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Swimming spring tides: an explanation by Michael Oram

The following is a copy of correspondence on the google channel swimming discussion website. Used with permission.

There are 3 separate articals below in which I have tried to explain Chartwork and it's specific connection to different parts of a Channel swim.

It is a great misconception that you swim the course as seen on the chart.
You actually swim the distance you travel forwards on what is known as your WATER TRACK.
That means you only swim around the 18 to 20 nautical miles BETWEEN England and France.
Your swim course that includes the effect of the tides is known as your GROUND TRACK.

As an example if you walk the corridors at an airport and know the speed you walk at you can measure the distance and work out the time it would take to get to your boarding area.

If, like me, you stand on the moving walkways and just step off the end you will travel the same distance to get to your boarding station but you will not have walked very far as the moving walkway is transporting you to your destination. The tide is like a moving walkway and transports you up and down the channel while you are swimming across it.

If you swim from one side of the river to the other you only swim the distance required to cross the river. You do not swim the distance the currant carries you up or down stream.

Right so now we have established you only swim the required distances to CROSS the Channel from one side to the other. Any sideway movement created by the tides is an "outside influence" you do not have to personally work for.

Swimming the Channel can be calculated and plotted in advance if you know a few factors like --

  • Distance to travel
  • Tidal speed for any moment on the route
  • Tidal direction at the time intervals during the crossing.
  • The course on which you are going to cross
  • The Swimmers speed.

This is what your pilot is calculating through out your swim, whether it be a mental calculation through knowledge and experiance or with instruments and information on a chart

The shortest distance across is 18.2 nm (about) and the direction is about the 145°/150° true mark. If the water had no tidal influence that would be both your WATER TRACK & your GROUND TRACK. ( See the expanded explanation further down the page.)

The tidal influence predictions for every day can be found by calculation once you know the height of the tide and you have a chart or tidal atlas to get the information for the area of the chart you are travelling through.
You must remember however that these are only predictions and very seldom match exactly what will happen. It's this slight variation and the ability to make a calculated guess as to what happens on the day that is part of the reason why not all the pilots choose to use the same course.
A good pilot will be matching the swimmers ability to the conditions and selecting the best course for the day.

You are paying your pilot to plot the course, guide you through the commercial traffic and be there for safety purposes.
Anything else you get in the way of assistance or advice is a bonus.

Below is a SPRING tide plot to show you how your start place is effected by the tide and the course your pilot takes when they leave the beach.
We now start swims at a different place (and often a different time) depending on the height of the tide and the speed of the swimmer.
When these swims were plotted Lance and I were the only pilots using the Sanfire Hoe start and swimming SPRING tide regularly.

This was an actual tidal plot of 2 swims form a long way back. It was on the SPRING tide after a long spell of bad weather.
This day it was still a confused choppy sea but both swimmers were running out of time and had planes to catch.
The swimmer that started from Shakespeare was a good one and afterwards he could not understand how my slow relay managed to get where it did.
He travelled along the coast while we went across the Channel.

I had taken my team to Sanfire Hoe / Abbott's cliff for their start. Our course from the beach was 165° T
The Shakespeare beach start was on a course of 135° T -- the usual and accepted start from that beach at the time
The lines going up towards the top right hand corner are the tidal streams - as you can see they vary considerably from one start to the other.

I did originally post a much longer article about these two swims but can not find It now to post it.
It is most likely in the chat site archives somewhere if anybody wants to search it out.

PART 2 a expansion on the navigation involved

A basic and brief description of the navigation surrounding a Channel swim.
The Chart we use for the normal Channel crossing is "Dover Straits Western" 1892 Admiralty.
If anyone wants one you can get them from marine outlets off the internet or we can arrange to send you one for a nominal fee.

The shortest distance across the Channel is Between Shakespeare Beach, to the South West of Dover and Cap Gris Nez.
This is 18.15 Nautical miles -- 36200 yards -- 33521 metres (give or take a bit)
If you draw a line between the two points mentioned above you will find that the course between them is about 145°true, plus or minus a degree. This course is roughly 90°degrees to the tidal flow up and down the Channel.
The general idea is to get from A to B by the shortest route, that means crossing the Channel as close to 145°True as possible but without swimming into the tide (that's more than 90° towards the tidal direction.
"T-ing" the tide is what we call it.
To do this you need to take into consideration the movement of the tide up and down the Channel. To determine the time the tidal flow changes you need to look at the tidal diamonds in the area you are considering (for instance see below on chart insert - diamond "K" in the separation zone to the NE of the Colbart North buoy).
Tidal diamonds show the tidal flows at that point on the chart Spring & Neap tides for the periods from 6 hours before HW to 6 hours after HW -
Over that 12 hour period the average of the tidal direction and flow are shown for each tidal hour.
From "K" (it's typed in below) we can see that the tide changes direction by 180° at 1.5 hours before (-) High Water to flow towards the NE (around the 30°true) until 4.5 hours after (+) HW. It then turns and flows back to the SW (around the 205°true) from HW (+) 4.5 to HW (-) 1.5 hours. There is actually a little more than 12 hours per tidal period so the tides move back in time a little each day about 45 or so minutes.
When the tide is flowing towards the NE it is called the "Flood tide" because it is running in that direction over the HW time.
When it is flowing towards the SW it is called the "Ebb tide".

The tidal height is related to the position of the moon and also changes every day. The moons cycle is about 28 days plus a few minutes. In that 28 day period there are two Neap tides and 2 Spring tides. (Explained in more detail in the previous e-mail)
The Neap tides are when the moon is at 90° to the sun (a quarter moon) this is when the tidal movement is at it's lowest.
The spring tides are when the sun and moon are in line.
There are High Springs when the sun and moon are on the same side of the earth (a new moon)
There are Low springs when the moon is on the opposite side of the earth to the sun (Full moon).
It would be too involved to list the tidal heights for every day so we use a set of standard measurements to calculate the tidal flow in relationship to the tidal height or tidal range. (depending on how accurate you want to be).
MHWN (Mean High Water Neap) height - this is 5.3 meters for Dover
MLWN (Mean Low Water Neap) height - this is 2.1 metres for Dover
Neap Range is 3.2 metres (5.3 - 2.1 = 3.2 metres)
MHWN are always about 0600 and 1800 GMT at Dover (add an hour for BST)

MHWS (Mean High Water Spring) height - this is 6.8 metres for Dover
MLWS (Mean Low Water Springs) height - this is 0.8 metres for Dover
Spring range is 6 metres (6.8 - 0.8 - 6.0 metres)
MHWS are always about 000 (Midnight) and 1200 (Mid day) GMT at Dover

For every hour in a tidal period you have 3 sets of figures in your tidal diamond information.
The tidal direction - in °Degrees True
The rate of flow for MHWS periods
The rate of flow for MHWN periods

information in the tidal box "K"
(position - 50°58'.6 N 001°.26'.7E)

tidal hour Dir Springs Neaps
-6 208 1.7 1.0
-5 204 2.5 1.4
-4 208 2.7 1.5
-3 209 2.1 1.2
-2 221 0.9 0.5
-1 017 0.7 0.4
HW 026 2.0 1.1
+1 028 2.6 1.5
+2 030 2.4 1.4
+3 033 1.7 0.9
+4 028 0.6 0.3
+5 214 0.4 0.2
+6 209 1.4 0.8

For this tidal area you can add up the flow figures and you will see that it is
10 nm in a SW direction on the Ebb tide on Mean Spring Range
10 nm in a NE direction on the Flood tide on Mean Spring Range

5.6 nm in a SW direction for the Ebb tide on Mean Neap Range
5.6 nm in a NE direction for the Flood tide on Mean Neap Range.

For Channel swimming we call the
Neap tides anything with a High Water up to the 6.1 metres
Spring tides anything with a High Water above 6.1metres

If we want to follow the shortest course to France then we have to balance out the tide so that it carries us up Channel for about the same amount of time as it carries us down Channel. We do this by calculating the speed of the tide and the speed of the swimmer and picking a course that will take us to where we want to be at the end of the swim time.
Without a good set of diagrams this next bit is possibly hard to understand but I will try to explain.
Using the above tidal diamond figures for "K" @ 3 hours before High Water.
HW - 3 209°T Spring rate 2.1 Neap rate 1.2

If you are swimming on a boat course of 145° True at 1.5 nm (3000 yards or 2800 metres) per hour the tide will be carrying you and the boat in a direction of 209°T at a speed of 1.5 if the Tidal range is 3.2 meters (HW about 5.3 metres / Low water about 2.1 metres on the tide tables)
That gives you a course over the ground of 175°True at a speed of 2.6nm. (see dig. below)
This is the reading you would see on a GPS.
You are actually swimming in a direction of 145° True at 1.5 (3000 yards ) an hour.
Your swim is made up of these types of calculations made by your pilot all the way over from England to France using the tidal information from the tidal diamonds scattered all over the chart. They have to adjust them for the tide, wind etc and consider what is happening in the North Sea and at the South Western approaches.

Where we start and end up is dependant on the swimmers speed and the tidal height, the interpolation of the tides, plus a bit that has to be calculated by "seat of the pants" and added for the wind and the weather variables.
Unless you are very fast or very slow you will usually start somewhere between an hour before high water and an hour after high water. This start time is often "Gut feeling" and worked back from the estimated crossing time and landing place.
For instance --- If you start at one hour before High Water then you have 5.5 hours of "flood carrying you North Easterly followed by 6 hours of a South Westerly flow that's 11.5 hours to reach Cap Gris Nez. (It's actually a little less than that, say 11 hours most days - but it can also be a little more sometimes)
36200 yards ÷ 11.5 = 3150 yards an hour = 1.575 nm = 2900 metres of swim distance per hour - (including feeds and stops) to reach the point. I actually think you have to cover 3400 yards (1.7) plus nm or at least 3000 metres to land at Cap Gris Nez, if everything goes according to plan.
Remember that at this speed (1.7nm) every minute lost is costing you 60 yards. That adds up to a lot of distance for 11 /12 hours. If you loose an average of 5 minutes an hour over 11 hours it will cost you 3300 yards - that's 1.65 nautical miles or an hours swim time, and possibly a landing on the point at Cap Gris Nez.
Just because I have used an hour before HW do not think that is the best time to start.
It might not be best for you or for the day or for the tide. Your pilot might not be planning for you to land at Cap Gris Nez. If you swim slower than 1.7 or the tide is higher than predicted or the wind is in a different direction and he/she wants a sheltered landing place - the list of "ponderables" is a very long one of.

--- Remember that this is a very basic explanation and there is a lot more involved "on the day" ---

Unfortunately it's not very often that everything goes to plan anyway and there are other many factors that determine what happens.
For instance I like all my swimmers to be at least 3 nm from the shoreline by the time they are in line with the Eastern Entrance to avoid the tidal split and flow into the Downs. We will often start at Abbotts Cliff to make this happen.
There is a tidal flow out of the Calais approaches and from the Dunkirk area that we try to avoid (unless we are doing a slow 2-way attempt) and the same goes for being on the wrong side - Westerly / seaward - of the Colbart (Ridge) bank in certain conditions.
As I have said before " a little knowledge can be dangerous". Try and think of yourself as the pilots engine - all you have to do is go forward at a steady pace. Let him do the navigating and driving.
A Channel swim is one of the only open water swims where you will have to work as a team with your pilot. Success is a joint venture.
Michael O

Chartwork -Transcript from an e-mail dated 5th July 2007 "Chartwork 1"
This was the original reply to questions about negative tides at ZC2

If you are happy with what you have read above -- or totally confused -- you do not need to read further.
This piece is a more detailed bit of "technical waffle" and the chart extract reference the tides around ZC2 and Cap Gras Nez.

The unfortunate thing about charts is that they supply information that has to be interpreted by the user. Interpretation takes skill and understanding and a basic knowledge of the area and the subject.
All is not what it seems when you first look at the chart information.
What you have to do is read the chart, look at it and try and understand what it is trying to tell you. It's the overall picture that is important not the spot reference from the diamonds (although the spot reference is required - that's why it is put there).
Try and visualise the chart in 3D and the seabed as if it was without water.
That way you will get a better insight into where and how the water flows. The topography of the seabed is the same as it is on land. On land water flows through the bottom of valleys and is directional according to the contours of the ground around it. The underwater view of hills and valleys is just the same only they are covered with water instead of air.
Water does not flow up and down hills on land but twists and turns it's way around the high spots. The same happens under the water, it flows around the obstacles on the seabed not over them.

Diamonds are an individual measurement that only give a record of what is expected to happen at one small point on the chart. They are placed on a chart for 2 reasons.
1. The first is when they are placed in areas of open water such as mid channel, or where the water flow pattern is unrestricted as much as possible.
Here they give an indication of the speed and direction of the water.
They are also found in the middle of smaller areas like the Downs (the area of water in front of Deal between Dover and Ramsgate) and diamond "R" off Sangatte on the French coast for the same reason. These Diamonds are to give an indication of what you can expect for an area when the conditions are ideal and the tidal height is as predicted for the day and are seldom what actually happens.
2. Secondly diamonds are placed at points where there is an anomaly or change in tidal direction or speed that mariners need to know about.
Such as in the front of Dover harbour where diamond "Q" shows the fast flow on the harbour entrance ---
Diamond "N" at the ZC2 buoy 3 miles out from Cap Gris Nez Where there is a change in coastline direction ---
In the approaches to Calais ----
At the tidal split on the South Goodwin lightship where the tide is different depending on where you are reference the Goodwin sands etc.

The other problem with diamonds is that they are an average of measurements taken at a point on the chart divided into periods of Tidal Hours.
To get a slightly better picture of what tides are doing in an area there are also "Tidal Atlases" that can be referred to. These give arrows that indicate the general direction and speed of the tidal flow in an area - and the arrows bend to indicate whet is happening over a bigger area. Both "tidal Atlases" and "tidal diamonds" work to tidal hours.

Tidal Hours are calculated as an average of the direction and flow of the water over the seabed for every 60 minute period.
There are 12 tidal hours in a Tidal period - from 6 hours before High water to 6 hours after high water. The cycle then starts all over again for the next High water and repeats itself but at a slightly different rates as the tidal height will have changed.

During a Tidal period on the Dover chart the tide will turn through 180° at some stage. It travels for 6 of the hours in a direction towards the NE (for about 1.5 hours before HW to about 4.5 hours after HW) and then another 6 hours down towards the SW (about 4.5 after HW to 1.5 before HW). These are approximate times as they vary with the weather, tidal speed and your position.
Tidal rates change each day as well according to tidal heights. They are directly related to the HW height on the day and the difference between the MHWS (Mean High Water Springs) & MHWN (Mean High water Neap) readings.
See the extract for the Diamonds "N" & "R" information from the tidal box for the Dover Straits Western chart (chart 1892) below ----
Diamond "N" is the diamond for just above the ZC2 buoy.
Diamond "R" is the diamond a couple of miles offshore to the west of Sangatte and just above the CA3 buoy. Charts will have diamonds all over them for you to refer to. The Diamond position is at the top of the boxes.

"Dir" - is the direction in degrees true of the tidal flow as read from the compass rose on the chart. (All charts are "North up"- 0° degrees at the top)
All tidal information for each chart is based on one port on that chart. In our area on the 1892 chart that port is Dover.

Tidal information based on Dover copied form 1892 "Dover Straits Western" chart

 

Tidal

Hour

 

“N”  50°53’.9N

     001°31’.9E

 

“R”   50°57’.8N

       001°41’.1E

Position of  tidal diamond on chart

Hours

Dir

Spring -- Neap

Dir

Spring -- Neap 

Spring & Neap tide rate

-6

222

   2.6         1.5

265

   2.1         1.3

     

Tidal hour and speeds for MHWS & MHWN before High Water Dover

-5

216

   3.4         2.0

265

   2.9         1.9

-4

214

   3.3         1.9

265

   3.2         2.2

-3

213

   2.4         1.4

265

   2.9         2.0

-2

213

   0.7         0.4

269

   1.5         1.0

-1

010

   1.4         0.8

053

   1.0         0.7

High Water

017

   3.0         1.7

051

   2.0         1.3

          Rate in knots

Nautical Miles per hour

+1

019

   3.4         2.0

050

   2.8         1.9

     

Tidal hour and speeds for MHWS & MHWN after High Water

Dover

+2

019

   3.0         1.7

050

   2.8         1.9

+3

016

   2.0         1.1

051

   2.0         1.4

+4

003

   0.8         0.5

035

   0.8         0.6

+5

250

   0.7         0.4

263

   0.7         0.4

+6

223

   2.1         1.2

264

   1.7         1.0

 

Further down the page is an extract from the chart for you to look at.

The direction at Diamond "N" for the NE flow is around the 15/20° mark but changes very quickly as you move up above it towards Calais. It comes around to the 50°True direction -- that is shown on diamond "R" off Sangatte.
The Diamond is there because the tide splits in this region.
Out to sea on the Western side of the NE shipping lane the tide goes up Channel towards the Sandette light ship.
To each side of ZC2 the tide is spreading out to fill the widened area of water that goes around to the East and past Calais to Dunkirk. The tendency is for the water to flow North East here.
If you are less than 2 nm from the point you are affected by the change in direction of the water that is flowing into Wissant Bay and filling the Bay and the shoreline towards Calais
The tidal information for Diamond "N" is partly influenced by the overfalls caused by the tide coming out of Wissant Bay and meeting the tide moving up the Channel when it is travelling Northwards past Cap Gris Nez.

The overfalls off the Cap run up to 2 nm out to sea and are a result of the tidal flow coming out of the Bay at Cap Gris Nez. This happens because the water in the Bay flows back up very close inshore in front of the restaurant beach and to the South side of the sandbank in this area to fill it up. This sandbank stretches for about 1800 to 2000 metres North East of Cap Gris Nez - and acts like a roundabout. (It can also cause confused and choppy water along it's seaward edge). When this tidal eddy gets to the point at Cap Gris Nez it is forced into the main tidal stream going NE up channel hitting it at 90% and disrupts it's flow.
Back out at Diamond "N" the tidal flow to the West (English side) of it is influenced by the water running up the inside of the le Colbart bank (The Ridge) about 5 miles off shore. (The Colbert is an underwater ridge that forms a wall and comes up from 30 metres to very close to the surface (within 4 metres) it's long and thin and runs at about 25 ° true).
See the extract of the 1892 chart below.
The 30 metre and 20 metre contour lines (depth lines) run at about 15°True parallel to the French coast and to the ZC2 buoy on the shore side of the NE lane. They change direction at ZC2 to run at about 50° true towards Calais. This change in direction of the bank (shown by the contour lines on the chart ) prompts the water to turn and travel in the same direction. To do this it changes direction in a long curve towards Calais.
Broken down a bit more ---
If you are on the inside of the ZC2 buoy but more than 1.5 to 2 miles off the shoreline you will be helped along by the tide and carried (hopefully on the inside of the Abbeville buoy) into Sangatte and the Calais approaches as the water fills up this area. Exactly what happens depends on how much effort you put in and what the height / strength of the tide is.
If you look at the tidal direction on Diamond "R" (which is in a much more stable place off Sangatte) you will see the tide is running at around 050° there, that's parallel to the depth contours as they cross Wissant Bay and are running gently towards the shore as you approach Sangatte.
If you are in the area up to 1.5 nm off the point at CAP gris Nez when you pass it You should end up being carried into Wissant Bay on the tidal eddy that is going around the end of the sandbank.

As I mentioned earlier water finds it's own level. It does not flow up or down underwater banks. It will spill over the top of the banks to fill the Bay but slows down considerably when it does. This means it fills in the areas towards the shore to the North of Cap Gris Nez. The closer you are to the bank and 20 metre contour line the more it will help you when it flows into the bay.
The distance across the Channel between Shakespeare beach at Dover and Cap Gris Nez is 18.2 nautical miles. This is a bottle neck in the Channel that acts like an hour glass Between the Central Channel area and the North Sea. The Channel above and below this line is much wider (45 nautical miles Dover to Dunkirk - 60 miles from the English coast to Dieppe). The volume of water that passes back and forth through this gap is very prone to the influences of the wind and the conditions above and below it. It is important to remember that the information on the chart is only a guide and refers to stable conditions and good weather. They are something we do not often get in this area.

These days with all the modern navigational equipment based around GPS (Global Positioning System) we can get a much more accurate picture as to what is going on.
All the tidal data is updated within seconds to give a reading as to exactly what is happening. This information can be transferred to chart plotters and computers where it is shown as speed and direction over the ground (SOG)and course over the ground (COG). We can recorded this information in both chart overlay and figure read outs and store it for future reference.
You as a swimmer do not have to worry about negative tides - unless your pilot gets it terribly wrong or points you in the wrong direction. The general rule is quite an easy one when it comes to direction. -- We try to never go above the "T" with the tide. That means we try not to go more than 90° into the tide as that is wasted effort - (Sometimes we might ask for a short burst of speed to reach a point).
If we can we use the tide to give the swimmer more speed we will.
We are swimming you to your "place of landing" this is not usually the closest point or what you and your team are looking at. It's more a case of --- Go with the flow --- until you get there.
You do have to swim across the tide however as you are swimming across the Channel.

What a lot of swimmers do not realise is that if you are 3 miles from the beach and tired you might only be swimming at .7 or .5 of a knot (including your feed and stop times). This means you are still 5 to 6 hours to the beach. Even a mile off can be a 2 hour swim. That is demoralising when you are looking at land that is so near and yet so far.
What the water does not do is flow up the beach or directly away from the beach so the nearer you get to land the more the flow is parallel to the shore line.
The only way to get there is to swim across the tide.
As I said at the beginning - "technical Waffle" - hope you can understand it. If not just leave it to the pilot and think of yourself as his/her engine. You supply the propulsion and let them supply the chartwork.
Michael O