Aerodynamics in Sports Equipment, Recreation and Machines - Knuckleball

The Aerodynamics of the Knuckleball

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Knuckleball

First, we need to understand that a knuckleball is different than a "standard" curve ball. The knuckleball is the most difficult pitch to master. The knuckleball is thrown with no spin and at a slower speed. The ball appears to "dance" in the air as it leaves the pitcher's hand and on the way to the batter.

The ball moves through the air in an unpredictable fashion. At first, some people thought the "dance" was caused by passing breezes. However, the most likely reason for the "dance" is a very slow spin. Researchers have learned that a slight change in the orientation (position) of the ball with respect to the air flow results in dramatic changes in the forces acting on the ball. Not only does the magnitude (strength) of the force change, but the direction also changes. This is why the ball seems to dance.

How the forces change magnitude and the direction of the baseball is not known. However, one theory is that the stitches in the ball play an important part. The stitches most likely cause turbulence on and around the surface of the ball. The flow separation is reduced. As the ball slowly rotates it may change to laminar (smooth) flow, which is more separated. This altering state of flow from laminar to turbulent (and separated to attached) could cause the forces on the ball to fluctuate (change) thereby causing the ball to "dance".

It is important to note that even if the pitcher throws the ball without any spin, the flow changes created by the stitch pattern of the ball will cause the ball to spin.

Spitball and Vaseline Ball

The knuckleball is very difficult for a pitcher to learn, much less control. An easier and more effective way is to use a lubricant such as saliva (spit) or vaseline. This causes the ball to slide through the pitcher's fingers and thus have little spin. Therefore, the ball moves like a knuckleball, but at the speeds of a fastball. This makes a spitball next to impossible to hit. That is why they are illegal in professional baseball.

Other dirty tricks used by pitchers include scuffing (scratching) the surface and/or polishing the surface. Just as a rough surface promotes turbulent and, therefore, attached flow, a polished surface will maintain laminar and separated flow. By roughening one side and polishing another, the effects of various pitches will be exaggerated (made greater), helping to confuse the batter. A pitcher, then is not just a ball player, but an amateur aerodynamicist as well.

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