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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