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phorvati · 14-05-2021, 09:37

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?

SailRN · 14-05-2021, 10:01

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...

Dsanduril · 14-05-2021, 10:13

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.

phorvati · 14-05-2021, 11:14

Quote:

Originally Posted by Dsanduril

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.

Dsanduril · 14-05-2021, 11:44

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.

P_Dub · 15-05-2021, 06:56

phorvati;_ does this picture sound more like it for th currnt at warren?

Dsanduril · 15-05-2021, 07:19

Quote:

Originally Posted by phorvati

...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.

P_Dub · 15-05-2021, 07:41

Sorry I send you the wrong date print out. How that sound like now?

P_Dub · 15-05-2021, 08:20

I have to correct the time zone, sorry.

P_Dub · 15-05-2021, 08:38

I apologise for my error. Here is the correct print out.

CarCode · 15-05-2021, 08:52

Quote:

Originally Posted by P_Dub

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?

P_Dub · 15-05-2021, 09:06

He wanted to see the 13 not the 15. I suppose we have the same calender.
Bye.

phorvati · 17-05-2021, 06:27

Quote:

Originally Posted by P_Dub

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.