...The letter "Z" (zulu) is conventional shorthand for Universal Time (UT) or "GMT".
....Clocks set to GMT read 1200hrs as the sun 'traverses' the Greenwich Meridian
... and a shadow cast in Greenwich Reach lies along a true North/South line
.................as Big Ben rings out the first of his midday chimes ... in "Winter Time"
"Zulu" is the only [maritime] signal flag to use four colours.
"ZULU" signifies Greenwich Mean Time is in use.
Here ends the brief instruction on how to read time.
Let us proceed now to reading Thames Estuary tides.
"Nae man can tether time nor tide". (Rabbie Burns 1791) ... and tides read time from the moon.
In theory, tides should flood for a little over six and a quarter hours and ebb for about the same time.
On the open sea this is reasonably accurate. Tides are Gaia's breathing in and out to the beat of the moon
On the Thames they do no such thing! ... Tides vary enormously in timing on the River.
Rivermen often observe the flood to be over seven hours and the ebb nearer five.
The flow rate at a given point tends to be faster on the ebb than on the flood.
Under "ideal" conditions the water flows at almost 8kts (10mph) in places.
The following example was plucked from predicted tides some years ago and
illustrates the fickleness of Thames tidal waters over time and distance
To keep it simple only the first high for each day is recorded here.
When the daily variations were averaged, it seemed the tide was an hour later each day
but the extremes showed one day at 82min and another only 42m!
FRI 0245z + 42m
= SAT 0327z + 46m
= SUN 0413z + 52m
= MON 0505z + 66m
= TUE 0611z + 82m
= WED 0753 !!!
Dealing with Thames tides in reality is erratic to say the least. Study the tidal predictions by all means but also observe what is happening in the River day by day. London VTS broadcasts actual tidal measurements as part of their half-hourly River Broadcasts. (H+15&+45 Ch14 "upriver" and H+00 &+30 on Ch68 "downriver"). Among the data supplied on the River Broadcasts may be how close the height of the tide at that moment is to 'predictions'. It is frequently over 'prediction' on the flood, showing that the flood is running 'earlier' than predicted. Observation of slack high water under Tower Bridge shows Top'O'Tide sometimes 20/30 minutes after prediction. Eventual top height is rarely as predicted either. As a student, always couple theory with practice and don't get cocky. Listen to the watermen. Make your own observations. The better you observe the better you will be. You need to keep a broad mind about tides but with all the variations the rules are the same.
Springs occur at more or less the same time of day for a given place. For the Thames Estuary this is 'afternoon'. Watermen say "the nearer a London Bridge tide is to 3oc the higher it is likely to be" but I have not managed to prove them right or wrong. In general, night time tides lift higher than daytime tides and Springs usually peak a couple of days AFTER full moon and AFTER new moon. Also, "new moon" Springs tend to be higher than "full moon" Springs.
Thames Estuary and River tides top progressively later (and higher) as you go upstream towards Teddington. As the tide progresses upstream, the available space becomes narrower and 'shallower'. Tide gets progressively deeper and also move faster ... but not on a linear scale. This may be important to know and understand in navigating between Tilbury and London Bridge and also between Limehouse and Brentford.
Tide/time predictions from Walton on the Naze, via Margate, to London Bridge for the same random day mentioned above.
The numbers speak for themselves.
Note that when tide is full out it is much the same height above chart datum for all locations.
Walton, 0327z 4.2m 0949z 0.3m
Margate, 0345z 4.8m 1022z 0.5m
Southend, 0419z 5.7m 1042z 0.4m
Coryton, 0431z 6.1m 1054z 0.5m
Tilbury, 0449z 6.5m 1117z 0.4m
Woolwich, 0520z 7.0m 1146z 0.4m
LondonBr 0558z 7.1m 1223z 0.5m
Depth of water above chart datum is pretty much the same for all points at Low Water, but High Tide at London Bridge is almost 4m higher than at Walton-on-the-Naze. Even Southend is 1.5m higher! Water is somewhat elastic and more than somewhat unpredictable. Your business is to try to understand it; leave predictions to 'experts' and listen to the river broadcasts on Ch14 and Ch68 for exact figues at set times to compare reality with calculations.
Very accurate methods of working out a height of tide for a given time are fine for RYA exams. Reality is something else.
Tide tables give predicted times and heights of high and low water for each day of the year. They are arrived at by use of historical data, mathematical formulae and, nowadays, computer models. Events frequently happen as predicted and give a starting point for what to expect and when. A reasonable way of figuring how much water is under your boat is the "rule of twelfts"
The Rule of Twelfths.
Starting from slack water at low tide, flow rate is not in any way 'linear', but speeds up at an increasing rate until half tide and then 'slows down' until full tide. It has been observed that the tide lifts 1/12th of its range during the 1st 'hour', 2/12ths in the 2nd 'hour'. 3/12 during the 3rd AND 4th hours, 2/12 during the 5th hour and finally 1/12th in the 6th hour and this observation has been shown to be reasonable everywhere.
1/12 + 2/12 + 3/12 + 3/12 + 2/12 + 1/12 = 12/12 ! An 'hour' is taken to be 1/6 of the time between low and high or vice-versa!
The following predictions were published for North Woolwich in Jan 2006 (recall that the letter 'z' means UT or GMT)
20/1709z 6.4m, 20/2327z 1.2m, 21/0511z 6.3m, 21/1121z 1.1m, 21/1749z 6.3m, 21/2327z 1.2m
The range we were interested in that day was between 0511 and 1121 on 21st. We needed to float into a 'dock' at slack water and High Tide. Slack water so we did not have to contend with currents and High Tide so we had as much water as possible under us. Once we were in and moored the tide had to fall til the keel rested on the cross beams. This would occur with less than 3metres of water over the beams. (the draft was 2.9m, it being the Brixham Trawler, Kenya Jacaranda!) We then waited for the tide to drop 2.9metres until the beams 'dried' and we could walk on them. For what it's worth the drying height of the cross-beams upon which the ship rested is about 2.3metres above "Chart Datum" and all this had been checked out previously
Using the Rule of Twelfths for the tidal range of 5.2m, 1/12 is 0.433m. For the time range we used a rule of sixths between 0511 and 1121 (6hrs10mins) to get six time intervals of about 62min. each (for that day and for that tide and also a prediction!) WE predicted that at about 0635z, the [rear end of the] keel would touch the beams. By 0745 she would be firmly grounded and would begin to lean over against the WALL because we had sparred her to shore. The beams should 'dry' about 0915 on 2.4m of tide. But we would not be able to stand under her port side til the water had receded another 0.5m or so about 0945. We would then have 1hr45min til turn of tide and a similar interval til the water was around our ankles again. So the total 'working time' available to the team was 3hrs 15mins between 0945 and 1300.
On that day that was EXACTLY what happened!
Predictions became facts to our great relief.
We mended a hole in the bottom which had been letting in 600gals of water in 24hrs!
"Reassembly was a reversal of the dismantling process but much more relaxed"