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How many dimples are on a golf ball?

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2015-05-27 02:43:21

Most balls on sale today have about 300 to 450 dimples.

There were a few balls having over 500 dimples before. The

record holder was a ball with 1,070 dimples -- 414 larger ones (in

four different sizes) and 656 pinhead-sized ones. All brands of

balls, except one, have even-numbered dimples. The only

odd-numbered ball on market is a ball with 333 dimples.

Officially sanctioned balls are designed to be as symmetrical as

possible. A ball can have six rows of normal dimples on its

equator, and very shallow dimples elsewhere. This asymmetrical

design helps the ball self-adjust its spin-axis during the flight.

The USGA did not sanction it and changed the rules to ban

aerodynamic asymmetrical balls. The ball supplier sued the USGA and

the USGA paid U.S. $1.375 million in an out of court

settlement.

The number of dimples on a golf ball varies, depending on the

manufacturer and may even be different for different models made by

the same manufacturer. The dimples are usually the same size as one

another, but some golf balls have several different sizes of dimple

on the same ball. Any number between 300 and 500 dimples is

reasonable, and 336 is a common number. Not just any number will

do. Golf balls are usually covered with dimples in a spherically

symmetrical way, and for many values of N, it is impossible to

cover the golf ball uniformly without gaps. Symmetry is important

or the ball will wobble or its flight will depend on which part of

the ball is forwards or sideways as the ball spins. You can get an

idea of how to space dimples uniformly around a sphere by thinking

about the "platonic solids" -- the tetrahedron, cube, octahedron,

dodecahedron and icosahedron, and placing a dimple at the corners

of an inscribed platonic solid. Variations on this theme give the

corners of Buckminster Fuller's geodesic domes, and also the

possible symmetrical locations of dimples on a golf ball.

Why does a golf ball have dimples on its

surface?

When a golf ball is flying through the air, it has three forces

on it. Two of them are gravity and drag from the air. In addition,

if the golf ball is spinning, there will be a Magnus force on it

(which may point up or down or either side, depending on how it is

spinning -- the Magnus force cannot point along the flight path of

the ball however -- see our answer on the Magnus force for more

information).

The main goal of making a golf ball go farther however is to

reduce the force of drag as it flies through the air. In general,

turbulence increases drag, because the energy needed to stir the

air up and make it swirl around is energy that the ball has lost.

One might think that a rough ball will induce more turbulence in

the air than a smooth one, but it depends on how fast the ball is

going. For balls going slowly through a viscous fluid, then the

fluid just moves a bit to the side as the ball passes, and then it

returns more or less to where it was. If the fluid motion is

smooth, we call the motion "laminar", otherwise it is "turbulent."

A ball moving quickly through a fluid like the air will have air

flowing in a laminar fashion in some places and in a turbulent

fashion in others. Directly behind the ball there will be a

turbulent "wake", and surrounding that will be smoothly flowing

air. The whole idea behind reducing the drag is to make the

turbulent wake small.

The air that slides past the ball very close to it is called the

"boundary layer". At the place where the turbulent wake starts is

called "separation of the boundary layer" where the smoothly

flowing air departs from the ball and does not close up behind the

ball nicely but rather swirls around in small vortices. If the

boundary layer can be encouraged to stick to the ball a little

longer, then the turbulent part of the wake can be reduced. It

turns out that adding a little extra turbulence in the boundary

layer itself all over the ball allows the main smoothly-flowing air

currents to stay closer to the ball and delays the separation of

the boundary layer. Some nice pictures of balls in wind tunnel

showing this effect can be found here.

There is also an increase in the Magnus force, giving the ball

some lift when it is spinning in the correct direction. This force

helps keep the ball in the air longer, allowing it to travel

farther.

People have thought of putting dimples on everything from

swimsuits to cars to airplanes. You only get an advantage from

these dimples if the boundary layer can be made to stick longer to

the object. Some cars just have vertical flat ends to them where

the trunk comes down and there is no way to reduce the turbulent

wake of these no matter how dimpled the paint is. And the boundary

layer stays with airplane wings except maybe a bit at the ends

(some gain can be made by putting small rods out on the tips or on

the trailing edges of the wings).


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