Current Charts

R

rawidman

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In trying to predict slack current, I can access a current station prediction near my marina with my Garmin plotter or Homeport.

If I select "Profile", this is what I see:

388427853.jpg


and the prediction for "0 kt" is for 8::35 AM.

If I select "Properties", this is what I see:

388427852.jpg


and the prediction for slack current is 9:34 AM.

The difference is approximately one hour. Can somebody please explain this to me? Which is correct and why?
 
You re asking about slack tide but using a current chart
 
quote:
Originally posted by PascalG

You re asking about slack tide but using a current chart

Click to expand...

OK then, slack current. I want to be able to determine when the water is not moving in either direction. I don't care how high or low it is.

I should have phrased my question better. And it's a "current station", not a tide station. I will edit the post.
 
teh current chart in OpenCPN is showing slack current for that location/day as 9:05am

in Navimatics Charts & tide, 0kts is between 9:13 to 9:24

i've always seen minor differences in tide/current tables for locations other than the reference station.
 
Looks like one is not adjusted for daylight savings. You'll have to ask Garmin why that is. Everything else on the chart and table looks OK.
Also "slack" always refers to current. You have "high tide" and "low tide" to indicate when the water level is not moving up and down.

Edit: Just to satisfy my curiosity and to subsequently make things more amusing, both the Mr. Tides program on my Apple and my Navionics chip on my Furuno NavNet VX2 show slack tide on the 12th as 9:07 AM.. so just take the average on your Garmin!
 
quote:
Originally posted by caltexfla

Looks like one is not adjusted for daylight savings. You'll have to ask Garmin why that is. Everything else on the chart and table looks OK.
Also "slack" always refers to current. You have "high tide" and "low tide" to indicate when the water level is not moving up and down.

Edit: Just to satisfy my curiosity and to subsequently make things more amusing, both the Mr. Tides program on my Apple and my Navionics chip on my Furuno NavNet VX2 show slack tide on the 12th as 9:07 AM.. so just take the average on your Garmin!

Click to expand...


Yes I meant current and edited the post a while back.

Both charts show GMT -5:00.
 
I was responding to Pascal's comment, Ron; you were fine from the beginning. Besides, I know you know perfectly well how it all is defined. As for you, sounds like a Garmin specific issue on a number of fronts. Perhaps one applet does not adjust for European Summer Time. Either way, they are both off the NOAA prediction. I didn't check to see if this is a measured station or an extrapolated sub-station.
 
Ron, I may be mistaken (it IS late at night for me), but I think we're in Quebec time (Z-4), at least here in New England. Perhaps that has something to do with the difference? A lot of pubs such as the Tide Tables and Tidal Current Tables aren't corrected for daylight savings time, so you have to math it out to factor daylight savings time. Not familiar with the program you're using, but maybe it's not factoring that? Or maybe it's using a different reference station than it seems.
 
Sidenote:

In your location, using the predictions charts may be practical. But in some areas ( my stomping grounds are certainly one of those areas ) the tides represent only one relatively small factor. In this area, wind speed/direction/duration is the primary factor. Tides normally represent a max of 3 ft range, with 1.5 ft typical. Wind effects can easily range over +/- 3 or 4 ft. It is not uncommon to see an "official slack tide" only to discover that you are fighting a heavy current.

Just keep this in mind when relying on tide predictions. It depends on the area, and wind speed/direction/duration will, in all instances, have at least some effect. Sometimes the effect is the dominant factor.
 
Like Bill alluded to, Tidal Current predictions are only that. Add in all the other factors that influence total water current, and those Tidal Current predictions may not be so accurate.
 
I understand that these are only predictions but that is about all I have to go on. My marina is very tight and it takes more skill than I have to get my boat into the slip with the current we see at maximum flow. Or any significant flow for that matter.

I understand the possibility that the one of the predictions might not take DST into account, but a couple references posted above put the predicted slack current in the middle of the two I have.
 
I am very familiar with the current situation on the Ashley and it is not to be trifled with, especially the ebb. I speak as someone who underestimated its effects once.... but never again! Up where Ron is it will really get jiggy on you as the river goes through the twisty-turnies; it's bad enough down towards Charleston Harbor.
 
Ron on my tides program I do not have a tide station for West Marsh Island. I do have a "current" listing. I wonder if you computer picked the wrong station and the tide and current are not the same location. I looked at the South Ashley bridge and the tide and current were identical.

Ron you are so correct you must work the tides to get in and out of a slip on the Ashley.
 
quote:
Originally posted by PJ

Ron on my tides program I do not have a tide station for West Marsh Island. I do have a "current" listing. I wonder if you computer picked the wrong station and the tide and current are not the same location. I looked at the South Ashley bridge and the tide and current were identical.

Ron you are so correct you must work the tides to get in and out of a slip on the Ashley.

Click to expand...


There is no tide station listed. There is one downstream at the Cosgrove bridge and one upstream at Drayton hall. My marina (and that current station) are about halfway between them.
 
There are many areas where slack current does not occur at or usually even close to the time of high or low tide. That's certainly very true here in Vineyard and Nantucket Sounds where water enters a funnel area before squeezing out the other side , only to turn around and do the same the other direction , plus Buzzards Bay has some drainage here too. If it was completely unimpeded the times might match at least a lot closer .

That tide level chart indicates it is near a river that would effect current and water levels beyond only astronomical influences, and other local hydrographical factors may come into play as well.
 
quote:
Originally posted by Sandy

There are many areas where slack current does not occur at or usually even close to the time of high or low tide. That's certainly very true here in Vineyard and Nantucket Sounds where water enters a funnel area before squeezing out the other side , only to turn around and do the same the other direction , plus Buzzards Bay has some drainage here too. If it was completely unimpeded the times might match at least a lot closer .

That tide level chart indicates it is near a river that would effect current and water levels beyond only astronomical influences, and other local hydrographical factors may come into play as well.
Click to expand...

Yes, but it's a current chart from a current station. And the two different views from the same chart are just about one hour apart.

If I look it up here:
http://www.tides.info/?command=view...east of, Ashley River, South Carolina Current

I get something between the two values on the Garmin chart. It's not Garmin but whoever does their charts.
 
My apologies-you're right, my bonehead mistake. I initially just looked at the chart and thought I was seeing the typical tide level chart in spite of it's title and your careful description.

That slack/0kt difference is indeed peculiar. However, FWIW my experience is that even if you are right at a particular listed tide/current station, the predictions should be taken as general reference only since present or recent past weather can throw the timing way off, more than an hour in some cases. That said , it sure looks like the 2 times you refer to at the same GMT -5:00 zone should be identical assuming they are for the same station as one would expect going to "properties" of the sine & square wave current chart . Good catch. Do you see the same issue with tide level data?
 
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