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Old 26-05-2022, 10:19   #1
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Confusion about tidal current

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?
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Old 26-05-2022, 10:24   #2
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Re: Confusion about tidal current

Tidal flow is basically sinusoidal, with other factors added on top. In the general case, maximum flow will be half way between the high and the low, just like the change in the length of day is maximum on the equinoxes.
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Old 26-05-2022, 10:39   #3
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Re: Confusion about tidal current

When you have a lagoon/estuary with a constricted entrance to the ocean the lagoon level will lag behind the ocean level - the amount of lag depends on how severe the constriction and the tidal swing, so neap lag will be shorter than spring lag.
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Old 26-05-2022, 10:45   #4
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Re: Confusion about tidal current

You should get a set of tables for your area that show BOTH tide heights and current strengths, with times. You will plainly see the differences. In many cases, time and height adjustments need to be made for intermediate stations. You should also read basic navigation and boating books or websites which explain all this in more detail than is available in a forum.
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Old 26-05-2022, 10:47   #5
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Re: Confusion about tidal current

the time of maximum flow depends on several factors including but not limited to:

1) weather conditions
2) exact location of moon (varies over several different cycles, consider super moons)
3) the specific geography, as sometimes a lot more water has to rush at the very end of high tide to fill a larger area for example.


for these reasons, the time of maximum flow from start of tide switch can vary from place to place, and even at the same place to some degree depending on several factors
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Old 26-05-2022, 10:58   #6
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Re: Confusion about tidal current

Port Philip Heads in Australia - look it up - Port Philip - not Australia - involves a very large bay and a very skinny entrance. Slack water occurs a full 3 hours after high or low water at the outside of the entrance but at the same time as high and low water at the head of the bay - some 30 miles away.
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Old 26-05-2022, 11:11   #7
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Re: Confusion about tidal current

The most simple ebb/slack/flood model is rarely what actually happens, at least where I live. Many quirky complications can be found.

High and low tide times are not always the same as slack current, as you have found out. Slack current at my usual shore occurs an hour or more after the actual high or low, many variations in other local spots. I can watch low tide come and go and see that the water level is clearly rising again, while the current continues to flow out. I think of it as a sort of teeter totter effect with some complexities under the surface.

Best advice is just to ask someone local to fill you in.
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Old 26-05-2022, 11:15   #8
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Re: Confusion about tidal current

An old rule of thumb for tidal flow is the rule of 6. For a three hour period, the first hour will get 1/6 of the flow, the second hour will get 2/6, and the third hour will get 3/6. For the next three hours, the pattern will be reversed as the tidal flow slows down.
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Old 26-05-2022, 11:29   #9
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Re: Confusion about tidal current

The strongest tidal currents usually occur before, or near, the time of the high [3RD & 4TH hour of flood] and low [3RD & 4TH hour ebb] tides. The weakest currents occur between the flood and ebb currents, and are called "slack water" or "slack current".

Most tides are semi-diurnal, meaning there are two high, and two low tides a day, 12:25 minutes apart. [6+ Hrs rising + 6+ Hrs falling]
The Rule of Twelfths, for tidal height, applies to a semidiurnal tide period [12 hours and 25.2 minutes], from low to high tide, and then repeating to low tide again.
During the first hour, the water level rises by 1/12th of the total tidal range. In the second hour, it rises by an additional 2/12ths of the total. During the third and fourth hour, it rises by 3/12ths. Then the increase begins to slow down. In the fifth hour, the water only rises by 2/12ths, and in the sixth hour it rises by 1/12th.
The pattern is 1,2,3,3,2,1.
The two important points to note are that in the third and fourth hour there is a lot of tidal movement, and in the first and sixth hour, there is very little.

See also ➥ https://www.cruisersforum.com/forums...ml#post2641083
And ➥ https://www.cruisersforum.com/forums....html#post1320
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Old 26-05-2022, 11:33   #10
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Re: Confusion about tidal current

Many regional variations to the textbook ideal.

East Narrows near where I live, connects Hecate Strait to the west coast of Haida Gwaii. It’s mostly just a very narrow boulder-strewn ditch, through which the current gets very fast.

The Hecate has high tides up to about 25 feet or so. The west side highs rarely get above 16. In the narrows, tides rise and fall every six hours or so, but the current is mainly always running strongly to the west only. Current flows east only for two brief periods during the day. Exhausted and confused kayak tourists on shore are a common sight.

Ask locals to get your info.
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Old 26-05-2022, 15:01   #11
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Re: Confusion about tidal current

Quote:
Originally Posted by markcouz View Post
When you have a lagoon/estuary with a constricted entrance to the ocean the lagoon level will lag behind the ocean level - the amount of lag depends on how severe the constriction and the tidal swing, so neap lag will be shorter than spring lag.
Basically this imo. The lagoon level is unlikely to have the same tidal range as outside given the narrow entrance. As others have said, you need some local knowledge re that channel. But, if there is significant traffic through there, it might be listed as a current station (or secondary one) in;
https://www.charts.gc.ca/publications/atlas-eng.html
We keep our boat in a marina in Campbell River on Discovery Passage in BC. Currents are high ( up to 8-9 knots at max in the Passage)and are based on nearby Seymour Narrows which runs at up to 16kn. The current highs and lows are between the 3rd and 4th hours between the slacks at Seymour. These are often more than an hour different than high/low tide levels in Campbell River. I pay no attention to tide tables coming and going ... it's the current tables that matter.
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Old 26-05-2022, 15:06   #12
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Re: Confusion about tidal current

Quick edit. Sorry ... assumed for no good reason you were Canadian. I suspect that there are also US current tables ... NOAA?
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Old 26-05-2022, 15:28   #13
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Re: Confusion about tidal current

Going by the OP's name:

NOAA Tides and Currents for Maine

The other states are in the menu to the left. There are some easier sites to use, but NOAA is the horse's mouth so to speak.

A narrow entrance to a large body of water can shift times significantly from outside to inside.

You can see from this sample image (station picked at random) that flood and ebb are entirely different beasts:
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Old 26-05-2022, 15:38   #14
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Re: Confusion about tidal current

Ask a local professional fisherman. Local conditions invariably muck up rules of thumb. Wind conditions are typical as they can also have an effect. Not available from any chart or table.
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Old 26-05-2022, 15:42   #15
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Re: Confusion about tidal current

For the purpose of understanding, imagine the water, it spreads out in the bay, but then is constricted by the narrow entrance, and it will scream through the constricted area, at a higher rate of flow than in the wide part, but it takes time to let the backed up water in or out. That is why true slack doesn't necessarily follow the rule of twelfths. Who knows when slack water happens are fishermen. We "yachties" usually use tide tables, but coupled with local knowledge of various areas, gained by arriving at the wrong time. Heavy rainfalls will also affect ebb duration; sometimes the outflow will last the whole day. This is the day it will take you hours to get through the narrows.

Hope this helps.

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