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Jonah
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I've been trying to get my head round tides, and their implications for dive planning etc. I think I understand the basics, but I was wondering if anyone had a handy website or other reference for a complete explanation.

I think I understand that on spring tides there's more water moving about than on neaps. High tide will be higher on springs and so forth. Does that imply that slack water will be shorter, and currents stronger, on spring tides?

Am I correct in assuming that there is a period of slack water when the tide is at its highest and lowest levels? (i.e. when the current is changing direction).

Assuming there are two tide cycles in a 24 hour period (two high and two low waters) does that mean there are four periods of slack water in the average 24 hour period?

(e.g. see http://www.bbc.co.uk/weather/marine/tides/20040120/0069_20040120.shtml  - would there be slack water at 03.00, 10.00, 16.00 and 22.00 in that example?)

I assume that actual time and duration of slack water will depend on tidal flows and so on. Is there any handy rule of thumb for estimating how long any given period of slack water will last, other than relying on local knowledge?

If any of these assumptions are wrong I'd really like to know!

Thanks in advance for any enlightenment.
Tom
 

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A complete answer  to your post could take a long time - or perhaps you should take a course? RYA Level II boathandling should teach you all you need  see Eclipse boat handling courses
But in summary. Charts have tidle diamonds which will give you the strength and direction of the current through the 12 hours of the tide cycle. These can be used for dive planning but are never as good as local knowledge. you can also purchase tidle stream books which will give you a pictorial view of strength and direction in your area. Slack water does not always occur around high and low water. it depends on the land mass and the effect that mass has on the water movement. Once you get offshore (20 miles or so) slack will be more consistant with high and low water.

Regards


David
 

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<font color='#0000FF'>or Taurus Diving , we don't do courses YET
, but do have a yachtmaster instuctor & a Thames / Harwich pilot , that will be willing to sit down and go though it with you ,as divers here at Dover

& Tom, next time we meet, there are some books on the boat that can help you.

Andy

PS No Dave , i'm not trying to take work away from you  
 

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You'll  find pretty much all you need to know here
http://www.sailingissues.com/navcourse6.html


[b said:
Quote[/b] ]Am I correct in assuming that there is a period of slack water when the tide is at its highest and lowest levels? (i.e. when the current is changing direction
a common misconception, but slack isn't simply at the turn of the tide, you find slack from tidal info on nautical charts

[b said:
Quote[/b] ]Assuming there are two tide cycles in a 24 hour period (two high and two low waters) does that mean there are four periods of slack water in the average 24 hour period?
not quite, a full tidal cycle is about 12hours and 25mins so sometimes there's 2 HWs and 1 LW in a day and vice versa

[b said:
Quote[/b] ]I assume that actual time and duration of slack water will depend on tidal flows and so on. Is there any handy rule of thumb for estimating how long any given period of slack water will last, other than relying on local knowledge?
In a word, no. Have a gander at the online Nav course in the link above. If there's any points that you want to know more about gimme a shout 'cos I'm halfway through my RYA Day Skipper theory course (evening class at my local marine college) and so far we've done about 8 hours all about tides and tide prediction, it's fascinating how much variance there is, once you know how much work has gone into gathering all this info,  you really have to admire the work of the UKHGO and the history behind it

I can highly recommend the RYA DS course, BSAC DL and chartwork courses cover the essentials of tidal predictions etc which is enough for diving purposes, but for other agency divers, the RYA will teach you the whole kit and kaboodle.

HTH
Steve
 

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<font color='#000F22'>Tom, well done for bringing this up mate.

The number of folk that jump in at various dive sites - some of them who should know better - and end up hanging in the breeze - is growing. Some get into real trouble.

I'd advise the RYA couse as it's great fun too but if you;'re not sure of slack then check with someone who KNOWS.

Again, thanks for bringing this up.
 

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[b said:
Quote[/b] (Heads Up @ Jan. 17 2004,16:34)]And don't believe that tides are caused by the moons gravity either! It is only one (and a small one at that) of a large number of variables including currents and centrifugal force of the Earth's spin.
And theres me thinking that tides were responsible for currents.

The moons effect is much greater than the sun, the moon is soooo much nearer. Both, when working together, account for about 16 inches in height of water.

The earth spins upder the water, but there is lots of land in the way, of different sizes and depths. This is responsible for nearly all the height. Just imagine all the water funnelling between England and Wales as the channel gets narrower and shallower, forcing the tidal bore up the Severn.

Understanding the opposite humps of the tides gives the most grief. I always remember my physics teacher explaining that 'centrifugal' force does not exist, it is a reaction to centripetal force you feel. However centrifugal force is easier to understand.

Adrian
 

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DUE CEO, Booking agent, Coffee maker & Dogsbody...
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Tom

Next time you are down i can show you how it all works and where to get all the info off charts and tide tables. It is actually a Dive Leader lesson (DT8), so i can do it on power point and some charts etc in a cafe off my laptop.

That will give you enough to understand and plan. I am actually teaching that at Chatham for the Medway club tomorrow morning.

Dive Safe

Paul
 

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Jonah
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Thanks for the info and the websites, people. I had a suspicion it was a little bit more complicated... things are a little clearer now; at least I know what I don't know if you know what I mean! A couple of my misconceptions have been cleared up so thanks.

I can't see me doing a course at the moment (though I appreciate their value, and the thought of getting to whizz about in David's boat does appeal!). I think the main situation where I'll need to know about this stuff at the moment, given that I'm not driving club ribs about etc, is in planning when I want to dive.

Say I want to book a dive on (say) Eclipse or Taurus. Obviously, in those circumstances *I* personally don't need to work out exactly when slack is on the dive site etc - that's the skipper's job - but I do want to be able to decide, for instance, whether the second or third weekend of march would be a better bet. Similarly, say I'm planning a shore dive. I'd make sure I found out when slack water was, but I assume duration and the strength of any current would be affected by the state of the tide.

I guess what I'm really asking is: Are neap or spring tides 'better', and why?


It seems to me that this stuff is really quite important for planning dives, but I've really never been able to identify how one learns about it other than some kind of boathandling course. That's why I was hoping someone would be able to point to a book or website or other reference that said 'this is what you need to know to dive safely'. It's certainly something that's missing from the PADI system that I came through. Nobody ever told me explicitly - and I had pretty good instructors, I think. I don't know about other agencies.

If it hadn't been for looking at diving-related stuff on the web, the first time I went off diving independently with a similarly trained buddy, I wouldn't even have  known that there was such a thing as 'slack water' and might well have thrown myself into a raging current and ended up in France or somewhere. I get the impression there are growing numbers of non-club, independent divers about nowadays - people like me - and we just don't know about this stuff.

Now, can someone explain quantum mechanics next please
 

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<font color='#0000FF'>Also , other effects comes from Weather & Atmospheric pressure..

Mean atmospheric pressure is 1013 mb. A 1 mb change in pressure results in a 0.01m change in water level. high pressure depresses water levels and low pressure raises them.
 

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[b said:
Quote[/b] (tom @ Jan. 17 2004,19:58)]I guess what I'm really asking is: Are neap or spring tides 'better', and why?
Neaps.

At Neaps, the difference between HW and LW (the 'tidal range') is less, therefore less water movement, ie weaker currents.

Try and book your dives around half-moons (neaps) and avoid full and new moons (springs)

Laters,
   Janos
 

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Charter Boat Skipper, Salvage Diver & YBOD abuser
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<font color='#000080'>You'll also find that Neap LW occurs around midday- HW 6.00 am/pm, and on Springs HW occurs around midday- LW 6.00 am/pm- Handy rule of thumb for guesstimation.
 
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