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"Bringing Seamanship to the Creeks from the River and the Estuary "   


TIDAL EBB & FLOW by Pogue Muhone


...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. 

"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 flood for a little over six and a quarter hours and ebb for about the same time.

On the Thames they do no such thing! ...  Tides vary enormously in timing on the River. 

Rivermen often observe the flood to be nearer seven and the ebb nearer five hours.

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 was recorded.

When the daily variations were averaged, it seemed the tide was an hour later each day 

but the extremes showed one day (82m) 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 tides 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 as the tide floods, showing that the flood is running 'earlier' than predicted. Observation of slack 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.

Each month Springs occur at more or less the same time of day for a given place. For the Thames Estuary this is 'afternoon'. In general, night time tides lift higher than daytime tides and Springs usually peak a couple of days AFTER the full moon and AFTER new moon. Also, the "new moon" Springs are usually higher than the "full moon" Springs. Estuary 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 as you go up and also moves faster but not on a linear scale. This is 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'.

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 do 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 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 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!  

 





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This project is "Work in Progress", subject to frequent and constant revision.

Everything in it is open to public scrutiny and comment.

To dispute or discuss any aspect contact:-  e-mail .. pentargonspringer@gmail.com
 

All comment is welcome and will be dealt with, to ensure the article is absolutely accurate.




During 2014, a forward looking Kentish Man realised that Dartford Creek,  (which, at one time, had been a vibrant port) was now completely derelict and totally disused as a marine asset. Neglect, mismanagement, opportunism and inertia had all played their part over many years, but essentially the challenge seen by the visionary was total lack of accessability to boats from the sea to the ruined lock and no accessability from the land side for small craft even down to the size of kayaks.

The whole of the west-side waterfront from Mill Pond Road to Steam Crane Wharf
had been sold off and sealed off in successive deals over the preceding forty or more years and the danger was that something similar would happen on the eastern bank. The Countryside and Rights of Way Act 2000 had been ignored or flouted. Access to public footways and rights of way (see Definitive Map) supposed to have been protected under that legislation was tenuous or blocked off on the west bank south of Ben Dunn Bridge. On the eastern bank the right of way DB06  (part of The Darent Valley Path) was effectively impassible at times due to the intrusion of brambles, nettles and even dog-roses for which neglect Dartford Borough Council and Kent County Council would have been collectively responsible and the Environment Agency might be passively implicated. The actual track across the marshes in 2016 is down to a width of less than 18"
 

Literature in Dartford Library shows that water has been siphoned off upstream for years and sold.
There is still a shortfall of supply in the Creek but it could be negated by servicing the cill.  
 


The River Darent itself (or at least the tidal section between the railway and the A206) has been allowed to silt up to such an extent that a creek once able to float 50tonne barges of over one meter of draught inside the lock has been reduced to a trickle. The water was frequently less than 6" deep in a meandering centre littered with debris and had built up mud banks over almost 80% of the original port area to heights varying from 0.5m to 1.5m. A cill fitted in the top end of the old lock to maintain a minimum of 24" (0.6m) draught at all times, is being circumvented by easily repairable leaks so that even a shallow draft boat can not stay afloat when the tide is out. 

By July of 2016 the cill itself was leaking between the beams and mud was building up behind it to such an extent that the tidal outfall had chosen to escape via the (rotten) ruckings.


Literature in Dartford Library shows that water was siphoned off upstream for years and sold.
There is still a shortfall of supply in the Creek which could be neutralised by sealing the cill.  
 

Meanwhile back in 2015!

The Kentish Man decided that, rather than watch things [not] happening, he should make things happen and set up "Friends of Dartford and Crayford Creek aka Steam Crane Wharf" a  [Facebook] group on social media which attracted the attention of like-minded people and "interested parties". By mid-July 2015,
a canal boat (which usually cruised the South East waterways and the Lee and Stort Navigations) .the first boat in over thirty years had moored up on bollards literally dug out of the overgrown quayside.    Its skipper had been invited to join the "Friends" and his boat was designed to be able to deal with tides and the sea. By coincidence he was experiences on the wily estuary.

The creek had been visited irregularly over the years, by boats scuttling in on a rising tide and escaping on the fall, usually skippered and crewed by intrepid and skilled canal boaters often under the umbrella of the Inland Waterways Association and in search of a new challenge or in making a stand for maintaining a right and ability to navigate a 'forgotten' waterway.  They are 'ships which passed in the night' but to the great credit of the IWA the visits were made.

What was needed by the "Friends" to top up the IWA activity was for a boat to stay overnight and this happened (for the first time since the port was abandoned) on the xxth April 2015, when a 36' Springer narrowboat, skippered by an experienced blue-water sailor, and crewed in by a team of experienced narrowboat owners from London's waterways, entered the lock at 5.30pm and tied up to wait for the tide to rise sufficiently to cross the cill about an hour later. The Rubicon, in a manner of speaking, had been crossed. The crew returned to London on the next train, but the Springer and its skipper stayed for over a week before returning to London waterways.

 London Boaters involved in that escapade were eager to discover Dartford's potential as a port of call or a destination.
It was not lost on them that Dartford is end of the line for their Oyster Cards and the station a five-minute walk from the creek and 25mins from London bridge .  The Springer returned in June and, on a suitably high tide,  to perch a ten-tonne narrow boat on a mudbank just inside the lock from where the skipper spent weeks studying tides and river conditions, surveying and assessing the potential of the Creek as a marine facility.

Enterprising volunteers within the "Friends of Dartford and Crayford Creek " group  progressively dug out the lock side and the quay above it, excavating bollards, pushing back thirty years of dereliction and making a usuable quayside of over 200'. They even managed to move a yawning lock gate to increase the "gap" to some 20' towards the end of the Summer.

The group eagerly awaits a boat of such girth to arrive but the fear is that such width (pro rata) would have air draught challeges and be too tall to pass under Bob Dunn Bridge. 

Links were  forged, casually and informally, with Port of London AuthorityThe Environment AgencyDartford Borough Council,  ( Dartford Museum) among others. Local MPs and Councillers have been approached ...  as part of the group's  avowed mission to .....  "see boats back on this navigable part of Britain's Inland Waterways and celebrate the Darent and Cray's rich nautical heritage". .. Network Rail engineers opened up Crayford Creek head of navigation over which the railway line passes. Crayford's "Jolly Farmer" wharf  had been closed to boats by a grove of alder trees getting out of hand over decades. Early in 2016, trees which prevented boats from reaching the head of navigation were cleared and "Pentargon"  tied up there in May 2016. A Gravesend RNLI crew patrolling Crayford Creek for the first time ever, passed under the railway and met "Pentargons skipper, quietly gauging "The Jolly Farmer" wharf and figuring how to moor in a creek which dries out between tides, but has a lift of over 2.5m and totally lacks facilities other proximity to the 428 bus.

EDITING ONGOING. 2016-10-10


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