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One of the most common questions one hears amongst divers around Seahouses and Beadnell car parks is "how do you know when slack water is?".
This article should provide the answer to this question......
As most divers will know, the times of high and low tide can be fairly accurately predicted and tide tables are readily obtainable for most ports around the UK. Also, most will know that "Slack-water" rarely coincides with the high and low water times; large volumes of water moving over shallows or through narrow channels can be slowed significantly and can delay or advance the "turning time" by several hours.
Again these times can usually be predicted, however, what can't be predicted are the effects of weather conditions up to several hundred miles distant. These can further affect the "slack" times so the golden rule is: once you've calculated the theoretical time of "slack", plan to arrive early; it's better to sit and wait twenty minutes than to miss the dive!.
All you'll need are tide tables for the area (I prefer tables for Tynemouth/North Shields because the tidal data on the chart is referenced to the River Tyne - this means you don't have to add or subtract the tidal differences between ports).
The data from the tidal diamond closest to the Farnes on my old chart is shown in the adjacent table. From left to right it shows the times before and after HW, the direction of flow and the speed of the water for Springs and Neaps.
I've added the column "Gen Dir" for clarity and from it you can see that the flow changes direction sometime between 5 & 4 hours before and again sometime between 1 & 2 hours after high water.
We can get an even better idea if this data is presented in graphical form (after all, a picture's supposed to be worth a thousand words).
By plotting the values of the current's speed/direction against the times we end up with the following......
The light blue curve represents springs and the dark blue represents neaps.
Where the blue curves cross the "0-knots" horizontal line is the theoretical time of "slack".
The green horizontal band across the centre represents tidal speeds of less than 0.4 knot - speeds less than this can be regarded as diveable, beyond this should be considered difficult for any but experienced divers.
Dropping from the green band are yellow and maroon lines, these show the diveable periods around "slack", yellow for springs and maroon for neaps. By comparing the widths of these bands you can see that during neaps "slack" lasts twice as long as during springs. You can also see that "high water slack" lasts slightly longer than "low water slack".
Most importantly you can see when the diveable periods start - for "low water slack" from about 5 hours twenty minutes to 5 hours before high water at Tynemouth and for "high water slack" from about 50 minutes to 1 hour twenty minutes after high water at Tynemouth (for neaps and springs respectively).
For those with Seahouses (North Sunderland) tide tables I've added a red vertical line marked "Seahouses" just to the left of the vertical centreline marked "Tynemouth", this gives some indication of the tidal difference between the two ports; tides at Seahouses being about 40 minutes earlier than Tynemouth.
The Seahouses tide tables I've got are already corrected for BST so there's no need for me to compensate for BST/GMT differences. This means that at H.W. neaps, if I add one hour 30 minutes, or at springs, two hours, I should arrive at the beginning of "slack". If by chance your Seahouses tables are in GMT, you'd have to increase these figures by one hour (but only during summertime).
Finally, and as an example for those who still can't grasp the arithmetic.....
At the very bottom there are two rows of times labelled GMT and BST coinciding with the horizontal time scale.
If say, we plan to dive the Somali in June (BST) and our Tynemouth tide tables show high water as being at 1pm GMT & they indicate a neap tide (a small tidal range), we should aim to be on-site with our first wave of divers ready to enter the water no later than 3pm BST. This assumes a typical club-type RIB dive with two or more waves of divers entering at staggered times i.e. the first wave enters while the current's starting to slacken and they'll probably surface right on "slack", the final wave go in at this time and they'll surface when the current's starting to pick up in the opposite direction.
So, as a very general rule, if you'e using tide tables for Tynemouth (which give times in GMT format) simply add two hours to the high water time to find the time of "slack" in BST. Using this you'll always be on time for neaps and about 20 minutes early for springs.
And for low water, subtract 4 hours twenty minutes.
A final word of advice. Because they usually dump all their divers in one wave, the charter boats out of Seahouses usually aim to arrive on-site right on "slack" so it's not unusual to think you've got the wreck to yourselves when your first wave enter the water, only to find that twenty-five minutes later there are three or more hardboats manoeuvring amongst your surfacing divers!. It's important to brief your divers to expect congestion on the surface and to use SMBs on the ascent.
These figures are good for the wreck of the Somali off Beadnell, they're also quite useful for the coast from Howick up to Holy Island including the outlying Farnes area around the Knivestone BUT please don't assume they'll be valid within the islands themselves!.
Be safe!! And don't be too proud or arrogant with your new found knowledge to assume that it's accurate. It's never let me down ..... but you shouldn't hesitate to confirm your calculated times with other divers on the day.
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Contact: Dave Cordes.