Slack tide and Slack Current

[Lost Horizons]

Originally Posted by a64pilot

I read the Wiki article, it describes it, but does not explain why.

Yes, it does. The very part quoted by StuM provides the explanation. The current between two adjacent bodies of water is slack when the levels of water in these two bodies are equal, which has little to do with the phase of the tide.

By the way, tidal currents in the Bahamas? Seriously? You should visit PNW.

[desodave]

As several of my PNW colleagues have noted, we learn to look at the current tables first. There are different factors to consider in each situation. In our area, we are where the waters coming in around the N end of Vancouver Island meet the waters coming in around the S end, ie, flooding S meets flooding N. Currents in places can appear somewhat perverse at times but often reflect the geography, the freshets on big rivers or the prevailing weather pattern. The current can be in one direction on one side of an island and the other way on the other side.

But take Nakwato Rapids on the BC mainland across from the N end of Vancouver Island - perhaps the fastest flowing set of tidal rapids in the world;
https://www.google.com/maps/place/Na...!4d-127.503239
There are miles and miles of inlets with depths approaching 2000ft (also with inflowing rivers) to the E of the Rapids and 2 short channels to the open ocean on the W side. The Rapids are about 1400ft across (navigable much less) and about 70ft in depth. There are huge differences in the tidal range on each side of the Rapids and slack is nowhere near high or low tides but when water levels equalize on either side of the rapids.
The tidal range inside is about 55% of the range at a station just outside the Rapids.

[Clivevon]

Ah yes. Coming in to Portsmouth Harbour against a big spring ebb can be a lot of fun. Sometimes Happens when you have come back from France & arrive in dark in early hours. All OK with Good engine & power in reserve. Of course you dont muck about on a Sunday afternoon - too many boats too close.
Sorry. Just reminiscing. Missing the boat already. Roll on 2021.

[jewt]

Over the years I have that at Current Cut there is no way to predict the tide the that cut that I have found. It is always a surprise!

Either motoring through at 2 kts. or 7kts.

With it against you it is fun watching the turtles tumbling as they as they pass you. If it is with you trying to dodge the rocks at the end as you try to slow down.

It is what it is.

jewt S/V Bifrost 8

New Bern NC

[Cheechako]

makes no sense to me. If there is no current, the sea level can't change right?
If sea level is changing, there must be current to drain or fill it.

[Lost Horizons]

Originally Posted by Cheechako

makes no sense to me. If there is no current, the sea level can't change right?
If sea level is changing, there must be current to drain or fill it.

The levels are constantly changing independently. The current is an indication of the difference in those levels.

[Cheechako]

Originally Posted by Lost Horizons

The levels are constantly changing independently. The current is an indication of the difference in those levels.

Yeah, "bulges" happen before movement occurs I guess.

[a64pilot]

Originally Posted by Lost Horizons

Yes, it does. The very part quoted by StuM provides the explanation. The current between two adjacent bodies of water is slack when the levels of water in these two bodies are equal, which has little to do with the phase of the tide.

By the way, tidal currents in the Bahamas? Seriously? You should visit PNW.

Yes tidal currents in the Bahama’s. Current cut that I posted the picture to can have a 10 kt current at times, which is real significant when you have at most an 8 kt boat.

Actually the danger is going down current where the eddys can cause you to loose steerage and your going 14 or more kts, you have to have significant forward speed thru the water to maintain steerage.

[a64pilot]

I know that winds even maybe barometric pressure can or will affect tides and currents, but I can tell you that the Bahamas cuts it’s about 2 hours after the high or low tides that get slack water, that in the entrances in the East coast of Fl, it’s about 2 hours, and the St John’s river if you want to go up river from the ICW, it’s best to wait until 2 hours after low tide when the current is slack and of course the rising tide helps offset the river flow.

Maybe it’s coincidence that all of that is about 2 hours, but I don’t think so, it certainly can be location specific as all those areas are only a few hundred miles apart, with I assume similar tidal ranges.

[conachair]

Originally Posted by Cheechako

makes no sense to me. If there is no current, the sea level can't change right?
If sea level is changing, there must be current to drain or fill it.

What if there is current coming from more than one direction though...

[dougweibel]

A64,

Lots of answers here and most are not wrong as there are many things that affect tidal current flow. The guys in the PNW have a very complicated current picture up there as the current is winding its way between many obstructions, and often coming from or going to multiple directions in the bigger picture.

But for the simple case I think you are asking about, where the current flow from the ocean into/out of a sound through an inlet that restricts the flow, the answers in posts #7 and #9 are pretty good. It is all about restriction of the tidal flow coming in from or going out to the ocean. The situation can be hard to see but you can simplify the situation in your mind by imagining a big sound of water connected to the ocean by a single long channel. The channel restricts the flow of water so that the level at the two ends of the channel can be quite different. When the tide is at peak height in the ocean the water level at the other end of the channel will be significantly lower and current will be flowing quickly in. An hour after peak tide (for example) the ocean level will have gone down, but will still be above the level of water in the sound so water continues to flow in. Another hour later the ocean level is down so that the water level on both ends of the channel is equal and the tidal current is slack. Etc. How late after high tide the current goes slack is a function of how restrictive the channel is and how big the sound is. In the extreme slack current occurs at mid tide.

In some locations it is this simple but often, as in the Bahamas, you have multiple cuts connecting interior sounds with the ocean and then you can get all sorts of other things going on due to momentum, wind, bottom contours and all sorts of things.

[Jim Cate]

Originally Posted by dougweibel

A64,

Lots of answers here and most are not wrong as there are many things that affect tidal current flow. The guys in the PNW have a very complicated current picture up there as the current is winding its way between many obstructions, and often coming from or going to multiple directions in the bigger picture.

But for the simple case I think you are asking about, where the current flow from the ocean into/out of a sound through an inlet that restricts the flow, the answers in posts #7 and #9 are pretty good. It is all about restriction of the tidal flow coming in from or going out to the ocean. The situation can be hard to see but you can simplify the situation in your mind by imagining a big sound of water connected to the ocean by a single long channel. The channel restricts the flow of water so that the level at the two ends of the channel can be quite different. When the tide is at peak height in the ocean the water level at the other end of the channel will be significantly lower and current will be flowing quickly in. An hour after peak tide (for example) the ocean level will have gone down, but will still be above the level of water in the sound so water continues to flow in. Another hour later the ocean level is down so that the water level on both ends of the channel is equal and the tidal current is slack. Etc. How late after high tide the current goes slack is a function of how restrictive the channel is and how big the sound is. In the extreme slack current occurs at mid tide.

In some locations it is this simple but often, as in the Bahamas, you have multiple cuts connecting interior sounds with the ocean and then you can get all sorts of other things going on due to momentum, wind, bottom contours and all sorts of things.

Excellent answer! Well explained and correct. Good one!

Jim

[a64pilot]

Yeah, I think from my observations Steamgoat answered it in easy to understand language.
And yes, I’m only talking about the simple one inlet scenario because once it gets complex then the answer will be different for each situation, and may be a complex answer also.

But yes logically it’s the height difference in the two bodies of water that exists due to restricted flow, and the 2 hour part is likely Coincidental as less restriction would seem to shorten the time and more restriction lengthen it.

Part that hit me is that you can’t necessarily time your arrival at a narrow entrance at peak tides to ensure no or low flow, maybe need to be an hour or so late, maybe two.

[JPA Cate]

Exactly. It will vary with the amount of water that has to get out through the restricted entrance, through which the ebb is coming. Key is quantity of impounded water and narrowness of entry. This is why different harbors are different for the difference. Yes, also affected by the size of the tides (one reason the English Channel is so very complicated). The breakers stop when the water's all going the same way, and you can ride the incoming tide in to wherever you're going.

As we are a deep draft boat, we choose to move on rising tides.

Ann

[MicHughV]

Some many years I cruised the Chesapeake Bay over a summer.

After one grounding too many, where I should not have run aground, I found out from a local Chesapeake lobsterman that the wind direction and strength in the Chesapeake Bay very much influences the tide.

ie, A strong s-easter tends to pile the water up in the myriad of bays and cuts that line the Chesapeake. Conversely a strong n-wester tends to blow the water out.

From my experience there, it seemed like there was little rhyme nor reason to it all. There where a few times I was anchored in a small bay unable to get out because even at high tide, I was lacking the 6-9" or so I needed to get out across the entrance bar as I had unknowingly entered the bay post strong s-easter wind..which gave me the necessary depth to get in.

In my depths of gloom and despair, a local lobsterman told me not to worry, as a new strong s-easter was coming, which would raise the tidal water above the norm, which it did, a few days later, at midnight, but I little cared for the time, as I made my escape.

Though I studiously studied the tide tables and current direction for the water of the Chesapeake Bay I was stymied by the constant differences influenced by the wind.

Thereafter, I always made it a point to consider wind direction and speed when entering a tidal cut.