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Slacks are notoriously difficult to predict, and rely mainly on local knowledge from the "old salts" and people who are out there every day. However, the following represents a half reasonable prediction. As always, get there early, and be prepared for the slack to vary according to the wind, weather, and what you had for breakfast in the morning.

Slacks (very roughly) according to my interpretation of NP250 - Channel Tidal Stream Atlas

 From  To  HW Slack  LW Slack
 Chatham  Margate  HW Dover + 1h 00  HW Dover - 5h 00
 Margate  Dover  HW Dover - 1h 00  HW Dover + 5h 00
 Dover  Dungeness  HW Dover - 2h 00  HW Dover + 4h 00
 Dungeness  Hastings  HW Dover + 3h 00  HW Dover - 4h 00
 Hastings  Beachy Head  HW Dover + 1h 00  HW Dover - 5h 00
 Beachy Head  Brighton  HW Dover  HW Dover + 6h 00
 Brighton  Littlehampton  HW Dover - 1h 00  HW Dover + 5h 30
 Littlehampton  Chichester  HW Dover - 1h 30  HW Dover + 5h 30

If you need to calculate slack times a bit more accurately, then you need to base them on more accurate Tidal Stream data such as that which appears on Admiralty Charts. See the sections below for methods of doing this.

Calculating Slack Water

Before you start, you need to find out what the symbols and terms mean. You need to know the difference between a neap tide and a spring tide, and to understand tidal movement and the concept of high and low water.

Pick out the most appropriate (usually the nearest, but not always the case) tidal diamond from the chart that you are using. Tidal diamonds appear as small magenta diamonds, and are lettered, usually starting from the bottom left of the chart.

Having selected the most appropriate diamond to use, refer to the Tidal Streams Table, also on your chart, for details of what the current will be for every state of the tide from 6 hours before high water to 6 hours after. In our example, we have found the table for Diamond C. The table contains three columns representing the direction of travel (the direction the current is going to), and the rates for (mean) spring and neap tidal ranges. Note that the currents will be greater when the tidal range is greater than the mean, and less when it is less.

Reviewing the table, we can look for the point where the current appears to be the weakest, and in this example we can see that the spring rate is 0.2 knots at HW-1 and 0.4 knots at HW+6. Now this area of the chart is between Littlehampton and Chichester, and my rough estimate based on the tidal streams atlas was HW-1:30 and HW+5:30

However, this doesn't yet translate into a clock time that we can plan for, and we must refer to the tide tables to discover when the High Water will be.

Important Note: The Tidal Streams table will refer to a particular port (e.g. Dover, Portland, etc) and it is very important to use the tide tables for that port, and not the tide tables for the nearest port to your dive site.

The tide tables show a high water at 07:22 and we must add one hour in the summer months as most tide tables use GMT rather than BST for their times, giving us 08:22. This in turn gives us calculated slacks at 07:22 (HW-1) which is a high water slack, and at 14:22 (HW+6) which is a low water slack.

You will also want to discover the depth of water over your wreck at the times when you want to dive. Refer to the tide tables for the nearest port, and work out (using the rule of twelfths) the height of the tide at your chosen slack water time. Add this height of tide to the stated depth of the water on the chart, and you're pretty much there.

Write it all down so that you don't have to calculate it all again later, and get someone to check it for you!

Getting a more accurate picture

I use Microsoft Excel to help me with this. It only needs to be done once, as the tidal streams don't really change from year to year (although they sometimes change the letters when they reprint the charts!).

Firstly, select your Tidal Diamond, and copy the information from it into your Excel worksheet, as the example below.

Note: Using the technique we described above, we calculated the slacks as occurring at HW-1 and HW+6, being the times that the current flow appeared to be the least.

Luckily for us, (and this is why the technique below works well) the tide runs one way (072) for about half of the time, and then the other way (252) for the other half. We need to arbitrarily select one direction as the positive direction, and the other as the negative direction. In this case we will select 072 as the positive, and 252 as the negative.

Next we need to change the data so that all the 252 rows have their rates expressed as a minus number, as in the example below.

Now we will create a graph based on columns 1, 3 and 4 - and after a little bit of playing around with graph options, we arrive at the following graph.

We can see from this graph that the slacks occur when the rate is zero, and in our case this occurs at about HW -00:50 and at HW +05:40. This is a little different from the HW-1 and HW-6 that we calculated earlier, and shows how this technique can help produce a more accurate picture. Remember the original calculation from the Tidal Streams Atlas was HW -01:30 and HW +05:30.

In this example the slacks are at almost identical times for springs (the blue line) and neaps (the magenta line), but this isn't always the case and is why we need to plot both of them.

Remember that this is only an approximation. The graph indicates that there is only a single point in time where the current is zero, but we know from experience that the current does really stop for a period of time, more so on neap slacks.

I have highlighted an area in yellow on the graph (you won't be able to get Excel to do this!) which indicates the rate between +0.5 and -0.5 knots, rates which should offer no problem to a reasonably experienced diver. Viewing the graph with this overlay then tells us that the diveable high water slack on springs lasts from HW -01:15 to HW -00:25 (50 minutes), and the diveable high water slack on neaps lasts from HW -01:40 to HW -00:00 (1 hour 40 minutes).

Remember that these times will only be reflected when the tides match the mean spring and neap tidal ranges and will expand and contract as those ranges alter.

If you want to try this out for yourself, then why not start with the spreadsheet I used in this example.

Two notes of caution:

  1. Slacks are notoriously difficult to predict. Get there early, and be prepared for the slack to vary according to the wind, weather, and what you had for breakfast in the morning.
  2. Remember that Tidal Streams will refer to a major port (in our example it refers to Dover), and it is likely that the HW time at your local port will be different (although in our example the difference is very minor).

Lastly, we must remember that the length of diveable slack varies with the tidal range, and our graph only shows us the details for the mean spring and neap tides. However, if you want a slightly more accurate indication of the diveable slack for a particular day, then look up the mean spring and neap ranges for the port in question, calculate today's tidal range, and then put those figures into a second spreadsheet which will plot today's slacks for you.

This page was last updated on : Saturday, 21 August 2004 14:18

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