So why does a dimpled golf ball travel further than a smooth golf ball?
The answer has everything to do with drag, even though that may seem a
bit counterintuitive at first. As air moves past a sphere, two types of
drag develop. The first is drag due to friction, or the drag of the
air hitting the front of the ball. The second and more prominent type
of drag, however, comes from the separation of the airflow behind the
golf ball (see top of figure). As the air loses its ability to stick
close to the ball (known as the boundary layer), it separates from the
ball and creates lots of little eddies that basically try to pull the
ball backwards. Thus, to make a ball fly further, the separation of the
boundary layer must be delayed as long as possible. To do that, there
are basically two options: increase the speed of the ball through the
air (which isn't really possible since a player can only hit the ball so
hard) or find a way to force the air to stay in the boundary layer
longer. And that, Mr. Hobson discovered, could be done by roughing up
the ball.
Why does a rough ball help delay airflow separation? The rough parts
(or dimples) create very small amounts of turbulence near the ball's
surface that actually add energy to the air in the boundary layer. More
energy means more sticking power which means the airflow separation
occurs later. The later the separation, the fewer eddies behind the
ball thus decreasing the force trying to pull the ball backwards and
allowing the ball to fly further. (See bottom of figure).
Obviously, the physics involved are more complex than this basic
explanation, but hopefully you get the idea. Golf ball manufacturers
are constantly doing more tests to figure out the ideal number of
dimples, but it's generally accepted that between 300 and 450 dimples is
ideal. The USGA also stipulates that the dimples must be symmetrically
placed around the golf ball because asymmetrically placed dimples help
to keep a ball spinning upright and thus nearly eliminate the chance for
hooks and slices.
