The adventure continues.
The Sting in the Tail of the DW
Unlike most other long-distance events, the DW has a unique 'sting in its tail' for those contestants on the 'Overnight' (non-stop) event, brought about by the 'celestial' influences of the Moon and tides. It's a factor that doesn't manifest itself until the closing hours of the race, but has to be taken into account in the planning stage in order to steer away from having to withdraw from the race in its last few hours. It all comes to a crunch at mile 107 where the last of the 77 locks has to be negotiated.
That last lock is at Teddington which is on the west side of London, 17 miles from the finish at Westminster, and it has one major difference from all the other locks that would have had to have been negotiated previously. Being the last in the chain it separates the river that's levels and flow is largely controlled by the lock system, from the tidal flow of water out to the Thames estuary and the English Channel.
As previously mentioned, in the four-day event contestants are set off each morning in groups and make their way through the 30 to 40 miles of the stage to the next overnight stop. If however the contestant takes more than ten hours to complete that days mileage they will not be allowed to start on the following day's stage. That insures that a minimum standard and speed is maintained, which is quite low and equivalent to an easily achievable brisk walking pace of about 3.5 to 4 miles per hour. But the overnighters don't have that worry, they have another worry all of their own.
The overnighters have to get to Teddington at the time of the high tide to take advantage of the ebb flow as the tide turns back towards Westminster. Due to the safety restrictions imposed on the competitors the inviolate rule is that entrants are only allowed to progress through Teddington and onto the tideway for a period of a half-hour before high tide, to three and a half hours afterwards. If an entry can't make that time window they have to wait for the next tide and sit around counting the hours ticking away while being added to their race time... or retire. Any attempt to carry on after the window has closed, results in an automatic disqualification.
The water levels, and to an extent the speed of flow of the water in both the canal and the Thames, is governed by the locks along the way. In the canal section the water flow is not very fast and might add about one mile-per-hour to the boat's overall speed after periods of heavy rain. In the river the water flow can add another half to one mile-per-hour. The tideway however, is of a different complexion in that the water flow and level is uncontrolled. Flow is at its lowest around the high tide but can ebb or flow at a much greater rate. The water is also much more turbulent, especially when passing under bridges where standing waves can build up at bridge pillars. Couple this with the widening of the river it becomes much more exposed to wind and where the banks are built up to accommodate the varying height of the river, makes the tideway a not very much fun place to be when conditions are at their worst.
This then calls for a bit of basic arithmetic and knowledge of the average speed, including all stops and delays that are likely to be experienced throughout the race, so that a start time at Devizes can be calculated to bring about the arrival at Teddington to the optimum moment. This is judged to be about a quarter hour after high tide to gain the maximum assistance over the final 17 miles. The contestants have a choice of start time anywhere between 7 am and 12 noon on the Easter Saturday. So to calculate an individual start time you would need to estimate the times it will take to complete the canal section, add it to the river section's time to Teddington, and subtract that from the high tide time.
Generally speaking, to achieve a fairly average race time of 24 hours, you would need to arrive at Teddington 21 hours after starting. Given that the Easter high tide at Teddington is usually somewhere between 4 am and 6 am on the Easter Sunday, the start time at Devizes would then need to be between 7 am and 9 am.
But the race's final sting in the tail comes from the turn of the tide from ebb to flow. Any crew just making the very end of the four hour window and embarking on the last 17 miles in an tired state can run into trouble if they don't make Westminster before then. In fact, the water flow reverses within minutes and uncovered banks are quickly under a couple of feet of fast flowing water as the tide turns. This means that exhausted crews are now paddling against the flow in the last few miles. This is an almost impossible task and crews have been known to leave the water and portage their boats through to the finish.
These are all factors to be taken into account and allowed for before the race begins, and are just some of the reasons which in my opinion make this race almost unique in the sporting calendar.