Swing bowling
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Swing bowling is a technique used for bowling in the sport of cricket. Practitioners are known as swing bowlers.
Swing bowling is generally classed as a subtype of fast bowling.
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Physics of swing bowling
The essence of swing bowling is to get the cricket ball to deviate sideways as it moves through the air towards the batsman. In order to do this, the bowler makes use of two factors:
- The raised seam of the cricket ball.
- Asymmetry in the ball caused by uneven wear of its surface.
The asymmetry of the ball is encouraged by the constant polishing of one side of the ball by members of the fielding team, while allowing the opposite side to deteriorate through wear and tear. Over time, this produces a marked difference in the aerodynamic properties of the two sides.
As a fast ball is bowled with the seam held vertically and rotation about a horizontal axis, there are two possible factors that can cause the ball to move through the air:
- If the seam is aligned to point slightly to one side or the other, the slipstream of air moving over the raised seam causes an aerofoil effect, pushing the ball to one side.
- The flow of air over the two different sides of the ball causes differential pressure based on the surface qualities of the sides. This causes the ball to curve through the air towards the rougher side.
In combination, these two effects can produce considerable sideways movement of the ball through the air, known as swing.
Note that swing of a cricket ball is not caused by the Magnus effect, which gives rise to a force perpendicular to the axis of rotation (in this case up or down). The swing of a cricket ball is parallel to the axis of its rotation.
Normal swing
Typically, a swing bowler aligns the seam and the sides of the ball to reinforce the swing effect. This can be done in two ways:
- Outswinger: By aligning the seam to the left and placing the roughened side of the ball on the left, the ball will swing to the left. To a right-handed batsman, this results in the ball moving away to the off side while in flight, usually outwards from his body.
- Inswinger: By aligning the seam to the right and placing the roughened side of the ball on the right, the ball will swing to the right. To a right-handed batsman, this results in the ball moving in to the leg side while in flight, usually inwards towards his body.
The curvature of swing deliveries can make them difficult for a batsman to hit with his bat. Typically, bowlers more commonly bowl outswingers, as they tend to move away from the batsman, meaning he has to "chase" the ball in order to hit it. Hitting away from the batsman's body is dangerous, as it leaves a gap between the bat and body through which the ball may travel to hit the wicket. Also, if the batsman misjudges the amount of swing, he can hit the ball with an edge of the bat. An inside edge can ricochet on to the wicket, resulting in him being out bowled, while an outside edge can fly to the wicket-keeper or slips fielders for a catch.
An inswinger presents relatively fewer dangers to the batsman, but can result in bowled or leg before wicket dismissals if the batsman misjudges the swing on the ball.
An inswinger combined with a yorker can be especially difficult for the batsman to defend against, especially if used as a surprise delivery after a sequence of outswingers.
It is a common belief that balls swing more in humid weather conditions, although no objective research exists to bear this out.
Reverse swing
Normal swing occurs mostly when the ball is fairly new. As it wears more, the aerodynamics of the asymmetry change and it is more difficult to extract a large amount of swing.
When the ball becomes very old—around 70 or more overs old—it can begin to swing towards the polished side rather than the rough side. This is known as reverse swing.
Reverse swing is difficult to achieve consistently, as it relies on uneven wear of the ball, tends to occur mostly in hot, dry weather conditions, and requires bowling at high speed. Normal swing can be achieved at relatively moderate bowling speeds, but only the fastest bowlers can regularly produce reverse swing.
Reverse swing tends to be stronger than normal swing, and to occur late in the ball's trajectory. This gives it a very different character to normal swing, and because batsmen experience it less often they generally find it much more difficult to defend against.
Notable practitioners of reverse swing have mostly been Pakistani. Sarfaraz Nawaz and Imran Khan are often credited as the first bowlers to produce reverse swing, and they have been followed by bowlers such as Wasim Akram, Waqar Younis and Shoaib Akhtar.