A FEW MONTHS AGO, we looked at tides and how the time and height of high and low tide are predicted (Time & Tide, March). High and low tide heights and times are of course of direct use to divers in indicating how deep the water will be on any dive site and, on a beach or in a harbour, when it is deep enough to launch and recover boats.
However, on our coastline of strong currents the other aspect in which we are interested is slack water – the time when the current arising from the tide stops or drops low enough to
allow us to dive.

The first thing that confuses many divers is that slack water rarely coincides with high and low tide where the boat is launched.
It’s easy to assume that because the tide goes in and out, slack water should coincide with the time when the tide is all the way in or all the way out, but that would be incorrect.
Think of the tide as a six-hour-long wave that is moving along the coast at an angle.
Because water shallows to the shore, and because of the intricate shape of the coast, the crest of this wave will not be straight.
The oceans are not infinite, and at some point this wave reaches the limit of its travel and goes back the other way, adding to the next wave in places, and cancelling it out in others.
Slack water at any one place comes when the movement of the cumulative tide waves is minimal at that specific location. It sounds complicated, but in practice it can be approximated close enough by observation.
The tides are repetitive – so is slack water.
In its simplest form, to find the slack water time at any particular dive site, all you need do is sit in a boat above the site for a day, observe when slack water occurs, and make a note of that relative to a set of tide tables.
On that particular site and that particular day, we might observe that slack water started 1 hour before high water Devonport (Plymouth) and 5 hours after high water Devonport.
Because tides are repetitive, on subsequent days slack will continue to occur 1 hour before and 5 hours after high water Devonport.
It’s just the time of high water Devonport that shifts an average of 51 minutes later with the moon each day. There are complications, but just for now it’s as simple as that.

EVEN MORE CONVENIENTLY, we don’t have to sit in a boat all day to find this out. For hundreds of years, other people have been sitting in boats and watching the tides, working out how fast the current is, in which direction it is going and when it is slack water.
They may not have made these observations for exactly where we are diving, but chances
are that an observation will have been made somewhere close enough to where we want
to dive that we can use it as a starting point.
If it’s a little early or late compared to where we are, we can simply get there 30 minutes or
an hour early and wait. We drop a shotline and watch the current flow round the buoy. When the current drops, we dive.
We also make a note of the time relative to our tide tables, because we can use this on subsequent days to repeat the dive without having to be there too early, and without having to wait so long.
That’s how charter-boat skippers have such good details on slack-water time for the wrecks they visit regularly.
Observations that we can use as a starting point for slack water are the “tidal diamonds” on marine charts, as published by the UK Hydrographic Office.
A tidal diamond symbol is a purple diamond with a letter in the middle of it. Charts that have these also have a table that gives, for each diamond, the direction and speed of the current in knots for each hour of the tide with respect to a reference port.
Find the closest diamond or diamonds to where we will be diving. Then go to the purple table and look for the hour when the current is smallest and the hours between which it switches direction, which should be the same, and roughly when slack water occurs.
Suppose we want slack-water time for the wreck of the Kyarra, off Durlston Head in Dorset. Chart 2610, Bill of Portland to Anvil Point, has a tidal diamond “U” about halfway between the wreck site and St Alban’s Head.
The associated tidal streams table at the top of the chart shows the lowest current 1 hour before and 5 hours after high water Devonport.
It also shows a complete change of direction either side of these times.
Now you can see how contrived our abstract example was earlier.

NOT ALL CHARTS HAVE TIDAL DIAMONDS, or there may not be one close enough to our dive site, so an alternative method is to consult a tidal stream atlas. Like the charts, these are published by UKHO, and also printed in Nautical Almanacs.
A tidal stream atlas gives a sequence of large-scale maps for each hour of the tide, covered in arrows that show the current flow.
Flip through it and look for where the current is lowest, the arrows switch directions, or the word “Slack” is printed closest to our planned dive site.
For the wreck of the Kyarra, we need UKHO publication NP250, Tidal Stream Atlas: The English and Bristol Channels.
This shows the current dropping and switching directions between 1 hour before high water and high water Dover, then again between 5 and 6 hours after high water Dover.
NP250 also gives the times relative to Devonport, which are 5 hours 40 minutes before Dover or, to put it another way, pretty much the same but on the other tide, confirming what tidal diamond “U” on the chart says.
You can be above the wreck 30 minutes early and wait for the tide to slacken, but local knowledge is always best. Charter-boats in the area plan to dive the Kyarra 36 minutes before and 5 hours 44 minutes after high water Dover.
We have two lots of theory and local observations, and they all agree!

Why Devonport or Dover Why not plan relative to Swanage or Poole tide times
Firstly, the charts and tidal atlas for the area give times relative to Devonport and Dover. Secondly, Christchurch Bay suffers from a double tide, including Poole and to a lesser extent Swanage. The resulting wobble in the tidal curve results in local tide tables that do not relate consistently to slack water on the wreck.
Shallow water results in significant friction between the wave of the tide and the seabed.
In a large area of shallow water, this results in the deeper part of the wave being delayed behind the upper part, spreading out the tide.
Delay the deeper part enough, and the top of the tide will drop off a bit before the deeper part catches up and raises the level again.
This is the most common reason for double tides like that of Christchurch Bay. A similar
effect results in double low tides in other places, which is why dive-boats in this area work out slack water relative to Devonport or Dover.
Big headlands can also result in swirling currents. To either side of Portland Bill, the passing tide results in a swirling back-eddy on whichever side is downcurrent, to the east in Weymouth Bay on an incoming tide and to the west on an outgoing tide.
On spring tides, when the currents are biggest, this means that there is a diveable slack water only on one tide.
At Land’s End, outgoing tidal streams from the Bristol Channel and the English Channel are not conveniently synchronised to start and end at the same time.
On the Runnelstone, to the south of the peninsula, the resulting swirling current means that there is only one real slack, but that occasionally on good neap tides there may be another about three hours later.

The funnelling effect of the English Channel and southern North Sea results in the time separation of tides along the coast being conveniently compressed.
After a slack-water dive, by driving the boat a few miles along the coast you can rendezvous with slack water on the previous or next tide only a few hours later.

Because spring tides are bigger than neap tides, more water has to move. Springs give bigger currents, and usually shorter slack water.
Once you start thinking about tides as very long waves, all the other things we have mentioned that affect the wave of the tide also affect the time of slack water when the wave is bigger or smaller.
Between neap and spring tides, the time of slack water could easily be half an hour earlier
or later for the same wreck. So the skippers of our charter-boats keep notes on slack water for different heights of tide.
With this in mind, other things that affect the height of tide, like a big low-pressure area and
a storm, can also change the time of slack water. We don’t usually have to worry about this, as we wouldn’t usually be diving is such conditions.

Let’s go back to thinking of an incoming tide being a mass of water coming along a channel. One scenario is that all that water piles up at the top of the channel before turning round and going back out.
Another is that it circulates, going up one side of the channel and back down the other.
A further scenario is that it circulates vertically, coming in on the surface and out on the bottom. In practice, all these things often take place to some extent when we get close to the turning of the tide.
All the observations we see about current on charts and tidal stream atlases are what is happening on the surface.
While the time of slack water can vary by a few minutes (or even hours) over a few miles, under water it can vary over just a few metres’ depth.
When the skipper says jump in now, drift to the shot, and it will be slack by the time you’re on the wreck, it may be because the slack water is approaching fast.
However, especially on a deeper dive, there could also be a consideration that it is already slack, or even going the other way on the wreck.