The letter "z" (zulu) is conventional shorthand for Universal Time (UT) also known as GMT. Clocks set to GMT read  1200hrs as the sun 'traverses' the Greenwich Meridian, . A shadow cast in Greenwich Reach lies along true North/South as Big Ben rings out the first of his 12 midday chimes. Fancy that! 

Alongside and above is the only flag in the set which uses four colours. It also signifies that Greenwich Mean Time is in use. Here ends the brief instruction on how to read a clock. Let us proceed now to tides. The two are inextricably linked in that "Nae man can tether tide or time". Burns 1791

As a Rule of Thumb, tides ebb for a little over six hours and flow for about the same time. In reality they do no such thing. Tides vary enormously in timing in the Thames estuary. Experienced rivermen observe that the flood is nearer seven hours and the ebb nearer five, so that the flow rate at a given point tends to be faster on the ebb than on the flood.

Following are predicted high tides for a Thames estuary port over a random long weekend in February 2006. Only the first high is shown for each day to keep it simple. When the daily variations are averaged, the number is 59.6 minutes in the example plucked but at the extremes the variation on WED is almost twice that of the previous FRI!

FRI 0245z + 42m =

SAT 0327z + 46m =

SUN 0413z + 52m =  

MON 0505z + 66m =

TUE 0611z + 82m =  WED!!!

Dealing with tide in reality is somewhat like organising mercury with a fork. You should study tides of course but you must also observe what is happening in the River. London VTS broadcasts nav. info at H+15 and H+45 ish on Ch14. Among the data is spot tide height for Tower Bridge and how close it is to prediction. It frequently is over prediction as the tide floods showing that the flood is 'earlier' than predicted. Observation of the moment of slack under Tower Bridge shows that top'o'tide is sometimes 20/30 minutes after prediction. The eventual top height is rarely as predicted either.

As a student, always couple the theory with the practice and don't get cocky.

http://www.tcd.ie/Clubs/Subaqua/Tides/ is an excellent resource on the theory of tides; it relates to Dublin.

Each month's Springs occur at more or less the same time of day for a given place.

For the Thames Estuary this is 'afternoon'. Within that band, the tide is progressively later (and higher) as you go upstream. As the tide progresses upstream the available space becomes narrower and usually shallower. The tide gets progressively deeper as you go up and also moves faster. This is important to know and understand in navigating between Tilbury and London Bridge.

Here are a range of tide predictions from Walton-on-the-Noze, via Margate to London Bridge on 4th Feb 2006. The numbers speak for themselves if you get the drift. Note that when the tide is full out it is much the same height above chart datum at all locations and also note that the tidal stream for the outer estuary is predominantly SW on the flood and NE on the ebb. (Tidal streams come up again elsewhere especially when we are trying to get round the North Foreland or up to Lowestoft.) 

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, 0458z 7.1m              1223z 0.5m

Depth of water above chart datum is much the same for all points at Low Water, but High Tide at London Bridge is 3.9m higher than at Walton-on-the-Naze. Even Southend is 1.5m higher!

Tide tables give predicted times and heights of high and low water for each day of the year. They are worked out by a constipated mathematician [ie with a pencil and paper]. Events rarely happen as predicted but they do give a rough idea or a starting point of what to expect and when.

There is a tried and tested way of figuring how much water is under your boat called The Rule of Twelfths. Starting from slack water tide out, the flow rate is not 'linear' but increases at an increasing rate 'til half tide and then 'slows down' 'til 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. 

1/12 + 2/12 + 3/12 + 3/12 + 2/12 + 1/12 = 12/12 ! An 'hour' is actually 1/6 of the time between low and high!

The following predictions were published for North Woolwich in Jan 2006 (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 of tide we are interested in is between 0511 and 1121. We need to float into a 'dock' at slack water and High Tide. Slack water so we do not have to contend with currents and High Tide so we have as much water as possible under us. Once we are in and moored we have to let the tide fall til the keel rests on the cross beams. This will happen when there is about 2.9 metres of water over the beams. And THAT will be when the water is 5.4m above Chart Datum. We then wait for the tide to drop 2.9 metres until the beams 'dry' and we can walk on them.

For what it's worth the drying height of the cross-beams, which all 'dry' together, is about 2.4 metres.

Using the rule of twelfths for the tidal range of 5.2m, 1/12 is 0.433m. For the time range we use a rule of sixths between 0511 and 1121 (6hrs10mins) to get six time intervals of about 1hr2min.

6.300 at 0511z - 0.433 =

5.867 at 0604z - 0.866 =

5.001 at 0706z - 1.299 =

3.702 at 0808z -1.299 =

2.403 at 0911 - 0.866 =

1.537 at 1014 - 0.433 =

1.104 at 1116 with

1.1m at 1121 (predicted).

Thus WE can now predict that at about 0635z when the level has dropped to 5.4m above chart datum the keel should touch the beams for the first time. By 0745 she should be firmly grounded and should begin to lean over against the WALL. The beams should 'dry' at 0915, with 2.4m of tide above chart datum. But we will not be able to stand under her port side til the water has receded another 0.5m or so at about 0940. We then have 1hr and 40min til the turn of tide and another similar time period til the water is around our ankles again. So the total 'working time available to the team is 3 hours and 20 minutes between 0940 and 1300.