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Old 23-08-2010, 07:52   #1
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Challenge: Tide vs Actual Current

while making my way south on the hudson I am noticing pretty significant differences between when high tide is listed and when the current actually starts to ebb, ie flow south. Excuse me if this is sort of cruising 101 but I havenlt done much sailing in areas with significant tidal ranges. So is there a way to predict the difference between current change and tide change?
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Old 23-08-2010, 07:56   #2
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The difference probably amounts to how far away you are from the location the tidal charts are referring to. Think of tides as giant waves, the peak moves slowly(or quickly) through out the tidal areas.
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Old 23-08-2010, 08:13   #3
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Sailing the Hudson over the past two seasons, I've discovered that there are wide variations in where the tide and current coincide (mostly north, such as up here in Kingston) and where they don't (notably Haverstraw Bay). It is normal for the tide and current to be going in completely opposite directions in the southern part of the river.
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Old 23-08-2010, 08:25   #4
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Went down the Hudson right after Hurricane (by then just a depression) Floyd went through. There was no tidal current, just a constant flow downstream. I guess a lot depends on how much rain has fallen, and a lot fell then!!
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Old 23-08-2010, 08:29   #5
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So is there a way to predict the difference between current change and tide change?
Not accurately, unless there have been observations done over time. If there are sailing directions or a tidal stream almanac available for that area, they might have such a rule of thumb. The lag between change in tide and change in current direction is partly due to inertia - once the water is flowing in a certain direction, it takes some time to overcome the momentum to change its direction.
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Old 23-08-2010, 09:17   #6
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Tide is the rise/fall of the water. Current is the flow. At its most basic (cruising 101? ), current should be slack at low tide and high tide, and will run at its highest rate in the middle of the time between high and low.

All the other factors apply, moon phase, weather, position of the sun, your distance from the mouth of the estuary, etc.

Fair WInds,
Mike
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Old 23-08-2010, 09:25   #7
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The tide and current tables for your area can tell you with some precision the time difference between a Low and a Slack or a High and a Slack.

There are of course seasonal variations cause by river flow for example which of course cannot be very closely predicted. A multi-year average is taken. That's the best that can be done to predict currents affected by river generated current whose exact flow volume is impossible to predict into the future.

I work in SF Bay which is a tidal estuary whose currents are affected by both tidal changes and river flows. The further inland you go here, the greater the time difference for example between a High and a Slack. The difference can be hours far inland in the San Joaquin Delta and Sacramento Delta.

In times of extreme river flow, I have seen it a ways inland where the predicted Flood never actually occurs.
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Old 23-08-2010, 09:28   #8
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So is there a way to predict the difference between current change and tide change?
The short answer is NO, there is no easy way to generally predict current flow from the tide height. Sometimes slack is around high and low tide, but not always. It depends a lot on the shape and length of the body of water.

There is a loch in ireland where slack at one of the choke points is mid-tide. I initially thought that must be wrong and impossible, but it is correct. At that point, at slack, the water is flowing down from the head of the loch and up from the mouth and the water is rising with no currrent!
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Old 23-08-2010, 09:43   #9
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Another vote for no.

The Thames Estuary has several very confusing currents as well as extremely fast tides running between sandbanks which are drying heights - there is 5 or 6m height of tide across the 500 square miles.

Familiarity may take too long. Tidal diamonds (if you have such things on your charts), guesstimates, and allowing lots of room for error ...
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Old 23-08-2010, 09:52   #10
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Eldridge Tide and Pilot book will give you the times of current changes at various points on the Hudson based on the Narrows.

For instance, the current turns at the Statue of Liberty about an hour after it has turned at the narrows...1 hour 45 minutes at the GW bridge and and over three hours at West Point....

Eldrige also has some graphic illustrations of what the current looks like in NY harbor,
in one hour increments, Based on high and low tide at the battery.

For the Hudson, Eldridge will list times of current changes at the GW bridge, Spuyten Duyvil, Riverdale, Dobbs Ferry, Tarrytown and West Point. You can probably fill in the blanks for points in between, or there may be local information printed for specific areas.

Heavy Rainfalls, big snowmelts, hurricanes, nor easters, etc. will affect the river.
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Old 23-08-2010, 10:04   #11
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Other than for unpredictable river flow, Tides and Current Tables can tell you the time difference between the two. It really depends on whats going on locally with the river flow, or water releases or meteorological factors at the time.

I would not just throw out the tide book saying it is worthless because you are near a river.
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Old 23-08-2010, 10:06   #12
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I think a simple analogy that might allow one to understand how slack current is often not timed with high or low tide would be to consider the flow of a long line of traffic at a stop light. When you are far from the light, your lane may be moving when the light is red or stoped while the light is green. Take care and joy, Aythya crew
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Old 23-08-2010, 10:12   #13
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Tides and currents are the effect of a lot of factors. I live and sail in an area of large tides and strong currents. In some channels the current reverses before the tides. The shape of land mass and relative depths must all be having an effect. I use tide tables and current tables for the specific location. Current tables are important when passing through certain passages since the currents can be very dangerous and strong, especially for a keel boat. I've never sailed the Hudson so don't know if it applies there but being a river I would expect the flow of current and the tides would have a very interesting interaction.
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Old 23-08-2010, 10:36   #14
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Actually NOAA has tables that predict the max and slack current for the Hudson Tidal Current Predictions
These hold true barring freshets (large rainfalls) or strong winds.
And NOAA also has tide prediction tables for the Hudson: Tide Tables
If you check the predictions at Tarrytown the current will change direction about 2 hours after high or low water under 'normal' weather conditions.
We race off of Nyack, NY and the current in the river can be the biggest challenge, even more then the sometimes 'snarky' winds.
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Old 23-08-2010, 10:48   #15
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Another No vote. Although related the tide height is not the same as current flow. For example low tide off Smith Island in the Straits of Juan de Fuca is at 9:30 today whereas slack is at 8:45. We use the current predictions to plan transits from A to B to have the current helping us as much as possible. Tide predictions are useful for anchoring or deciding when to hike on the beach.
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