Wednesday 28 July 2010

Does not compute

From The Daily Mail:

After five years in training, Mrs Cobell had just been crowned the slowest person to cross the Channel under her own steam. In a remarkable marathon of grit and determination, she had struggled valiantly through changing tides that swept her first one way, then the other. It turned the 21-mile crossing into a gruelling, 65-mile slog... soon after leaving Dover a strong tide dragged her south instead of east. Then it took her north... south again... northeast... south-west... and, eventually, subsided to allow her to carry on eastwards.

The article is accompanied by a chart plotting her 65-mile swim, and the captions suggest that she actually swam 65 miles:


Nonsense.

Imagine you are trying to cross a particular long and wide escalator from the left handrail to the right handrail (representing the English and French coasts). Does it make much difference to you whether the escalator is moving up, is moving down, or is stationary (representing the tides, which move up and down the English Channel), if all you care about is getting from the left to the right?

Nope.

Furthermore, if she really had swum 65 miles in 28 hours, then that would be an absolutely heroic 2.3 miles per hour average speed. For comparison, an ironman triathlon event starts off with a brisk 2.4 mile swim, for which they are allowed a maximum of 2.2 hours, so even at this stupendous level of fitness they are 'only' expected to be able to swim at 1.1 miles per hour (and Mrs Cobell managed at least 0.75 miles an hour on average, probably a bit more, so she must be pretty damned good).

But never mind all that... Jackie Cobell, you rock!

5 comments:

SimonF said...

Its not quite that simple, Mark. If you think the crossing will take 12 hours, or any multiple, then you can just swim (sail) on the same bearing. You then get the effect you describe where the tide washes you back and forth and you end up in the right place.

Anything outside these multiples and you'll miss the destination by an amount determined by the strength of the tides. Around Dover they are quite strong and and if you are unlucky you can end up doing some swimming against the tide.

What you mustn't do is try to swim (or sail) down the direct path as this means swimming into the tide at all times and correcting the bearing of swim.

Mark Wadsworth said...

SF, I'm sure nothing is that simple, but to claim she swam 65 miles in 28 hours is clearly hokum.

Let's also not forget that Mrs C was indifferent as to where she landed - had she been aiming for a specific spot then it is quite possibly the case that she would have had to swim more than 21 miles relative to the water itself.

Bayard said...

"but to claim she swam 65 miles in 28 hours is clearly hokum"

No it isn't. She may not have swum 65 miles through the water, but she swum 65 miles over the ground. Yes, she achieved a average speed that an Iron Man contestant would have envied, but she did this because the tide was helping her. If I sail down the Menai Straits at 10 mph, it matters not whether my speed through the water is 3 mph or 10 mph. it still takes me the same time to get from one end to the other.

And yes, the newspaper article was misleading in this respect, but it's a newspaper article. In the Mail. That's what they do.

J said...

Her physical distance travelled would have been 65 miles, but she would only have swum the cos component of the little triangles that you can draw over the legs of her journey (approximately) assuming that her body did not change its heading and the deviations were due to the direction of travel of the water.

Mark Wadsworth said...

EKTWP, is that your fancy waying of saying "she was swimming in the same direction relative to the water around her"?