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Thread: how to estimate slack water

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    how to estimate slack water

    One of my main drivers for buying a rebreather is to get more bootom time in on dives. Based on experience / v-planner, measuring SAC rate, and swim speed in caves, I have a good idea of the run times (limited by baiilout gas) and the distances round wrecks I can cover in a given time. Have a few fudge factors for bottom time based on how high the wreck rises from the bottom too. I understand that spring tide slack is MUCH less than neap tide slacks, and that high water slack windows are often longer than low water slack windows. Are ther any easy / basic ways to estimate / predict the slack water window available for UK sea diving? I don't mind going in 5 to 10 mins before slack, coming up 5 to 10 mins after slack water, so only looking for an estimate +/- 5 mins resolution. Have looked to tide plotter software etc, but they are accurate at prdicting tides, can't see any obvious way to predict slack duration. Not even sure what the normal ranges are... (20 mins spring tide to 1hr+ neap tide?)

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    Re: how to estimate slack water

    Quote Originally Posted by gallathan
    Are ther any easy / basic ways to estimate / predict the slack water window available for UK sea diving?
    Depends what you call easy / basic.

    The only way to get the 'accurate' times of slack (this is a relative term as they are always at best a SWAG or a historical average) is to use a tidal atlas, or a chart of the area (use the tidal diamonds).

    Each area will have a different slack time depending on lots of factors (tidal range, topography, moon cycle, recent/present weather) so giving a general best time is not possible - you need to know how to work out the tides and then use the methods mentioned.

    Of course local knowledge is always good for small areas, fishermen or local divers generally know when the 'best time' is for a certain wreck, dive site or localised area but otherwise it is not that simple!

    Hope this helps

    Neil

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    Re: how to estimate slack water

    I've got to agree with the above post. Work it out using tidal diamonds where you have to but if you can get local info then that is always more accurate.

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    Re: how to estimate slack water

    I agree. Ask a fisherman. Or a harbourmaster. A wealth of knowledge, and right more often than any other source in my experience.

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    Re: how to estimate slack water (around the British Isles)

    Apologies if the following is stating the obvious...

    Firstly around Europe Spring Tides occur about three days after a full moon. A lot of diaries carry the phases of the moon so that can be used as a reference. Also Spring Tides have a greater range in the Northern Hemisphere winter than the summer (the sun is further away in the summer so there's less gravitational effect). So the good news is that slacks are generally longer in the summer.

    Using a suitable Admiralty Chart and the tidal diamonds is a good start (get the one that covers just the area you are interested in rather than just the English Channel) but bear in mind these are always referenced to a specific port (i.e. Dover) so High Water at the diamond will be given in the related table as so many hours before or after High Water at that port. So you also need a set of tables for the port for the period in question (i.e. the 2006 Silk Cut Almanac) and then apply the offset.

    An alternative is to buy a set of local tables (dive shop, fishing tackle shop or similar). These are often given in local time (i.e. corrected for BST/GMT) whereas tables like the Silk Cut work in GMT only.

    If you don't want to plan too far ahead (i.e. <7 days) then
    http://easytide.ukho.gov.uk/easytide...ide/index.aspx
    may help with the basic tidal information.

    These tools will only give you the time of HW/LW it may not give you the time of SLACK water. There are many places around the British Isles where the slack is before or after HW/LW by a substantial period of time and places such as Lyme Bay are notorious for having two HWs in one 12 hour period. Some wrecks even have their own tables!

    For wrecks getting a copy of "Dive...." (i.e. Dive Dorset) will help in determining when Slack water occurs with respect to HW/LW and when the longest slack occurs.

    If you really want local diving info' then try and find the local dive club in the area (i.e. start with www.bsac.org) as they will tend to dive the area you are interested in regularly and may be able to fine tune the "two hours after HW at..." that you can get in the manner described above.

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    Re: how to estimate slack water

    Quote Originally Posted by gallathan
    ... so only looking for an estimate +/- 5 mins resolution. ...
    As the currents are related to the rate of rise of the tides, the following may help you. You will still need tide tables, and you have to accept that the tables are not always accurate for the exact spot where you are diving (underwater contours, ...). If you need to be more accurate, consult the appropriate charts with tidal diamonds.

    Rule of Twelfths
    If a tidal chart is not available, the rule of twelfths can be used to estimate the water’s rate of rise. First calculate the tidal range using the difference in water height between the tide's peak and trough. Divide this number by 12. If one assumes the tide will rise over a 6 hour period, it can be estimated to rise 1/12th during the first hour, 2/12ths during the second, 3/12ths during the third, 3/12ths during the fourth, 2/12ths during the fifth, and 1/12th during the sixth hour.
    Although based on the assumption that the tide follows this symmetrical flow pattern during a 12-hour cycle between successive high waters, the rule nevertheless provides enough accuracy for diving purposes.
    Hence, two hours after the HW the water has fallen 3/12ths of the full range.

    (IART CCRII manual)

    So if you have some idea of the max current, you can use the maths to calculate how it changes (approximately) during the tides.

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    Re: how to estimate slack water

    Quote Originally Posted by gallathan  View Original Post
    One of my main drivers for buying a rebreather is to get more bootom time in on dives. Based on experience / v-planner, measuring SAC rate, and swim speed in caves, I have a good idea of the run times (limited by baiilout gas) and the distances round wrecks I can cover in a given time. Have a few fudge factors for bottom time based on how high the wreck rises from the bottom too. I understand that spring tide slack is MUCH less than neap tide slacks, and that high water slack windows are often longer than low water slack windows. Are ther any easy / basic ways to estimate / predict the slack water window available for UK sea diving? I don't mind going in 5 to 10 mins before slack, coming up 5 to 10 mins after slack water, so only looking for an estimate +/- 5 mins resolution. Have looked to tide plotter software etc, but they are accurate at prdicting tides, can't see any obvious way to predict slack duration. Not even sure what the normal ranges are... (20 mins spring tide to 1hr+ neap tide?)
    not to put too fine a point on it slack water at any given position has very little to do with high and low water and everything to do with how water is moving around an obstruction such as the UK or a large rock or when the outflow current from a river is balanced by the inflow current of the tide.

    I dive one wreck that has about 8 slack water periods a day as it is in the confluence of two tidal streams.

    actual tidal streams are not that predictable because of the complex variables in topography so you really need to measure it. Luckily the Royal Navy have a vested interest in this subject and have spent hundreds of years surveying and recording and tabulating tidal flows.

    buy a copy of the Admiralty tidal stream atlas for the UK or look at a website such as http://www.visitmyharbour.com/articl...pentland-firth and look at the one for the area you wish to dive and look for when it says SLACK (some places are never 'slack' the tide continues to flow and just changes direction) these are referenced to hours before or after HW dover so you need a copy of Dover tide tables (or correct to a local port)

    If you really want to get into dive planning and gain some basic form of understanding of the way the sea flows around the UK* I'd recommend doing a BSAC chartwork and position fixing course or an RYA coastal skipper theory course.

    *you'll never understand how it works all the time as storm surges etc cock it up but it will give you a basic understanding of how we think it works.

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    Re: how to estimate slack water

    OP posted in 2006,

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    Re: how to estimate slack water

    bollocks. don't I feel a knob.

    it was at the top of the most recent list so I replied without looking at the date of the OP. the message from the spammer that put it to the top of the list has since been deleted.

    ah well thats five minutes of my life I won't get back.

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    Re: how to estimate slack water

    It was still a good reply

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