at 10AM....?
P_Dub, check your date.
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Old 15-05-2021, 07:41   #8
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Re: harmonics data for current off by 1.5hours for my spot

Sorry I send you the wrong date print out. How that sound like now?
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Old 15-05-2021, 08:20   #9
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Re: harmonics data for current off by 1.5hours for my spot

I have to correct the time zone, sorry.
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Old 15-05-2021, 08:38   #10
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Re: harmonics data for current off by 1.5hours for my spot

I apologise for my error. Here is the correct print out.
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Old 15-05-2021, 08:52   #11
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Re: harmonics data for current off by 1.5hours for my spot

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Originally Posted by P_Dub View Post
I apologise for my error. Here is the correct print out.
Still not correct. Today is not the 13. but the 15. May.
You have problems with date and time?
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Old 15-05-2021, 09:06   #12
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Re: harmonics data for current off by 1.5hours for my spot

He wanted to see the 13 not the 15. I suppose we have the same calender.
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Old 17-05-2021, 06:27   #13
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Re: harmonics data for current off by 1.5hours for my spot

Quote:
Originally Posted by P_Dub View Post
I apologise for my error. Here is the correct print out.
What harmonics file are you using. I have the original from when i first installed openCPN. And i get this, which was wrong. Slack was at 10AM. From your grapsh its off by 45mins not 90 mins.
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Old 17-05-2021, 12:21   #14
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Re: harmonics data for current off by 1.5hours for my spot

I will fooward it by some ti to make it fit. Thank you.
Bye.
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Old 17-05-2021, 12:40   #15
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Re: harmonics data for current off by 1.5hours for my spot

Would this fits you better? It was adjusted to a closer station.
Bye
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