Arjun had been watching too much Test cricket. He spent the whole of the first over of our Sunday match polishing one side of the ball on his trousers, until his left thigh had a red smear on it and the ball had one side that shone like a cherry, and then he bowled six deliveries that went perfectly, disappointingly straight.
“It’s the pitch,” he said.
“We’re playing on a car park,” I said.
He bowled the next over with even more polishing, and the result was the same: fast, straight, easy. Kabir hit two of them over the wall. By the drinks break, Arjun was sitting on the kerb asking whether swing was even real or just something commentators said to fill the time.
It is real, and it is one of the strangest things in sport. When a ball flies through the air, the air right next to it doesn’t just slide past. A thin skin of it, less than a millimetre thick, is dragged along by the surface. Engineers call this the boundary layer, and on the front half of a ball it flows smoothly round the curve. Somewhere on the back half, it can no longer hold on to the curve and peels away, leaving a churning wake behind the ball.
Where it peels away depends on what kind of flow it is. A smooth, orderly boundary layer, called laminar, gives up early, not long after the widest point of the ball. A turbulent one, full of tiny swirls, has more energy close to the surface, so it clings on further round the back before letting go. That difference is the whole secret of swing. If the air peels off early on one side and late on the other, the wake behind the ball is pushed off-centre, towards the side where it left early. Pushing air one way pushes the ball the other.
The seam is how a bowler chooses which side goes turbulent. Angle it, and on one side the air meets the raised stitching, trips over it, and turns turbulent; on the other side the air never touches the seam and stays smooth. The turbulent side clings longer, the wake is deflected away from it, and the ball curves towards the side the seam is pointing. That is also why fast bowlers keep one side shiny. A polished half helps the air on that side stay smooth, so it keeps letting go early, and the contrast with the seam side stays sharp.
So the polishing wasn’t Arjun’s problem. His grip was. He had learned to bowl with a tennis ball, and he still held the leather ball the same way, with the seam bolt upright, pointing straight down the pitch at the batter. With the seam dead straight, both sides of the ball meet the air in exactly the same way. Both boundary layers peel off at the same place, the wake sits straight behind, and there is nothing to push the ball sideways at all.
After drinks, I made him tilt the seam towards the slips, just a little, and keep his wrist firm so it stayed that way in the air. The first ball did nothing. The second one did nothing much. The third started on middle stump and drifted, late and lazy, towards the slips, and Kabir edged it straight to me.
Arjun ran down the car park with both arms out like he had won the World Cup. I’d like to tell you the next ten balls all swung. Two of them did. One of the others hit a scooter. But two was enough for Arjun to start explaining boundary layers to everyone at the chai stall, which, considering he’d learned the phrase an hour earlier, was impressive.
Try it yourselfMake a tape ball
- Cover exactly one half of the tennis ball with smooth, tight tape, leaving the other half fuzzy. The edge of the tape is your "seam".
- Throw or bowl it at a steady, fairly quick pace with the taped side facing one way. Watch where it drifts.
- Turn the ball round so the taped side faces the other way and throw again.
- Try slower and faster throws too. When does it move most?
What's going on?
Street cricketers have used tape balls for decades because they swing. The fuzzy half and the smooth half handle the clinging layer of air differently, so the air lets go of the two sides at different points. The wake ends up off-centre and the ball is pushed sideways. If the direction changes as you throw harder, you have found the same effect that lets fast bowlers reverse-swing an old ball.
Safety Throw in an open space away from windows and people.