How Average is your Speed?
Chris Jones
On Sunday morning, Kathryn and I were getting ready to paddle from the club to Grants Lock and back, as practice for Waterside A. “Any idea when you’ll be back?” asked Lisa. “Let’s be optimistic, say two and a half hours, that’s 5.2 miles an hour.” After a misspent youth doing silly mathematical puzzles, I really should have known better. We set off into a howling wind, clawing our way southwards “like a mule goin’ up a ladder” as the Byrds put it. After last year’s Thameside 2, nothing bothered us. It was slow work, and after the turn we were an overcoat warmer, and a gear faster. It felt like we were flying along, surfing through the bridge holes, and now and then we’d sprint just for the hell of it. But when we got by Tesco’s clock, it was clear we’d never make it in the time I’d said, not even with Ivan Lawler in the boat. (He wasn’t.)
We met Ray on the towpath in Cropredy. He had paddled, with Melanie, Graham and Alice, to Claydon Locks, then Bourton Lock and back, got changed, and swallowed two cups of tea, two slabs of cake and a pie. He kindly put our boat away. When we offered to help, he grinned and said, “No, you’ve been on the water for three hours.”
“Two hours fifty-eight minutes!” I snapped.
Back at the excuse factory, I sat down with pen and paper, and came up with some surprising calculations. Say you can paddle at 5 mph on calm, flat water, but there’s some factor- wind or current- that alters your speed by 1 mph, so you go one way at 4 mph and the other at 6mph. What’s your average speed? Not 5 mph.
Say you go 6 miles out, turn and come straight back.
6 miles at 4 mph takes 1.5 hours.
6 miles at 6 mph takes 1 hour.
Total 12 miles in 2.5 hours, average speed 12/2.5 = 4.8 mph.
If the factor affects you by 2 mph, it’s much worse: the average drops to 4.2 mph, which is about what we did!
This probably means that in there-and-back races like Windsor Vets and Waterside B, there’s more likely to be record times if there’s not much flow or wind. It also explains why you can’t do the Cherwell Trash in a stubby playboat or a poly pippin with the river flowing nicely, and still get home for lunch.
Measuring average speeds isn’t too easy. Time yourself over a known distance? A bloke in a pub told me that on the old sailing ships, they would throw some unwanted floating object off the sharp end, run to the stern counting seconds, and divide the length of the ship by the time….don’t try this with one of our boats, the Swimmers’ Page in January is a cold and lonely place.
Chris Jones 14 01 08