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How to calculate slack water?

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Initially Posted - 02/08/2013 : 08:30:15

I want to spin SL around in her slip so I can work on the stern easily. I know from experience that trying to move her around when the tide or river is flowing strongly is not a one person job, so I want to do it at slack tide (I'll have help, but it's still easier when the water's not pushing the boat around). I've searched online and found a number of references to calculating it for diving, but nobody ever actually gets around to telling you how (so far, I'm still looking, but I'm half a dozen sites in and no procedures yet). I know that low tide occurs at 10:16 tomorrow morning at the marina, but I'm also on a river, so I'd imagine the river current will affect when the tide goes slack.

Anyone know how to do this calculation?

David
C-250 Mainsheet Editor

Sirius Lepak
1997 C-250 WK TR #271 --Seattle area Port Captain --

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9081 Posts

Response Posted - 02/08/2013 : 09:41:47

From my observations here, there are too many variables--for example, how far up the river are you, how far up does the tide flow, with what speed (generating momentum) and how large an area does it fill. I go with local knowledge, or a copy of Eldridge Tide and Pilot Book for Long Island Sound (which doesn't cover my river location). The folks who work at your marina might be able to give some averages (flow-to-ebb and ebb-to-flow).

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JohnP
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1520 Posts

Response Posted - 02/08/2013 : 09:50:52

You would add the several factors to get the calmest conditions: <ul><li> tidal flow - pushes boat at 0-2 kt (for example) </li><li> river current - pushes boat at 3 knots constantly (for example) </li><li> wind - pushes boat at 0-2 knots (for example)</li></ul>

You need to estimate the strength and direction of each of these at your location, then time the manoever to use the tidal flow added to the river current or substracted from the river current. You may not be able to plan on the wind, and that can be a significant factor in pushing a boat around in a marina, too.

If you like vector diagrams for force calculations, this would be a good use of them to make sense of the additive forces on the boat, particularly if the 3 forces are in different directions.

If the conditions are extreme, it's helpful to use bow and stern docklines made fast to one or two pilings or dock cleats. You can control the boat's movement from the side deck by pulling or releasing each line and walking fore or aft with the line. It's easier with crew, but can be done singlehanded.

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redeye
Master Marine Consultant

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3476 Posts

Response Posted - 02/08/2013 : 10:01:41

From many years of scuba diving we have a foolproof calculation for tide high and low.

Show up at the location 2 hours before listed high/low.

Watch for high/low.

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redeye
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Response Posted - 02/08/2013 : 10:07:48

Additionally you want to look at the tide charts ( graphs ) for a period of time when the tides are changing the least.

I've seen a day when the outgoing tide was outgoing, until the incoming met it. Current all throughout.

Other days the tide would have still water for hours. Shut off the engine and never move.

Gulf Coast for me... I'm sure your location is quite different.. ask a local fisherman.

Atlantic coast in Georgia is an 8 foot tide.. never seems to quit flowing.

Edited by - redeye on 02/08/2013 10:11:45

Davy J
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USA
1511 Posts

Response Posted - 02/08/2013 : 11:02:52

I'm not sure if this will help you or not. Here is the web site for your area, I think.

http://tbone.biol.sc.edu/tide/sites_uswest.html

It has both tide and current predictions. Once you find the closest position to you, click on graphic plot. You should be able to get a good idea from that.

Here is the graph for my area for today. you can see I have a couple of opportunities for a slack or slow moving tide.

The site also has predictions for actual current.

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 02/08/2013 : 11:17:23

There's a nice discussion of the topic [url="http://tidesandcurrents.noaa.gov/faq4.html"]here[/url]. Look especially at FAQ #2. You can see that the phase lag between tides and currents can be almost anything, but is largely affected by whether the current is caused by standing waves or progressive waves, which depends on topology and hydraulogy.

The only truly reliable prediction is from NOAA's prediction tables. Maybe [url="http://tidesandcurrents.noaa.gov/currents13/tab2pc2.html#18"]one of these[/url] will be near you.

Many chart plotting programs (Garmin, OpenCPN, etc.) will also give current predictions, but the model that they use to make the predictions can vary widely. I looked into this a little bit and posted [url="http://www.sailnet.com/forums/chesapeake-bay/96138-currents-c-d-canal-3.html#post982551"]more than you may want to know here on Sailnet[/url].

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 02/08/2013 : 15:51:46

I found the NOAA tables, but as suggested, I pinged my marina manager. Since tomorrow morning's low tide is the "high-low-tide", there's only about 6' of tidal range as opposed to the "low-low-tide" that evening which is closer to 12'. He's of the opinion that the water will be slack for as much as an hour on either side of the low tide, and has offered to help me spin the boat as well. Nice guy!

I'll read some of the other stuff you guys provided when I get home this evening, but thanks for the help, I appreciate it.

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Voyager
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USA
5378 Posts

Response Posted - 02/08/2013 : 17:04:01

Definitely a vexing question since it depends on the hydrodynamics of your area. In my previous marina, we had both a NOAA tide station and current station nearby. In our case, time of slack water began about 1 hour after high tide and lasted about 1 hour. Depending on the height of the tide (spring vs neap), we would see both variations in lag and duration of slack tide (higher tides had longer lag with shorter slack time).
But in most locations you either get tide or current, but not both. NOAA's "TidesandCurrents" can be helpful. There is a guide in Eldridge that describes the hydrodynamics of the current in a channel responding to the height of the tide in the ocean and the amount of filling up of a bay, but it is very general. It does not take into account harmonics of the process, where water may pile up on one end of a long narrow bay then flow toward the opposite end like water resonating in the bathtub. As my physics professor at Georgia Tech used to tell me, there must be a second order differential equation for that!

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Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 02/08/2013 : 21:23:17

Rule of twelve. Divide the time between high and low by 6, pretty easy if you have 6 hour tide cycles. Tide rises or falls in sequence 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the total change over the 6 time units.. It is only a rule of thumb, but pretty reliable unless you have geographic aberrations.

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britinusa
Web Editor

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USA
5404 Posts

Response Posted - 02/09/2013 : 04:50:09

I have the Tide App on my Android, but when on a PC I have been using this site.
http://tidesandcurrents.noaa.gov/ Just keep clicking on the chart near your location.

And if you are in the Houston area (and use the Chrome Browser) then [url="http://apps.proto.cbi.tamucc.edu/transit/"]this might be useful

Edited by - britinusa on 02/09/2013 05:10:11

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 02/09/2013 : 08:22:43

Let's not forget that Dave was asking about currents, not tide levels. And a careful review of the historical data and information on wave propagation shows that slack water usually does not correspond to high or low tide. In fact, there are places where high tide actually corresponds to maximum flood current.

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Stinkpotter
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Djibouti
9081 Posts

Response Posted - 02/10/2013 : 09:47:57

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by TakeFive
 Let's not forget that Dave was asking about currents, not tide levels. And a careful review of the historical data and information on wave propagation shows that slack water usually does not correspond to high or low tide...<hr height="1" noshade id="quote"></blockquote id="quote">Correct. For example, here at The Race (where the tide enters and exits Long Island Sound like a huge wave), the current is going into the sound for over an hour after high tide at The Race, and is still flowing in the other direction, out of L.I.S., long after low tide here. The Eldridge book shows how the currents throughout the sound relate to the currents at The Race, in one hour increments. Cruising sailors who don't pay attention to that, particularly in this area, can have a frustrating few hours, and those who do will see GPS speeds that are pretty exciting!

Edited by - Stinkpotter on 02/10/2013 09:50:04

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 02/10/2013 : 09:56:57

That is why local information is so important. Slack water will be around the high and low in the vast majority of locations, but general rules are general, not specific.

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 02/10/2013 : 11:49:07

My marina manager was correct, I got there at roughly 10:45, about half an hour after low tide. He saw me walking down the dock & joined me. His first comment when we got to the boat (which is covered in a couple months worth of neglect), was "it looks pretty slack". I thought he was talking about the state of my boat, what with the streaks of green growing in the accumulated dirt from the two cement factories, and then I realized he was talking about the state of the current, which was indeed slack. And stayed so till I left, nearly four hours later. As I walked out, I could see some upwelling from the river current, but there was flotsam sitting in the fairway that wasn't moving at all.

We spun the boat around in less than ten minutes, including rigging lines, untying, and retying lines.

It was a good thing I went down there, I found that I had no power to the boat, and the batteries were down to about 12.1 VDC instead of their normal 13.4 or so. Plus my dehumidifier hadn't been running all winter, so it was a bit musty in there. Got that sorted out with my multimeter and now the batteries are charging again, and the dehumidifier is dehumidifying.

Thanks again for the insight, and this was good knowledge to gain, because knowing that it's slack-ish for the better part of four hours after a high-low-tide gives me a much bigger window for returning to my slip with minimal drama than I thought I had. I'd just never paid that much attention before.

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redeye
Master Marine Consultant

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3476 Posts

Response Posted - 02/11/2013 : 04:29:49

<< because knowing that it's slack-ish for the better part of four hours >>

In the Gulf coast the tides vary. We look at the graphs and see when it does what. Flat line means no current. Big humps mean current. I've seen the Destin jetties when the tide was coming in in the middle and out on the sides.

One day of the month you might have four hours, another you might have one hour. I look at the graph on my garmin chart software.

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