Slack tide and Slack Current

[a64pilot]

Originally Posted by Rasselas36

What you are questioning occurs daily on the Saint Johns Rivers, Florida. Normally, there is a hour+ lag in the time of slack current versus the high/low tide.

I’ve found it to be about two hours, I just came up the St John’s yesterday from Sisters creek to Ortega river, I got antsy to leave so we left about an hour after low tide and ran against about a kt of current until we go to downtown, there we even had about a .8 kts current pushing us.
Had I waited the extra hour we would have had a following current the whole time and may have arrived about the same time anyway, but less time underway, less fuel burned etc.

[Rothblum]

Tide and current in a bay are exactly analogous to voltage and current in an electric circuit. In a circuit with only a resistance and a varying sinusoidal voltage driving it, current lags voltage by 90 degrees. This would be a tide going into or out of an infinite bay. This lag can be increased or decreased by the addition of impedance or capacitance to the circuit. Capacitance is analogous in water to connecting a finite reservoir or lake to a fluid circuit. Impedance is analogous to the inertia of the water at a choke point, and resistance is analogous to the drag of the water on the boundaries, such as a channel at the entrance to a bay. So, depending on the values and arrangements of the components, you can have any phase relationship between the components -- voltage/height and current/flow, ranging from entirely in-phase and entirely out of phase and halfway between (closer to normal situations).

[GBR134]

I’m guessing really but in any body of water the tidal stream is taking the most direct downhill route. Thus depending on the topography the tidal flow can still be going downhill for sometime before or after the highest water in your location

[GordMay]

Originally Posted by Rothblum

Tide and current in a bay are exactly analogous to voltage and current in an electric circuit. In a circuit with only a resistance and a varying sinusoidal voltage driving it, current lags voltage by 90 degrees. This would be a tide going into or out of an infinite bay. This lag can be increased or decreased by the addition of impedance or capacitance to the circuit. Capacitance is analogous in water to connecting a finite reservoir or lake to a fluid circuit. Impedance is analogous to the inertia of the water at a choke point, and resistance is analogous to the drag of the water on the boundaries, such as a channel at the entrance to a bay. So, depending on the values and arrangements of the components, you can have any phase relationship between the components -- voltage/height and current/flow, ranging from entirely in-phase and entirely out of phase and halfway between (closer to normal situations).

What a unique and interesting analogy!

[makoman]

Originally Posted by a64pilot

I’m within 100 meters of the upper tidal station, there is significant current here, and I can assure you that slack current is about 2 hours after either high or low tide. There is no current station that I’m looking at, I’m looking at my boats orientation to the anchor.

Also interesting is that the tidal stations are .8 miles apart, yet the tide differences are about 35 minutes, lending I believe credence to Steamgoats post.

Where I lived in the Keys, the 7 mile bridge area, tide and current stations were close together. Best fishing was the slack tide that occurred about 2 hours after the highest high tide. I too go with Steamboat’s explanation.

[lo2jones]

At the Golden Gate at San Francisco, it’s slack water when the water moving out is equal to the water coming in. SF bay is a very complicated estuary. The directions, speed and timing of current depends on different geographical features. A famous San Francisco sailer once said, “slack water in San Francisco Bay is a state of mind.”

[Prairie 36]

Originally Posted by a64pilot

What does slack current not occur until a couple of hours after tide reversal? I observe that but it doesn’t make sense me, it would seem that current and tide should occur at the exact same times, yet current lags tide by about two hours, why?

Is it just inertia?

You received your answer it appears in the forum. Technically, when tide is neutral or doesn't seem to be moving it is called stand and when the current becomes neutral it is slack. Forever that's worth.

[merrydolphin]

The reason that there is often a difference is that we are dealing with fluid dynamics, not mechanical.
If the water were directly mechanically linked, then they would be the same. But they are not. As a fluid there is lag, acceleration, friction, wind, many factors. That keep them from being directly linked. In areas like SFO where tides and current are significant, you better have both tables! As they are almost always different.

[blazing928]

Live in Melbourne Australia, we have a large bay, Port Phillip Bay, with a small entrance about 1.5km wide to Bass Strait and the Southern Ocean.
Surface area 1,930 km2 (750 sq mi)
Average depth 8 m (26 ft)
Max. depth 24 m (79 ft)
Water volume 25 km3 (6.0 cu mi)
Shore length1 264 km (164 mi)

So we have smallish tide heights, about 1m but very different current directions down the bottom of the bay near the heads. ( where we boat/ fish)
Slack water at the heads is quite important for shipping and extremely important for small boats as wind against current can make it very dangerous.

The early explanation of a higher or lower water level in Bass strait vs the bay is what I’ve been told as to why slack and tides don’t match and makes sense to me.

The current runs pretty much clockwise around the bay and near our boats location, Blairgowrie, it’s almost always running towards the heads even on a flood tide.

If you look at a Google map satellite image of the heads, called ’The Rip’ you see these massive sand banks, called the Great Sands (!) so you get all sorts of weird currents there.

[Searles]

Check out the B O M warnings for Torres straight ,that's verry real, going through it a cape class bulky with 3ft under the keel is not for the faint hearted

[marcvilleneuve]

Originally Posted by a64pilot

Now that’s an interesting statement, I’m sure It’s correct, but what effects it? Maybe the size of the tide? Bigger swings more lag?

Here, in the St-Laurence River, we always cruise/race with a current between 2 to 6 kn. Our tides are between 4 to 6 meters.

Usually, the current will change direction 1 hour after the tide. It is a very good rule of thumb. But, when racing I have to be very precise with my calculation. Here is an example:
Let say the tide at K108 buoy (mid point of race Tour de l’Île d’Orléans 26 miles from the start) is 1200.

Tide at K108. 1200
Current change direction +0100
SW wind (same as current) +0015 (NW would mean -0015)
Tidal Range 5.5 m.(high) +0015
=====
I need to round the K108 at. 1330

This a very good calculation as even the Tidal Range and Direction of the wind are taken into account. We race the Tour de l’Île once year and I have perfected that calculation from observation and from speaking with the oldest and most experienced sailors aroun here.

Note 1: The water level will increase by 0,3 m (1 foot) before we observe the current changing direction by the shore. Very strange phenomenon! At first the current will change direction with a small band of water (10 m) on the shore. That band of water will progressively increase in width to reach the middle of the river in 30 minutes or so. After rounding K108 we sail in that band of water to maximise speed; or we lose the race!

Note 2: For thar atypical race we here to choose the starting time by the half hour. We have to inform the race committee 10 minutes in advance. It Is a real poker game on the starting line as we don’t want our opponents to guess our decision.

Hope this put some lights on this complex subject!

[seadago]

When looking at tidal inlets, river mouths and estuaries, and particular coastline features such as narrow coastal sandbanks and sharp rocky points, it depends not only on timing and prevailing winds, but also on the bottom topography and geomorphology of the area. It’s all to do with fluid mechanics.

Let’s take a river estuary for example. Firstly, predicted tidal current direction and speed are always estimated averages, based on assumptions of uniformity of a large number of parameters. In reality, you’ll find you’ll get significantly different readings (2 to 3 knots not uncommon) depending where you take your measurement, not only along the river channel but across it and with depth.

If the river drains extensive salt marshes or mangrove forests along its lower banks, the cross-section profile (called water prism) of the river will look more like a soup plate than a cereal bowl. Early on the ebb, the marshes and inundated banks will drain first on the topmost section (sometimes a very few cm from the surface) of the water column and at very low speeds. In this scenario, it is not uncommon to find the tidal current going out on the surface, denser sea water still pushing “upstream” in the centre of the channel, and a steep halocline between them.
Different types and structure of sediments on the bottom will also offer more or less friction to water flow, contributing to the observable differences.
In short, every river, creek, cove, inlet is different.

[Sand crab]

Originally Posted by SV__Grace

I would love to understand all this an be able to find a spot to anchor that has more stable motion, like we're able to sail or motor using eddies in different directions (e.g. local knowledge about Tacoma Narrows is that the east side always flows north and the west side always flows south).

Are you sure about that?

I live in Gig Harbor so the Narrows is in my back yard. I've been through there a couple of dozen times mostly by kayak. I always plan for the current to push me through and then wait for the tide to change and get a big push back. It's a 17 mile round trip so I need that current. I have done 6 knots through there without paddling. I've never noticed that the water is always flowing one way on either side.

I use Deep Zoom all the time too.

[SV__Grace]

Originally Posted by Sand crab

Are you sure about that?

I live in Gig Harbor so the Narrows is in my back yard. I've been through there a couple of dozen times mostly by kayak. I always plan for the current to push me through and then wait for the tide to change and get a big push back. It's a 17 mile round trip so I need that current. I have done 6 knots through there without paddling. I've never noticed that the water is always flowing one way on either side.

I use Deep Zoom all the time too.

Am I sure about that? Absolutely not!

When locals share their local knowledge I'm inclined to believe them, but I never do so blindly.

You're very lucky to live in Gig Harbor, great place and we love it. Thanks for sharing your back yard with us.