The FLOOD TIDE flows from the South West from 1.5 hours before HIGH WATER to 4.5 hours after HIGH WATER DOVER. That's up Channel towards Holland and the North Sea
The EBB TIDE flows from the North East from 4.5 hours after HIGH WATER to 2 hours before HIGH WATER DOVER. That's down Channel towards Folkestone and the Atlantic
As the tidal cycle is a little over 12 hours from one high water to next, the times of high water change every day getting later as the days progress. A good guide is high water Springs are at approximately midday and midnight(GMT), and high water Neaps are approximately 6am and 6pm (GMT). You must add one hour for British Summer Time to these times.
Because of this movement of water from one place to another, the Dover Straits are prone to strong tidal flows, and a large rise and fall in water between high and low tide. To complicate things a little more, the position of the moon relative to the earth and the sun affects the gravitational pull that is moving the water.
When the sun, moon and earth are in line we have maximum tides known as SPRING TIDES. This is every 14 days on the new moon or the full moon.
When the moon is at 90° to the earth, we have minimum tides known as NEAP TIDES. This is every 14 days when the moon is in its first and third quarter.
Thus we have 14 day cycles with the tides going from Springs to Neaps and back to Springs. From tidal atlases and nautical almanacs we find that at Dover: Mean High Water Springs is 6.8 metres. Mean High Water Neaps is 5.3 metres. For Channel swimming definition -
Spring tides are 6 metres or more
Neap tides are 6 metres or less.
Most swims take place on the Neap tides. These are the slacker tides and show as a more direct line on the chart. The lower the tide, the longer the period of slack water when the tide turns, and the slower the tidal flow. Spring tide swims are becoming a regular occurrence with very little difference in times (some swims are even faster), although they This require more planning by the pilot and good weather conditions the swimmer. The major factor on any swim is the weather and good weather on a Spring tide gives a chance a lot of people do not want to miss. The France/England swims (when permitted) are more prone to weather problems as there is at least three hours traveling time before the swimmer gets in the water. If the conditions are not right or the swimmer has suffered from sea-sickness it then takes another three hours to come back to England. The lack of sleep and the traveling means they will be too tired for the next tide and could even be put off altogether.
Because the tidal flow is parallel to the coast and the swimmer is swimming at 90° to the coast, the tides do not do a lot to either help or hinder the swimmer's progress, although they can appear to do so. The pilot's job is to guide you and put you in the right place at the right time. For this he needs to know your approximate swim rate over the period of the swim, in advance. There are places during the crossing where you can get a little help from the tide, and there are areas where the tide will hinder your progress. The idea is to get a balance between the two. If you waste two minutes on each feed and you feed every half hour, then on a 12 hour swim you will have lost 48 minutes. This can be the difference between landing with the tide or having to swim for another 2/3/4/ hours.