Slack water

[Drew MacIver]

Help please, i feel so dumb at the moment, everyone talks about slack water and how to calculate it, but when i ask how the explanation just seems to confuse me, can someone please give me it in plain english, when is it, how do you calculate it, is there a standard measurement and where do you measure from, thats the confusing bit, i can find tide tables, anyone please? help?
I assume some people will think i am an idiot, but when they mention it in the club they seem to assume it is that easy it doesnt need explaining,
Cheers

[Adrian Kelland]

Not easy to explain without a chart and some time tables in front of us;

Slack water at a location is a time before of after a particular state of tide, usually high water or low water, at another location.

So I might describe slack water on the Maine, off the S Devon coast, as 3 hours after HW at Devonport. I could also describe it as 2.5 hours after HW at Exmouth Dock, on a neap tide. (Exmouth Dock has a High Water about 30 minutes after Plymouth.)

Slack is an approximate time and condition. It is tide based and thus affected by local weather and currents. It can be earlier or later than predicted.

Tidal diamonds on charts can help you identify a likely slack time and speed - slack does not mean no current, just the least. However local knowledge can be essential on some site and the local land shape and water movements can mean a slack that is very different to that indicated by a diamond 5 miles away.

If they think tides are easy, ask them why, if the water is pulled by gravity towards the moon, why there is a hump on the opposite side of the earth? :D

[Nigel Hewitt]

If they think tides are easy, ask them why, if the water is pulled by gravity towards the moon, why there is a hump on the opposite side of the earth? :DBecause the Earth is falling towards the Moon faster than the water on the far side. Just like the water nearest the Moon is falling faster than the Earth.

Thankfully the moon keeps getting out of the way or the crunch would be 'Earth shattering'.

If things are pulled they move.

Don't get me started on space curvature...

[Adrian Kelland]

Because the Earth is falling towards the Moon faster than the water on the far side. Just like the water nearest the Moon is falling faster than the Earth.

Thankfully the moon keeps getting out of the way or the crunch would be 'Earth shattering'.

If things are pulled they move.

Don't get me started on space curvature...
Nice try.

I recall the problems start with an assumption that the moon goes around the earth...

[Drew MacIver]

Nice try.

I recall the problems start with an assumption that the moon goes around the earth...


This i understand......


....Still not sure about slack though...lol

Guess i need to be sat down with a load of charts then Adrian, and it explained using small words, or maybe i am just an idiot!!!!!!

cheers

[Ed Howarth]

I always suspect when you ask someone a question and they almost belittle you for asking it, that they don't know the real answer! Doubly so if they explain it, but do that in a way that is impossible to understand.

Way back when, someone would sit on a boat tied to an anchor and note when movement of the water was at its least. They marked the point on a chart, wrote down the time in relation to when high water, or low water, occurs at somewhere nearby. They then repeated the exercise for various states of the moon and noted it all. They then moved to somewhere else and did the whole thing over again.

So there is no "formula" to calculate when slack might be. But if you have yon chap's scribblings you can have a pretty good guess. The thing is, that wind, nearby pressure systems and even rainfall can alter the timings. Also, the readings taken are only truly true at the observed point. Move a few hundred yards just around the headland and the timings will be different.

When you look at the tidal diamonds on a chart, you often find them where they are most useful to boats and yachts and things, ie in the middle of a channel or somewhere near the entrance to a harbour. Wherever your dive site is, I bet there not a tidal diamond immediately on top of it. Its sod's law, that is.

So what can you do? Well, look for the nearest diamond, or better still find two diamonds on either side and average the readings. Bet you that doesn't work very well either!

Best thing of all is, ask someone to sit on a boat at your dive site and write down when slack is, just like they used to do. They can then put it in a magazine, book or internet for all to see. And if you can't find it written down anywhere, you can go and do it yourself and put it on the web.

I believe Chris Holden who writes some excellent dive books puts shot buoys on sites and watches them from the shore with binoculars over a week or two to note when slack occurs. Dedication.

Ed

[Drew MacIver]

I always suspect when you ask someone a question and they almost belittle you for asking it, that they don't know the real answer! Doubly so if they explain it, but do that in a way that is impossible to understand.

Way back when, someone would sit on a boat tied to an anchor and note when movement of the water was at its least. They marked the point on a chart, wrote down the time in relation to when high water, or low water, occurs at somewhere nearby. They then repeated the exercise for various states of the moon and noted it all. They then moved to somewhere else and did the whole thing over again.

So there is no "formula" to calculate when slack might be. But if you have yon chap's scribblings you can have a pretty good guess. The thing is, that wind, nearby pressure systems and even rainfall can alter the timings. Also, the readings taken are only truly true at the observed point. Move a few hundred yards just around the headland and the timings will be different.

When you look at the tidal diamonds on a chart, you often find them where they are most useful to boats and yachts and things, ie in the middle of a channel or somewhere near the entrance to a harbour. Wherever your dive site is, I bet there not a tidal diamond immediately on top of it. Its sod's law, that is.

So what can you do? Well, look for the nearest diamond, or better still find two diamonds on either side and average the readings. Bet you that doesn't work very well either!

Best thing of all is, ask someone to sit on a boat at your dive site and write down when slack is, just like they used to do. They can then put it in a magazine, book or internet for all to see. And if you can't find it written down anywhere, you can go and do it yourself and put it on the web.

I believe Chris Holden who writes some excellent dive books puts shot buoys on sites and watches them from the shore with binoculars over a week or two to note when slack occurs. Dedication.

Ed
Thank you Ed, that makes sense to me, and i think i understand it now, well sort of! lol.
Cheers
Drew

[Nigel Hewitt]

It a bit of local knowledge but for a lot of sites it is published.

Here we use the tide tables for Brighton Marina and for most of our range slack is about an hour. It starts on the low tide and 1 hour before high. Beware: Tide table are virtually always in GMT so you add an hour in summer-time.

When we're away from home we pull out the map and look for the diamond and then refer to the table. You still need tides for the reference point to use the offsets but a quick web search normally trawls them up in moments.

[PeteM]

Help please, i feel so dumb at the moment, everyone talks about slack water and how to calculate it, but when i ask how the explanation just seems to confuse me, can someone please give me it in plain english, when is it, how do you calculate it, is there a standard measurement and where do you measure from, thats the confusing bit, i can find tide tables, anyone please? help?
I assume some people will think i am an idiot, but when they mention it in the club they seem to assume it is that easy it doesnt need explaining,
Cheers

Bit more info...

This is a tide table from a chart
http://en.wikipedia.org/wiki/File:Nautical_chart_tidal_diamond.PNG

This one is showing three sets of tidal information. I will refer to A for this.

The first bit is the position on the chart 50 42.3N and 0 26.5E which from memory is Eastbourne/Beachy Head area. You don't need to worry about this as the diamonds are marked on the chart so you just find the closest one(s).

Left column-Hours. This is the time relative to high water at the reference port of the chart. Note this is important as it is not necessarily local high water. References ports are a number of normally big ports that official high tide is published for. Dover, London Bridge, etc. Which port to use is marked on the map so you can look up high water.

Column two-Dir. The direction that the water is flowing. Not really relevant to us as we normally do not care if it is going North/South/Round in circles. Might be important if you are doing a drift dive though.

Columns three and four-Sp and Np. These are the speed columns and are what we need. Sp is the speed on a Spring tide, Np on a Neap. You can roughly work out what things are going to be between spring and neap by averaging. As a general finger in the air rule we want to dive with less than a knot of current but less than half a knot is idea and may be a requirement for in experienced divers.

So lets look at the table. For the example let assume high water at the reference port is 14:00 today and it is a spring tide.

The first speed @-6 (14:00-6=10:00) is 0.8 which is diveable but -5 is better at 0.5. But -4 is running pretty fast at 1.9, so you would want people well out of the water for that. So we can say the predicted low water slack is roughly from 10:00 to 11:30. I would be planning to put divers in the water a little after 10:00 with a max dive time of an hour.

Now we scan down for the next bit of slow movement, this is at HW (14:00). This is a very nice 0.1 knot increasing to 0.9 @+1 and 1.4@+2. This is a nicer looking tide, the speed is slower but the build up is also gentler which makes life easier in the water. Because HW is only 0.1 and -1 is 1.2 knot it is a reasonable guess that it will be slack enough to dive from about 13:30 and will be OK for an hour and a half from there. So I would plan to be on site about 13:00 ready to put the divers in as soon as it was slack.

As others have said though this is just a prediction and is only really valid at the diamond, away from the diamond it is less reliable - add this to a vagaries caused by weather, pressure, the water swirling round the wreck etc. if you can get observed tides on the actual site it is far better. But it can be a good starting point

[Drew MacIver]

Bit more info...

This is a tide table from a chart
http://en.wikipedia.org/wiki/File:Nautical_chart_tidal_diamond.PNG

This one is showing three sets of tidal information. I will refer to A for this.

The first bit is the position on the chart 50 42.3N and 0 26.5E which from memory is Eastbourne/Beachy Head area. You don't need to worry about this as the diamonds are marked on the chart so you just find the closest one(s).

Left column-Hours. This is the time relative to high water at the reference port of the chart. Note this is important as it is not necessarily local high water. References ports are a number of normally big ports that official high tide is published for. Dover, London Bridge, etc. Which port to use is marked on the map so you can look up high water.

Column two-Dir. The direction that the water is flowing. Not really relevant to us as we normally do not care if it is going North/South/Round in circles. Might be important if you are doing a drift dive though.

Columns three and four-Sp and Np. These are the speed columns and are what we need. Sp is the speed on a Spring tide, Np on a Neap. You can roughly work out what things are going to be between spring and neap by averaging. As a general finger in the air rule we want to dive with less than a knot of current but less than half a knot is idea and may be a requirement for in experienced divers.

So lets look at the table. For the example let assume high water at the reference port is 14:00 today and it is a spring tide.

The first speed @-6 (14:00-6=10:00) is 0.8 which is diveable but -5 is better at 0.5. But -4 is running pretty fast at 1.9, so you would want people well out of the water for that. So we can say the predicted low water slack is roughly from 10:00 to 11:30. I would be planning to put divers in the water a little after 10:00 with a max dive time of an hour.

Now we scan down for the next bit of slow movement, this is at HW (14:00). This is a very nice 0.1 knot increasing to 0.9 @+1 and 1.4@+2. This is a nicer looking tide, the speed is slower but the build up is also gentler which makes life easier in the water. Because HW is only 0.1 and -1 is 1.2 knot it is a reasonable guess that it will be slack enough to dive from about 13:30 and will be OK for an hour and a half from there. So I would plan to be on site about 13:00 ready to put the divers in as soon as it was slack.

As others have said though this is just a prediction and is only really valid at the diamond, away from the diamond it is less reliable - add this to a vagaries caused by weather, pressure, the water swirling round the wreck etc. if you can get observed tides on the actual site it is far better. But it can be a good starting point


Thank you pete, i actually got that, thank you, 1 small thing though mate lol,

you said " The first speed @-6 (14:00-6=10:00)", surley that should be "The first speed @-6 (14:00-6=08:00)", lol, i actually did understand it, lol
Cheers mate, very informative,
Drew

[ChristianG]

Here in Oz most places are not tidally affected. Note "most", because there are some raging tides on this small continent, it's just that these areas are largely not dived, if only because of "black water" and, I suppose, also the fact that there's nothing much to see there anyway. Broome (http://www.broomevisitorcentre.com.au/), in the northerly part of Western Oz is one such.

Having said that, there are some very favourite places that are tidally affected and if you miss it you're on the way to New Zealand (in my instance). The mouth of Lake Macquarie (Swansea Channel) (http://www.michaelmcfadyenscuba.info/viewpage.php?page_id=145), just to my north, is one as is Halifax Park (http://www.panoramas.dk/fullscreen2/full30.html) a tad further to my north and these are two of my favourite places to dive. Here in NSW most tides are measured against that of Fort Denison in Sydney Harbour (too big to be tidally affected to any significant extent) and, from memory, the above two peak around four hours after Fort Denison. Regardless I always ask the locals before venturing there, better to be safe than sorry as you switch on that EPIRB that you don't actually have on your person.

[PeteM]

Thank you pete, i actually got that, thank you, 1 small thing though mate lol,

you said " The first speed @-6 (14:00-6=10:00)", surley that should be "The first speed @-6 (14:00-6=08:00)", lol, i actually did understand it, lol
Cheers mate, very informative,
Drew

Pete rules of forumdynamics #1:- In any sufficiently long and complex posting there will always be one stupid error

[Woz]

Help please, i feel so dumb at the moment, everyone talks about slack water and how to calculate it, but when i ask how the explanation just seems to confuse me, can someone please give me it in plain english, when is it, how do you calculate it, is there a standard measurement and where do you measure from, thats the confusing bit, i can find tide tables, anyone please? help?
I assume some people will think i am an idiot, but when they mention it in the club they seem to assume it is that easy it doesnt need explaining,
CheersIt's not "easy". Slack varies alot and rarely is it at high or low water. Off Weymouth, it's 3hrs after so slap bank in the middle!

Tidal diamonds are a help but local knowledge is invaluable. As is turning up a bit early and waiting!

[Drew MacIver]

Pete rules of forumdynamics #1:- In any sufficiently long and complex posting there will always be one stupid error

lol, is that a rule anyone can follow mate? cos i can think of a few times i needed it, rofl.

[Mike Halligan]

Fantastic explanation, thanks. I noted that the time of slack on the outside of the Mersey differed from that on the inside as it bends round from a NW to N direction at (roughly) the Irish Ferry berths before spilling into the Crosby Channel. Neither was very far from the time predicted at Alfred Dock, but the differ!

[m1cdq]

if diving an unknow area i would check with local fisherman, pilots, harbour masters ect .

Because where i work it not worth diving zero vis. but when the comercial divers have to go in often they might only have 30 mins in the water or less .
As we are on a river you may appear to have slack water but the tide may have stopped and gone slack but you still have the river water running out on the bottom.Like other have said local weather conditions can change things.more so on an easuary has there been a lot of rain inland and increased the flow of the river. is the wind with or against the tide

[neil_richardson]

Here in Oz most places are not tidally affected. Note "most", because there are some raging tides on this small continent, it's just that these areas are largely not dived, if only because of "black water" and, I suppose, also the fact that there's nothing much to see there anyway. Broome (http://www.broomevisitorcentre.com.au/), in the northerly part of Western Oz is one such.

Having said that, there are some very favourite places that are tidally affected and if you miss it you're on the way to New Zealand (in my instance). The mouth of Lake Macquarie (Swansea Channel) (http://www.michaelmcfadyenscuba.info/viewpage.php?page_id=145), just to my north, is one as is Halifax Park (http://www.panoramas.dk/fullscreen2/full30.html) a tad further to my north and these are two of my favourite places to dive. Here in NSW most tides are measured against that of Fort Denison in Sydney Harbour (too big to be tidally affected to any significant extent) and, from memory, the above two peak around four hours after Fort Denison. Regardless I always ask the locals before venturing there, better to be safe than sorry as you switch on that EPIRB that you don't actually have on your person.


ahh,, see you've forgotten to mention Port Phillip :-)

Possibly the most treacherous stretch of water in all of Australia.. Also with some of the best diving and wrecks.

Not unheard of to bump into Orca's, humpbacks, southern rights, bronze whalers, wobby's, port jacksons, pods of dolphins, mega rays etc etc etc... and yehh,, one of those dive spots where slack water can or cannot happen, and is not only tide and moon dependant, but wind dependant as well. Give us a strong Northerly, and there is no high tide, give us a southerly, and no low tide...

Sub surface currents have been known to reach up to 12-15 knots, or down to 1-2knots, while the surface speeds remain relatively constant at between 3-9 knots; and also on some days we may have 3 or 4 slack waters, other days 0...

incredibly interesting body of water is Port Philip bay,,, if you can understand the tidal streams around here, i reckon you can pretty much understand them anywhere....

To show you:
http://www.ppsp.com.au/passage-planning/environmentally.aspx

http://www.livenowfishtothemax.com.au/admin/FactSheets/Understanding_The_Tides.pdf

http://www.tide-forecast.com/locations/PortPhillipHeads-Australia/tides/latest
http://www.bom.gov.au/oceanography/tides/MAPS/lonsdale.shtml#form

In the last one, compare the port phillip heads table, with the port phillip streams table.. makes for interesting reading :-)

Neil

[ChristianG]

In the last one, compare the port phillip heads table, with the port phillip streams table.. makes for interesting reading :-)
Port Phillip? Isn't that "down Mexico" way? ;) :D

Curiously enough I today received a small book entitled "big tide country" all about the tides on the Kimberley Coast (Broome). Apparently these are the fourth highest in the world, the highest in the Indian Ocean and the highest in the southern hemisphere (I knew Oz was good for something ;)). Google it if you want to know more, it tells you in layman's terms how these tides are generated and why. Good stuff.

As for Port Phillip, I just don't know. Although I occasionally get to that neck of the woods, and first settled there when I came to Oz 50 :eek: years ago, I must say that I prefer other things about Melbourne, some of the restaurants for example. :D

[PeteM]

Curiously enough I today received a small book entitled "big tide country" all about the tides on the Kimberley Coast (Broome). Apparently these are the fourth highest in the world

7m according to Wikipedia. Baby tides :D

Bristol Channel gets double that and somewhere like Brighton or Eastbourne where the tides are considered "normal" are in the 6-7m range

[ChristianG]

7m according to Wikipedia. Baby tides :D
Oi, it's big by southern hemisphere standards. ;)

[Tristan Green]

7m according to Wikipedia. Baby tides :D

Bristol Channel gets double that and somewhere like Brighton or Eastbourne where the tides are considered "normal" are in the 6-7m range
Maybe Wikipedia needs an update. Spring tidal range 9.5 m according to the Port Authority:

http://broomeport.wa.gov.au/about-broome-port-authority

Maybe not in Broome itself, but nearby they experience 10 m tidal range at the 'horizontal falls' during spring tides:

http://en.wikipedia.org/wiki/Horizontal_Falls

Cheers,
Tristan

[Mark Mumford]

Help please, i feel so dumb at the moment, everyone talks about slack water and how to calculate it, but when i ask how the explanation just seems to confuse me, can someone please give me it in plain english, when is it, how do you calculate it, is there a standard measurement and where do you measure from, thats the confusing bit, i can find tide tables, anyone please? help?
I assume some people will think i am an idiot, but when they mention it in the club they seem to assume it is that easy it doesnt need explaining,
Cheers

You may find that the following link helps explain it.

http://www.bsac-se.org.uk/Slacks.asp

[Keith Littlebury]

I too was mystified by all the different slack times we would come up with on dive days and inadequate explanations. The reason people dont fully explain it is because it takes too long and is easier to give the quick way which you dont have to understand. But here is how you work out slack;

1. The current in the sea flows in one direction and it gradually slows down, turns and changes direction to go in the opposite direction. When it slows down enough to get in and dive without being swept away that is the start of slack. This is about 0.5 knots of current. When it speeds up again to 0.5 knots and you feel like you want to get out, that is the end of slack. Slack in my region lasts 45 - 78 minutes.

2. The tidal diamond nearest to where you want to dive will give you the speed and direction of current at hourly intervals before and after high tide for spring and neap tides so you can see when the current is at its slowest. Ideally you need to plot these figures into an excel spreadsheet and make a graph which will give you two curves - one for spring tide and one for neap tide. It is not as complicated as it sounds and once you have done the graph for that tidal diamond you dont have to do it again and it all becomes crystal clear.

3. From the graph you can tell when the current will slow down to 0.5 knots and can predict slack very accurately and at a glance by knowing the phase of the moon and where the tide is between spring and neap.

In my part of the UK for instance (Haisborough Sand) slack starts between 1h 37min and 1h 15min before HW at Immingham and ends 43 - 78 mins later.
The next slack is 4h 25min to 4h 45min after HW Immingham and lasts 44 - 60 mins. So the start of slack varies by about 20 mins between spring and neap tides.

So therefore we can say that slack at Haisborough Sand tidal diamond F is ROUGHLY 1.5 HOURS BEFORE OR 4.5 HOURS AFTER H.W. IMMINGHAM.
And as long as we get to the dive site in plenty of time to find the wreck and have enough current to help hooking in (i.e. an hour early if I'm driving), we can sit and wait for slack. Because, as they always say, its better to be too early than too late.

You now have to do graphs for all the other tidal diamonds on the chart near where you want to dive and find the best average depending on how accurate you need to be.

Hope that helps. BSAC SE region have explained all this on their website.

[rockhopper69]

Now im confused lol...I thought generally when snorkelling you would be snorkelling tight into the coast ?, so tidal diamonds and the likes would not matter , I thought they were aimed more at boating further out ? I know when im out kayaking and high water is at 14.00 the tide still runs from nw to se for quite sometime ive never done any courses or anything like that so carnt say for how long exactly it still runs that direction .
When im out snorkelling i just try to aim my dives around low water or hw , as these are the times ive always classed as slack water off the shore ? altho i know further off its a dif story ,
Strong Tide is something ive never experienced in the water , If i happened to be snorkelling around low water and continued diving well into high water ,if it happened to be an area with a bit of tidal movement ie you could see the water flowing from left to right or vice versa , would it be to strong for me to swim against it into the shore , even if i was wearing a good powerful pair of fins ?

[BigBlueTech]

First read this

http://www.coos-bay.net/understanding-tides.html

Then this

http://www.stripersonline.com/surftalk/showthread.php?t=551156

And this one for the uk version

http://en.wikipedia.org/wiki/Tidal_diamond

And if you still don't understand you can fly out here and do a BSAC course which will cover it :)

[ChristianG]

would it be to strong for me to swim against it into the shore , even if i was wearing a good powerful pair of fins ?
Well, I guess it depends how far from shore you are in the first place as well as how fast the tide is flowing but in my experience a SCUBA diver (not as streamlined as a snorkeller) is hard pressed to maintain even 1 knot for any significant amount of time. That translates into 1.149 mph or 1.85 km/h. For my part, I know that I wouldn't want to do 1 knot for more than a few minutes yet there are plenty of currents, including (particularly?) tidal ones, that can easily exceed that speed, often by some margin.

Oh, as for a "good powerful set of fins", all that means is that you'll tire more easily. Here in Oz swimmers are taught that to counter Rips (http://www.watersafety.vic.gov.au/CA256DC70017DA1D/page/At+the+Beach-Information+on+Rips?OpenDocument&1=10-At+the+Beach~&2=20-Information+on+Rips~&3=~) you should swim parallel to them until you get out of it. Unfortunately that doesn't apply to tidal "rips" or general currents.