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Old 02-23-2020, 11:51 AM   #1
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Currents at Slack Tide

Read these posts in the "Victoria BC to Portland, Or" thread and thought it might make for an interesting discussion because of the varied landscapes TF members live in:

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Originally Posted by mvweebles View Post
I didn't catch that - there was another recent thread where there was confusion about low tide being synonymous with slack current (despite it being intuitive and related, they do not correlate). I forget if it was off Noyo River (Ft Bragg) or Coos Bay, but the USCG got pretty concerned about a mariner who could not get it through their skull that the times for slack water did not correlate with high tide (or low tide, I forget which).
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What do you use for Current? I opened up Coastal Explorer for the area and really only found tides. I also have an App on my android but also gives tides, not currents? Any suggestions?

Thanks

Peter
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I know a few experienced boaters, even a licensed pro who can't get it through his head high and low don't necessarily coincide with slack. He even recommends only crossing bars at high slack, whatever he means by that because he doesn't understand the difference between high and slack.

I made a different kind of rookie mistake. Not reading the current graph correctly in OpenCPN.

Since Nobeltec bought and shutdown Tides and Currents software years ago I don't have any good recommendations for stand alone tide and current predictions.

To correct myself today I used NOAAs current predictions off the web.


We live on BC's north coast at the head of a 60 mile long channel that cuts into the Coast Mountains. It's off the Inside Passage, which because of the topography has some weird currents.

Prior to getting Badger we sea kayaked for decades, where if you read the currents properly you get a boost to your speed over ground, but if you read them wrong you pay for the mistake in increased effort or get into wild water you weren't planning on.

Channels here are 500' to 1000' deep. If you think of the momentum that much mass has, you can visualize how the water will keep flowing in despite the tide starting to go down.

It's actually written on the waters surface on calm days. Have you ever noticed thin lines of white foam that parallel the shores of a channel? Have you noticed how they first form along the shoreline when the tide turns, then slowly move towards each other in mid channel?

They mark the line where the two opposing currents pass each other. When you paddle across them your bow takes a definite nudge one way or the other, and the line is so distinct you sometimes see twigs going around in circles. Watch the waves on windy days and you can see the difference there as well.

In bays things can get weird as old back eddies compete with new ones trying to set up. Speaking of weird; seeing where the incoming tide meets itself coming around a large island on a calm day is truly bizarre...there's broad band of jumpy little wavelets across the whole width of the channel in otherwise flat water. On charts it'll say, "tides meet"

Things are pretty straightforward here, in that once you can estimate how long a current will be moving contrary to the tide and can read signs on the waters surface, you can avoid lumpier conditions. We don't have bar crossings here, but knowing when slack occurs when crossing one must be vital!
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Old 02-23-2020, 12:10 PM   #2
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Quote from the Canadian tide and current tables:

Quote:
The mariner should be aware that slack water and high
or low tide are not necessarily coincident.

http://charts.gc.ca/publications/tables-eng.html
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Old 02-23-2020, 12:30 PM   #3
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I use these tables
https://tides.gc.ca/eng/data/currents/2020#vol5
Each year I have to remind myself when we go to PDT from PST that these tables are an hour old. almost best to leave a ship clock running PST.
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Old 02-23-2020, 12:47 PM   #4
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Predicted Log contestants use NOAA current predictions. After twenty odd years of competing, I have found the NOAA predictions to be close, but no cigar.
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Old 02-23-2020, 12:51 PM   #5
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I understand high slack and I have been around the block.


I know the difference between tide and tidal current.


High slack is before the current reverses from flood to ebb...low slack from ebb to flood.


But the description of no tidal movement is "stand" meaning no more rise or fall while word to describe the period of no current is "slack" tidal current...and yes in many places the two don't necessarily occur at the same time of day.
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Old 02-23-2020, 01:14 PM   #6
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Quote:
Originally Posted by psneeld View Post
...
High slack is before the current reverses from flood to ebb...low slack from ebb to flood...
Ahhh, but words are slippery things, aren't they?

Tide refers to the vertical rise and fall of water, whereas current or tidal stream refers to horizontal movement...therefore...describing a flooding current after high slack tide would be correct, yes?

As noted earlier, the water here doesn't really 'stand' because the current changes from the shallows to deeper water which takes longer to lose its momentum. The water mid channel floods toward the head of a channel for quite a while (sometimes an hour or more) after the tide starts dropping.
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Old 02-23-2020, 01:47 PM   #7
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Good post Murray.

Once I enter Juan De Fuca I don't pay too much attention to "Tides" unless I am anchoring/letting out rode.

My CE shows the currents in BC very nicely as an arrow pointing in the direction of the current. The bigger the arrow the more current. As a back up I use Waggoner's Ports and Passes. They have improved it over the past few years
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Old 02-23-2020, 01:52 PM   #8
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When I first started traveling the Inside Passage, we used the NOAA books, fumbled our way through corrections, and sometimes came up with times that were pretty close. When Ports and Passes came out, we switched to using that as it seemed the information was the same as the NOAA books but the format was easier to use.
One thing that took a while to realize, and that we still havenít found a reasonable way to integrate into corrections, is flood conditions on the big rivers. We had a situation where we put the boat on the rocks (an uncharted area with a bad set of circumstances) but one of the factors that led to our trouble was the Taku River at flood stage, causing high tide and slack water to both be more than an hour later than it should have been. There are quite a few very large rivers that empty into the Inside Passage, and the amount of water they are adding to the local areas around them will affect tide levels and slack timing.
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Old 02-23-2020, 02:03 PM   #9
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Here's a good explanation from NOAA What is the relationship between "Tides" and "Tidal Currents"


Knowing the difference can be trivial or it can be important.
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Old 02-23-2020, 03:00 PM   #10
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There is a stand anywhere tides reverse, high or low slack described the "time" of a current....


not slippery at all...no crossover between vertical or horizontal movement concept....just describing when.


stand is whenever the going down or up is reversed.
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Old 02-23-2020, 03:30 PM   #11
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Quote:
Originally Posted by psneeld View Post
There is a stand anywhere tides reverse, high or low slack described the "time" of a current....


not slippery at all...no crossover between vertical or horizontal movement concept....just describing when.


stand is whenever the going down or up is reversed.

It appears we're both wrong (must be tons of local variations on this) at least according to the NOAA link given by Portage Bay;


Quote:
The vertical rise and fall of the tides, created by the gravitational force of the Moon and Sun acting on the oceans water, also creates a horizontal motion of the water in the bays, harbors and estuaries. These are tidal currents. In general, as the tides rise there will be a current flowing from the oceans into the bays, harbors and estuaries; this is termed a "flood current". As the tides fall there will be a current flowing towards the oceans; this is termed an "ebb current". There are also periods when there is little or no horizontal motion of the water; this is called "slack water".

https://tidesandcurrents.noaa.gov/faq4.html#15
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Old 02-23-2020, 04:04 PM   #12
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Then you currently stand corrected?
Ok I'll cut you some slack.
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Old 02-23-2020, 04:04 PM   #13
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Quote:
Originally Posted by MurrayM View Post
Ahhh, but words are slippery things, aren't they?

Tide refers to the vertical rise and fall of water, whereas current or tidal stream refers to horizontal movement...therefore...describing a flooding current after high slack tide would be correct, yes?

As noted earlier, the water here doesn't really 'stand' because the current changes from the shallows to deeper water which takes longer to lose its momentum. The water mid channel floods toward the head of a channel for quite a while (sometimes an hour or more) after the tide starts dropping.
All my learnin' is that there is a high and low tide, those are at a time and level. There are rates of rise and fall, but as boater's we don't measure those rates other than to note that a steep rise or fall of tide will have the obvious higher current implications. In referring to the height of the tide (or depth of the water) we are talking about high or low, or state of tide at a given time. That state is simply the measured (by us locally, with lead line or instruments) or predicted vertical level.

For currents there is the ebb and flood, and each have times of max and times of slack. As the discussion above indicates they are because of, but don't match, the times of high or low tide. Everything I've learned indicates those times are called "max (or min) flood" and "max (or min) ebb", and "slack before flood" and "slack before ebb."

Mr. Psneed, the term "high slack" seems a misnomer to me; unless it is specifically referring to the time that the "rate of rise of the tide", ie in vertical measure, is at a pause. It doesn't make sense to use that term to me because of the confusion with a "high slack current" time being different from "high tide" time. Does the "high slack" come from someplace? Or is that one of the regional differences this thread is trying to discern?

I agree with ASD, mostly. I'm mainly in that area inside the Juan de Fuca Strait. I do look at the tide first, but do most of my trip/route/weather planning and go/no-go based on current. I look at tide because right now, our marina is a little shallow at a 0 tide, and the Swinomish Channel I have to leave through doesn't really have a reliable current prediction, but a rule of thumb based more on whether the tide is rising or falling close to high tide or low tide. After that, my main need for tide data is anchoring. It seems that every activity I've every been involved in in the salt water here in the Puget Sound/ Salish Sea areas relies heavily on knowing and understanding the current predictions and how other conditions can modify those (Ie the huge river runoff mentioned earlier.)
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Old 02-23-2020, 04:22 PM   #14
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Not sure why someone thinks what I have posted is wrong?


What I have posted is not only a lifetime of water related jobs but also what the USCG teaches and I taught it for almost 15 years....


High Slack is a bit of nautical slang....not everyone uses it...but it's not really confusing or incorrect. I've never had anyone that really understood tides and currents to not get it pretty quick.


There are slack tidal currents that occur sometime between high and low tides...one between ebb and flood and one between flood and ebb. So why is using high slack or low slack that confusing? Slack before flood (low slack) and slack before ebb (high slack) are very common too....
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Old 02-23-2020, 04:30 PM   #15
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What I have posted is not only a lifetime of water related jobs but also what the USCG teaches and I taught it for almost 15 years...

I am shocked. shocked I say, that there is not homogenization between governmental organizations.
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Old 02-23-2020, 04:49 PM   #16
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I am shocked. shocked I say, that there is not homogenization between governmental organizations.
I don't know what difference you are talking about then....and if different...it's different just in the last few years and many organizations haven't adopted NOAAs new version.
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Old 02-23-2020, 05:49 PM   #17
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Quote:
Originally Posted by MurrayM View Post
It appears we're both wrong (must be tons of local variations on this) at least according to the NOAA link given by Portage Bay;



https://tidesandcurrents.noaa.gov/faq4.html#15
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I don't know what difference you are talking about then....and if different...it's different just in the last few years and many organizations haven't adopted NOAAs new version.
Hard to argue with the government written word!!!!

Dang Murray was RIGHT? Is the sky falling? Is the rising in the west?

Just kidding buddy
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Old 02-23-2020, 06:32 PM   #18
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Quote:
Originally Posted by Portage_Bay View Post
Here's a good explanation from NOAA What is the relationship between "Tides" and "Tidal Currents"

Knowing the difference can be trivial or it can be important.
This is an excellent link and I now understand why, in some locations, slack water occurs when you might expect maximum current if you were just looking at tide tables and not at current data.

"The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three "base case" conditions. The first is a "standing wave" type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid way between the high and low tides. The second is a "progressive wave" current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide. The third case is a "hydraulic current". In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same.

Progressive currents are most common at the oceanic entrance to many bays and harbors


A couple years ago in SE AK, we were transiting Tlevak Narrows, south of Craig, heading for Cordova Bay. Tlevak is near the north end of Dall Island and very near an opening to the ocean. Cordova Bay to the south also fronts the ocean, Dixon Entrance. In my haste to run these narrows to get to where we wanted to fish, I presumed that high slack water would give me minimal current.... which was dead wrong. Obviously, these are progressive currents in this area and at slack tide, the current was running like hell...the channel buoys were bent over and leaving a wake like a ski boat. We waited until the current slowed down before going through, but I was really scratching my head on how I could have been so far off in my current prediction. Tides in this area are in the 20+ foot range, which makes the effects even worse.
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Old 02-23-2020, 06:45 PM   #19
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"Slack before flood" "and slack before ebb" are the terms I've always used and heard or seen used by professionals, experienced amateurs and the government agencies. Exactly to avoid the confusion between tide height and current. For tide height, simply high tide and low tide as the baselines.

Maybe a revisit of Chapman's or Bowditch would be useful for review.

By the way, note that the direction of set usually is not a 180 degree opposite from ebb to flood. Current vectors anyone?
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Old 02-23-2020, 06:49 PM   #20
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Quote:
Originally Posted by Alaskan Sea-Duction View Post
Hard to argue with the government written word!!!!

Dang Murray was RIGHT? Is the sky falling? Is the rising in the west?

Just kidding buddy
Still wondering where the basics is different other than maybe the description of progressive and standing waves which has been known and recorded...just not separately discussed through the ages.
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