How Far Is It?

Wednesday 11/18/09

For someone with sufficient curiosity, the best English Channel visual aid available is the “Dover Straits Western” Admiralty chart number 1892. The chart provides a mariner’s view of what the French call Le Manche or “the sleeve.” It’s absolutely packed with maritime information including the location of firing practice areas. With some difficulty, Lindsay ordered a copy and we now have it mounted to a cork board in our dining room. We use different colored pins to mark significant spots. This gives us a handy way to familiarize ourselves with the challenge and train mentally for the undertaking.

The shortest distance between England and Europe is across the Dover Strait. This is a narrow band of water that forms an hourglass pinch in the English Channel. The bulk of cross-channel swims start at Shakespeare beach, just southwest of Dover Harbor. The most favorable landfall would then be at Cap Gris Nez in France, a rocky promontory south of Calais, which pokes its grey nose towards England. A straight line from Shakespeare Beach to Cap Gris Nez is 18.15 nautical miles. As with most measures, international standards bodies have been tweaking the various definitions but, as of 1929, a nautical mile is the equivalent of 1.15 statute miles, making it 20.89 miles or 33,123.75 meters across for us landlubbers.

Captain Matthew Webb was the first English Channel swimmer. He succeeded using breaststroke in 1875 with a time of twenty-one hours and forty-five minutes. The fastest verified swim crossing is just under seven hours in 2007, a record held by a Bulgarian, Petar Stoychev. Michael Phelps set a world record in the 200 meter freestyle at the Beijing Olympics in 2008 with a time of slightly less than one minute and forty-three seconds. If Mr. Phelps could string one hundred sixty-six of these performances together in open water, he could cross the English Channel in about four and three-quarter hours.

In addition to a swimmer’s speed, wind and tide contribute considerably to the crossing time. Le Manche is subject to semidiurnal tide flows. As in San Francisco Bay, there are two high tides and two low tides each day. This produces an oscillating current known as a tidal stream. Each day, the current flows north on the two flood tides and south on the two ebb tides. Since the swim is west to east (starts in France no longer being sanctioned), the tidal current is mostly perpendicular to the swimmer. If all goes well, the current will push the swimmer one direction for about half the swim. Then, the current changes direction and pushes the swimmer the other direction for half the swim, resulting in a safe landing at the closest point, Cap Gris Nez. The land recedes east on either side of the cape and with the complicating factor of tides, a swimmer missing the preferred landfall may take some extra hours to complete the journey. This is why it is so critical for the pilot to have a reliable account of the swimmer’s speed in order to calculate the proper jump time.

In addition to twice daily variations, the average amplitude of the tides also modulates roughly twice a month. When the moon and the sun are on opposite sides of the earth, the full moon appears in its entire splendor. With the sun tugging on one side of the earth and the moon tugging on the other, the Channel and San Francisco Bay experience the highest high tides and the lowest low tides. The phenomenon repeats itself when the moon and the sun occupy the same side of the earth and tug in unison. From the earth’s perspective, the moon is at its “new” or “dark” stage. Mariners call the extra large swings during these two phases of the moon spring tides. In between the full and new moons are the two half moons. The relatively smaller swings during this period are the neap tides. Most swims are launched during the neap tide windows in order to minimize the potential for miscalculation or mishap. However, strong (or desperate) swimmers and experienced pilots will happily tackle a spring tide crossing.

Viewed from above, a swimmer’s track across the face of the earth will be more or less serpentine depending on the strength of the tide and the speed of the swimmer. As witnessed from a global positioning satellite, this course is the “ground track.” The ground track may cover far more than the strict twenty-one statute miles across the ditch. That doesn’t mean that the swimmer is swimming any farther, though.

Imagine a river one mile wide with perfectly straight sides running exactly north and south. If the river is completely still and a swimmer crosses on a due east trajectory, the “water track” will be precisely one mile and the “ground track” will be identical to the water track. With a strong current flowing south, the swimmer will still swim one mile through the water by maintaining a 90 degree angle to the current. The difference is that the landing will happen farther south, making the ground track and water track diverge. If the swimmer completes a mile in thirty minutes, the crossing will take thirty minutes regardless of whether a current is flowing or not. If the current happened to flow the same speed south for fifteen minutes and then suddenly reverse and flow north at the opposite speed for fifteen minutes, the swimmer would wind up on the shore at the same place as if there were no current. In this case, the water track would be straight and the ground track would be convex or peaked.

Based on this trigonometry, pilot’s will generally try to “T the tide,” keeping the swimmer perpendicular to the current. This is not a simple matter of maintaining an east-west heading. Undersea ridges such as Le Colbart and The Varne will subtly alter the current direction. Also, depending on the time before or after high water, the current will shift as it fills or exits bays and rounds promontories. Mike Oram has posted a brilliant introductory exposition of the vagaries that shape a pilot’s decisions throughout the crossing in the Channel Swimmers Google group. He is especially lucid on the topic of tidal diamonds.

Wind is arguably the most important dynamic in a successful swim. It affects water movement and can make the surface rough and choppy. It takes some practice to maintain a smooth and powerful stroke when the water is bouncing around like a wash load set to “agitate.” Wind is also whimsical and capricious. As anyone who follows the local weather report knows, experts can often fail in their predictions.

Knowing these few details doesn’t alter the fundamental admonition to the aspiring channel crosser to “shut up and swim.” Even with the array of electronic navigation equipment available, it is the pilot’s instinct and experience that provide the reckoning to meld the myriad ingredients. Mr. Oram shrewdly suggests that the swimmer concentrate on being the engine that the pilot is steering to success.

No comments:

Post a Comment