U bent hier

Michael Oram explaines tides in the Channel

Used with permission - from e-mails exchanges on the google channel swimmers discussion group.

We have tides in the Channel - Neap tides & Spring tides

They are caused by the gravitational pull of the sun and the much smaller effect of the moon as the earth rotates. The tides vary in height and strength depending on the position of the moon relative to the sun.
When the sun and the moon are on the same axis you get neap tides. Maximum effect with a new moon (and high Spring tides) - This is when the moon is in line with the sun and they are both on the same side of the earth. An the calendar cycle of this event is 28 (and a bit)day.

Low Spring tides are when you have a full moon and the moon is on the opposite side of the earth to the sun.

Neap tides are when the moon is at 90° to the axis of the earth and the sun (every 14 days).

Hence the lunar cycle is

  • New Moon -- High Springs
  • 7 days later -- Neap tides
  • another 7 days to
  • Full moon -- Low Springs
  • 7 days to -- Neap tides
  • 28 day cycle is back to New Moon.

The cycles are regular therefore the time of High and Low water is the same for the lunar month -- for example the High tides at Dover on the Spring tide is about 0100 hours and 1300 hours UT

High tides at Dover on the Neap Tide is about 0700 hours and 1900 hours UT (Universal time or old GMT)

The higher the tide the larger the amount of water that is moved by the gravitational pull. Thus the stronger the tidal flow as a greater amount of water has to be moved from one place to another during the same time period (6 hours - give or take a bit between turns of the tide)

As the days progress the time of High water gets later - hence we have a different start time for our swims depending on the position of the moon and the tidal height.

The full explanation is a little more complicated but this is a good start to understanding what is happening.

Reason why moon can be seen in the day.

It is DAYTIME when it is light. The light comes from the sun. Night time is when the sun is out of sight and it is dark.

We see the MOON or part of it because the sun is shining on it. We only see the part of the moon that the sun is shining on. That is why it appears to change shape.

The moon goes around the earth anticlockwise every 28+ a bit days.

At the same time the Earth is going around the Sun.

The Earth is also spinning around on its own axis.

The spinning is why you get day light and darkness.

It is light when you can see the sun from where you are on the earth (for example in England) and dark when the sun disappears because it has moved around to look away from the sun. It takes about 24 hours for the earth to do 1 rotation.

The sun rises in the East and Sets in the west because the rotation is anticlockwise.

When the moon is on the opposite side of the earth to the sun it is big and white and can be mostly at seen at night. We call this a FULL MOON.

When the moon is on the same side of the earth as the sun just a small piece of it can bee seen as it is between the sun and the earth and it's surface pointing towards us is in the shadow of the sun. We call this a NEW MOON and can be seen a lot during the day.

When the moon is at 90° (each side of the earth) it can be seen either moving from the night to early morning or from late afternoon into the night -- depending on which side of the earth it is.

The tide (water height) has the biggest range when we have a new moon - High Springs & again, but slightly less, on a full moon (Low Springs). We call these periods SPRING tides. For swimming the division is around the 6.1 metre High Water Dover mark. Above that 6.1 High water figure is considered Spring range.

When the moon is at right angles to the sun and earth the tide (water height) is lower and called NEAP tides. On a Spring tide there is a lot more water moving from one place to another every 6 hours. The more water that has to be moved the faster the tide travels.

The tide changes it's direction about every 6 hours. The Flood tide travels towards the North East form about 2 to 1.5 hours before High water Dover to around the 4.5 hours after high water Dover.
The Ebb tide travel in a South Westerly direction from about 4.5 hours after High water to 1.5 hours before high water.

The wind can travel with the tide - against the tide or across the tide, and all points of the compass between the above.

The stronger the wind combined with the stronger tides the rougher the water can get. Wind and tide together produce a long sea. Wind against tide produces a short choppy sea, depending on how strong it is.

To get from England to France you have to swim across the tide -- the distance you swim is still only around the 18.2 to 19 nautical miles whether you swim on Neap or Spring tides
(that's -- 36400 to 38000 yards - or - 33700 to 35000 metres)

However when you swim on a Spring tide you are carried sideways (travel up and down with the tide) much further than on the Neap A 5.3 metre tide will take you up and down Channel about 7 nm in either a North East or  South West Direction. On a 7 metre Spring tide it will be double that, around 14 miles in each direction. (these are rough approximations only).

Each day and each tide is different in both height and the time of high water (It gets later by around 25/30 mins a tide).

The controlling factors are the atmospheric pressure, wind direction and the local weather conditions up to say 500 nautical miles each side of the Dover Straits.

The tide tables are only a guideline and based on -- an atmospheric pressure of 1013 milibars and no wind -- something we never see, hence the uncertainty of what is happening and what the tidal rate on the day is. It's the pilots experiance and their ability to access what they hope will happen that is the hard part of any Channel swim.

The higher the tide - the harder the prediction if the weather is, or has been unsettled.

This year Lisa Cummins did her 2-way on a 7 metre plus Spring tide in 35 hours - she was not a fast swimmer but her mental attitude was outstanding.
In the 7 days of good weather mid September the swims had a high success rate regardless of the speed of the swimmers.

The guiding factors:

  • Sorting out the weather is done by using all the parameters of the shipping and local forecast. Wind speed alone is not a good way to guess what is going to happen.
  • You need calmer weather for a Spring tide swim.
  • Your pilot has to work harder to get the course right.  Or you need to be a "hardier" swimmer. It's not speed that matters but mental attitude and the ability to swim in the conditions you encounter. This is open water swimming with waves.
  • The navigation equipment on the escort boats is much better and used properly the navigation is not a big problem these days.
  • Psychologically you have to be better prepared to the understanding of the sideway movement of the water with the tidal flows.

However the weather is often better on Spring tides. More and more swims are being completed on Spring tides and often they are faster. The fastest crossing records are attempted and created on the Neap tides at the moment. The next big step in breaking the records will be when swimmers that are good enough make their attempts on the top of Springs - but that course has yet to be perfected.

Addition to this text:

Tides flow up and down the Channel -- the Dover Straits is just like a section of a river -- as of yet I have not seen the tides flow across the Channel. Have you seen water in Dover or Calais High Street every 6 hours.

Think of the River Thames in Central London.
If you stand in the middle of London the water flows up and down the river. It does not flow across the river - just up it when the tide is coming in and and down it when it is going out.
That is because the Thames is tidal up to Teddington lock.
The Channel and the Thames work on a similar system.
The only difference is that the shoreline (river banks) that border the Channel are not exactly parallel.
The river Seine in Paris flows down stream only as it is about 200 KM above the tidal lock at Amferville.

Going one step further -- If you run a marathon you have to run the distance. The distance is the same if you are a fast runner or a slow runner. The only difference in the race result is the speed at which the runners move.

Now apply that reasoning to the Channel.
The shortest distance across it is between Shakespeare beach and Cap Gris Nez -- 18.2 nm (give or take a yard or two).
Starting at Abbotts cliff (Sanfire Hoe) is about 1000 yards longer.
The problem is that to land at the point at Cap Gris Nez you have to achieve a certain speed - why - because the tide turns and you are carried ALONG the coast.
Both sides of CapGris Nez the French coast drops away.
Not by much in everyday terms (1.5 to 2nm) but if you are swimming that is more ground you have to cover.
A good pilot will try and access your speed and adjust your course to land you in the best place working on a time scale.

Taking longer over feeds, dropping your swim rate, getting tired, not understanding what you have taken on are all factors that the pilot has to access.
If you land below Cap Gris Nez you swim further.
If you land at Calais you swim further.
If you are tired and have slowed down to 2000 yards or so that is at least an extra hour for every extra mile you have to swim.
The physiological effect of missing the point at Cap Gris Nez can terminate a swim if the swimmer lets it get to them. That is why we say forget your watches, forget your estimated crossing times and swim until you can walk out of the water.
Yes you do travel sideways along the coast but you do not travel backwards.
The shoreline is widening, that means it's a little further for you to swim. It does not mean you are swimming backwards.
The pilot tries not to swim you into the tide as that is lost ground and the tide will always win.
We try to swim you with tidal assistance or at a maximum of 90° across the tide.
There are places where the tide helps you and places where they agitate you. You have to accept and live with this fact.

Why do you swimmers ignore the "personal swim factor" and always look to some other excuse.
You have two options when you swim the Channel.

Prepare yourselves the best you can and be prepared to "Go the distance". Swim until you drop or walk ashore.
or
Live in a dream world, underestimate what you are taking on - convince yourself you are better than you will ever be and buy a book of excuses.

This is the worlds top swim. It's an Extreme swim. It's an endurance swim. It's a swim that pushes the limits.
Failure is a big possibility for many reason like the weather, injury, sickness etc.
It is also a possible failure because you are not prepared properly, not of the right constitution, not mentally prepared and most of all do not understand because you lack experience. Fast tracking is alright providing you take everything into consideration.
You have to clear your heads of negative vibes and doubt.

People like Lisa Cumming and Ross Hardiman prove that the swim is possible with the right frame of mind.
35 hours for a 2-way swim is a very good time when you are a slow swimmer (17.5 hours a crossing). It was done on the top of a Spring tide of 7+ metres. There was no whingeing, no excuses just continuous hard swimming.

Ross swam on the dropping end of a Spring tide and did it in 21 hours. It was her second attempt. Her first finished 700 metres from the French beach after a 25 hour swim.
Many of the swimmers quietly achieve their goal with tenacity and guts.

Why do some fail?
Why do they seam to go no-where?
Often they over estimate their abilities and their swim speed.
If your swim speed is 2000 yards an hour you have a very long swim ahead of you. If you mentally convince yourself you are faster you add disappointment to you problems.
At some stage there has to be a line and an understanding between truth and reality. If you are happy to spend a lot of money to find that point, that's up to the swimmer.
The pilot can only work with the materials he is supplied with. Reality says yards are yards you can not use a shorter ruler.

Some can not get the mental part in the right perspective.

Some, in fact quite a lot, do not pay enough attention to their feed pattern and just run out of fuel.
A swimmer is like a car, when the fuel runs out the engine stops.
A little less time spent on speed development and a little more time on studying electrolytes, carbs, water balance, calorie burning etc would improve the results tremendously.

Have a good, honest, support team is important. What is the sense of telling the swimmer lies when inwardly they know you are doing it. Why create a problem later by shrinking the distance?
On 90% of the swims I have rescued, and their have been a lot, it has been the heart to heart, honest, level playing fields approach that has given the swimmer the will to complete.

Yes this is a fine line and the pilot needs to be able to access the swimmer well as the swim progresses - but we only have a very short time to do that.
A good support crew has hours to sort out the logistics. It's hard and it requires a strong approach and a lot of understanding but it needs to be done.
All this " you doing great", "you look fantastic", "nearly there now", might sound positive but is often having the opposite effect.
You need 100%+ confidence in the fact that you are making the right decision when planning you Channel swim.
Do not book a Spring tide swim unless you believe you can do it.
If you choose to listen the the negative waffle them stick with a Neap swim.