Speed of current during tide cycle

[Dean Abramson]

I'm confused about something. At what point during the tide cycle is the current strongest?

Where we are, each ebb and flood takes about six hours. I have always thought that the maximum current is about at the 4-hour mark. Am I all wet, as it were?

I am reading conflicting things on this subject.

Where our boat is launched is a small bay; but it's long and skinny, and its entrance is a tiny funnel. The currents can be really daunting. This location is relatively new to us. I have assumed that if I wanted the least amount of current we should leave right at high, or right at low.

But despite what I've thought, even leaving the slip at those times, we've encountered very strong currents.

What's going on? Maybe what I've thought all along is just plain wrong?

Generally speaking, when is the max current flow? And in a bay as I described, which way, if any, does the pattern get skewed?

[mgphl52]

Dean, It sounds to me like you're fighting a continuous current. If so, the tidal range will simply slow it somewhat and then increase it again. I had this issue on the St Johns River in Jacksonville, especially near downtown where the river was heavily dredged, made much deeper and narrower. The river, which flows north, has a strong current and can be extremely strong when the tide is "going out."

[John Stone]

Tides and current flow are a funny business. Current strength is not consistent with tide cycle. I have some info on this. Let me look it up. It's on the boat and I am at home. Give me a couple days.

[Tom Keevil]

We sail in a region of very high tidal range and numerous restricted channels and harbor entrances. It is very clear that the tidal currents are only vaguely related to tidal heights, and there is no reliable way to connect them. That is why the government publishes separate tidal current tables. In the absence of official current tables you must rely on local knowledge - talk to your neighbors.

To understand this, imagine the tide rising outside the narrow entrance to your harbor. Water will flow in, but not fast enough to keep up with the rising water outside. When the outside tide is at its maximum, the harbor level will still be lower, so water will continue to flow in, even as the outside tide level drops. This will continue until the levels equalize, at which point the harbor water will start to flow out, but it will not be at the same time as the outside tidal change. This timing will also change depending upon how much water is running into the harbor from rainfall, rivers and glacial melt, so it is all a bit of guess.

We often have to transit narrows where the tidal current runs in excess of 10 knots. The idea is to arrive a bit early of the expected slack, and then watch the current. When it looks good, you go. If you’re late you cross your fingers and hope for the best. In England areas like this are called “tidal gates.”

[John Stone]

We sail in a region of very high tidal range and numerous restricted channels and harbor entrances. It is very clear that the tidal currents are only vaguely related to tidal heights, and there is no reliable way to connect them. That is why the government publishes separate tidal current tables. In the absence of official current tables you must rely on local knowledge - talk to your neighbors.

To understand this, imagine the tide rising outside the narrow entrance to your harbor. Water will flow in, but not fast enough to keep up with the rising water outside. When the outside tide is at its maximum, the harbor level will still be lower, so water will continue to flow in, even as the outside tide level drops. This will continue until the levels equalize, at which point the harbor water will start to flow out, but it will not be at the same time as the outside tidal change. This timing will also change depending upon how much water is running into the harbor from rainfall, rivers and glacial melt, so it is all a bit of guess.

We often have to transit narrows where the tidal current runs in excess of 10 knots. The idea is to arrive a bit early of the expected slack, and then watch the current. When it looks good, you go. If you’re late you cross your fingers and hope for the best. In England areas like this are called “tidal gates.”

Nice explanation. It also explains how you can have slack water after or before the high or low tide time.

[Chrisa006]

Where we keep our boat in Lewes De. the slack tide in the canal is 11/2 hrs. after the published tide change.

[fmueller]

There are some locations that feature tidal driven “gyres” - kind of like a giant whirlpool that always spins in one direction even though the tide switches, usually just speeding up and slowing down … this is the case between Deer Island and the shipping lane in outer Boston Harbor where there is a miles wide clockwise circulation. Some gyres are intermittent, but some persist right through the tide cycle.

[mgphl52]

And then you can visit the Bay Fundy where tidal currents completely reverse the flow of water!
Tides there range from over 45 feet to over 50 feet!

[Carl Thunberg]

Most of my experience with tide currents is on the Piscataqua River in Portsmouth, NH. Slack tide occurs roughly one hour after the scheduled low or high tide, which is close to what Chris said on the Lewes. This may not be technically accurate, but I have attributed it to momentum. That much mass of water at say, 6 knots doesn't stop and reverse at the tide cycle. It wants to keep moving in the same direction.

As for maximum, I was never particularly interested in that, because it's the minimum that you're aiming for.

[wikakaru]

Dean,

Check out this web site: https://tidesandcurrents.noaa.gov/noaacurrents/Regions. Look for the nearest prediction station to your location. Also, some chart plotters have tidal current predictions built-in if you turn that layer of the display on. I use OpenCPN, and this is what it looks like:

OpenCPN current predictions.jpg

The orange diamonds on the chart are the places for which tidal current predictions are available. I know the graph looks like a tide height graph, but it actually represents the speed of the tidal current at the selected station. Look in the little red box (#3 in the pic) and you will see the direction (AKA "set") and speed (AKA "drift") of the current at specific times.

The offset of the time of minimum current from the times of high and low tide differ, sometimes vastly, between one body of water and another, and even between different parts of the same body of water. Generally speaking, the farther "up the creek" you are from the mouth of the body of water the larger the difference the time of slack water (that is, zero current) is from the time of high or low tide. Once you are familiar with a given area you can come up with a rough rule of thumb for how long after high and low tides slack water is at that location, but it changes with changes to the earth/moon/sun relationship, and also with your location, so it is always best to consult the tables.

Smooth sailing,

Jim