Predicting Slack Tide

[Coldwater_Canuck]

Hey, this is my first post here and hoping some of you can help me out. Hopefully I'll stick around here, seems like a good place to discuss diving. Sorry if this has been asked before, but I wasn't able to find anything by search.

So far I haven't gone diving anywhere where current was anything more than a mild inconvenience (and actually worked to my advantage a couple of times). But now I'm looking at some sites where there could be a real danger due to currents, and I need to plan properly. There's 2 main questions I have:

1.) In general, how long before/after high/low tide is it considered "slack". I realize the correct answer is 'it depends', but that doesn't really help me, in general what are good times? For example, say low tide is at 9am, in an area with potentially deadly currents, what would be the general recommendation of when it's safe to dive?

2.) Reading a few dive site reviews, I saw some things like "there is minimal slack at low tide, you have to dive at high tide". Why would this be, and is there anyways to determine this from tide tables if there is no slack at a certain exchange; or do you need to find out from people who know the site?

and 3.) Any other tips related to current and slack?

Thanks in advance for your help.

 

[sschlesi]

While you can make some general assumptions based on tide tables as to the relative size and timing of currents, the tide tables tell you little about the timing and extent of slack. There are current tables for some areas, which will give you a much better idea of current speed and slack time, but that still varies dramatically depending on the area where you dive. WWW Tide/Current Predictor - Site Selection is the easiest reference I know of for good tide and current data.

A tide table may say there is a 10ft. exchange between 9am and 4pm. In one area that may mean minimal current and a "slack" period of 40 minutes or more, while another area may run 6knots, with less than 10 minutes of slack water. There is nothing in the tables to help you know the difference.

Local knowledge is necessary, ask local divers and dive shops about specific sites. Anytime I've visited a high current site a local dive shop has been happy to tell us the best times to dive.

You can quickly get a feel for sites you often visit by paying attention to the tide predictions and conditions on each day that you dive. Remember that conditions can vary significantly between a flood and ebb of similar size, they don't have to result in "mirror image" currents. It depends greatly on the local geography.

 

[flw]

Depends on where you are. Check the local pilot books/charts. Where we are, the N sea pilot books give guidance on the strength of the tides in a given area - there are charts which give tidal flows at hourly intervals - although they are slightly larger scale than ideal for diving, we use them a lot for planning kayaking trips.
In some areas the tides are fairly predictable, in others the pilot books will say things like 'unstable eddies' - which basically means you are only ever on a best guess, and it may not do what is predicted, possibly due to wind conditions offshore.
The best people to ask in a new area are often the local (commercial)fisherman - particularly those running creel boats for crabs/lobsters

 

[String]

Local knowledge is usually the only sane way. Whilst tidal diamonds on charts give an rough guess things like bottom topography and land shape can vary these fairly drastically over even a short distance.

Also slack water doesn't always necessarily fall on high and low water at a particular site.

 

[DA Aquamaster]

We dove the China wreck a couple weeks ago and slack tide occured about 90 minutes after high tide at Cape Henlopen. As others have said, its hard to predict and the high and low tides are not the end of the story.

If in doubt, show up a little early and wait for it.

 

[don Francisco]

I agree with the others. When it comes to tides and currrents there is absolutely no substitute for local knowledge, preferable divers. Tidal currents do strange things, many of which don't appear on the tide tables and charts.

In general, I've found that the best diving is near the end of the rising cycle, with a plan to end at high tide or just after. That's when the water is usually clearest, but I don't apply a general rule to unknown sites. Others have been there before me and I try to benefit from their knowledge.

 

[Darnold9999]

I use a current table that includes currents for the major spots in local waters and interpolate for the spot I am interested in. This only gets you a "best guess" and can be wrong. My last exploration dive in a high current spot (usualy the best dive spots) I dropped in 10 min before slack to find the expected light current going one direction so went with it knowing that it would reverse on the flood. 10 min later the current started the other way with almost no slack time at all and became too strong to fight within another 10 min and I had to abort the wall part of the dive or get blown by the boat.

You need great charts so you know what the underwater topography looks like to figure out what the water will likely be doing as it moves back andforth. Sitting and watching the surface for a couple of exchanges will give you an idea of what is going on and what to expect, however be prepared for the unexpected. You can get local eddies that are doing the exact opposite of what is happening on the surface.

This particular location was a great dive spot, but the expected "slack" simply did not exist. In future this will be a drift dive only as it needs a live boat to be more than a very short dive - or a very small exchange so currents are manageable.

 

[String]

Quite common for places with high tidal ranges (20ft+) to have no slack water on spring tides.

 

[lmorin]

Quite common for places with high tidal ranges (20ft+) to have no slack water on spring tides.

Isn't that physically impossible? Tide goes in, tide goes out. Current slows to zero, before reversing and picking up speed again. "Slack" is, perhaps, in the eye of the beholder with even small tidal movement not considered "slack."

 

[Saltair]

When it comes to your diving other divers are the best resource for this info. There are published tables that allow one to predict tidal current as opposed to tidal height. Current is the most important and it really has no relationship the the time of high or low water. As the water runs into a place the momentum does not stop right at high water. Water will continue to flow in after the tide peaks until the momentum is stopped and the water reverses direction. There is great variability with depth to the point that some current tables will give data for current speeds at depths below the surface. The variables that effect the times of these events are numerous and hard to predict making the tables tough to use for something as tide dependent as diving.

As to your question as to minimal or no slack at low water this could be expected in tidal rivers where there is a natural flow of water from the river combined with the tidal flow. As the tide runs out the current imparted from the tide will wane, but the current from the river remains.