Why Are Golf Balls Covered in Dimples Instead of Being Smooth?

Golfers Noticed That a Scuffed Old Ball Flew Farther Than a New One

The first golf balls with dimples were not designed with air in mind. For a long time golfers played with smooth balls made of gutta-percha, a hard rubbery material from tree sap. Players eventually found that scuffed balls traveled farther than new ones [2]. Nobody needed a theory to act on that. If nicks helped, then put nicks on the ball on purpose.

Around the same time a new kind of ball arrived: Coburn Haskell’s ball, made by winding rubber thread around a core and adding a cover, reached the market in 1901 [2]. Balls with dimples came soon after, and patents for dimpled designs date from the early 1900s (a Smithsonian photo caption gives 1905 for one of them) [2]. Today’s golf balls typically have between 300 and 500 dimples [2].

That is a rule of thumb from the golf course. It does not yet say what the dents do to the air.

Most of a Ball’s Drag Comes From the Swirling Wake Behind It

A ball moving through air is not mainly slowed by air rubbing on its skin. For a sphere, drag is dominated by what happens at the back: the airflow breaks away from the surface, and the point where it lets go is called the separation point [3]. Behind that point is a messy region of swirling air called the wake, where the pressure is low. The ball is pushed forward by high pressure at the front and pulled back by low pressure at the rear, so a bigger low-pressure wake means more drag.

You can see the same thing in a stream. A big rock in a river leaves a pool of churning water behind it, and the wider that churning region, the more the current pushes on the rock.

So the goal is not a slippery skin. The goal is to make the air stay attached to the ball for longer, so that the wake is narrower [3]. On a smooth ball, the air lets go early.

Dimples Push the Point Where Air Lets Go Toward the Back of the Ball

Here is a measurement that makes this concrete. In 2006, a team at Seoul National University put a dimpled sphere 150 millimeters (about 6 inches) across, with 392 dimples, into a wind tunnel and blew air at it at speeds from 5 to 28 meters per second (about 11 to 63 miles per hour). The sphere was built to behave like a real golf ball that is not spinning [1].

They painted the surface with colored oil. The oil is pushed along by the air and piles up where the airflow separates, which shows where the air lets go. Angles here are measured from the very front of the ball, so 90 degrees is the widest part of the ball as seen from the side. On the smooth sphere the air let go at about 82 degrees, just before the widest part. To read the oil clearly they used a sphere dimpled only on its front half, whose drag was nearly the same as the fully dimpled one. On that sphere the air let go at about 110 degrees, well onto the back half, and it stayed there over a wide range of speeds. The drag was reduced by more than 50 percent compared with the smooth sphere [1].

One more detail: the drag of the dimpled sphere stayed nearly constant as the wind got faster, once it had dropped. An earlier wind-tunnel study from 1976, by Bearman and Harvey, had also found that dimples lower the speed at which the drag drops and that drag then stays nearly constant [4]. “Up to half” is the best case, not a promise for every ball and every speed.

The next question is the odd part: how do dents get the air to hang on?

Dents Stir the Air Right Next to the Ball So It Clings Longer

Next to the ball there is a thin layer of air, called the boundary layer, that moves along the surface. On a smooth ball this layer flows in smooth sheets and does not have much forward energy left by the time it reaches the back, so it peels away early. A turbulent, churned-up layer mixes faster air from further out into the slow air near the surface. That gives it more forward momentum and lets it follow the curve of the ball longer, which leaves a narrower wake [3].

The dimples are what start the churning. In the Seoul experiment, the air first separates inside a dimple, which sets off a rapid increase in turbulence in the thin separating layer of air. Then the air reattaches to the ball, now carrying a lot of momentum near the wall, and it can push through the rising pressure at the back of the ball instead of peeling off [1]. The authors measured this directly with a very thin wire probe that records the air speed just above the dimples. Later computer simulations of a spinning golf ball also found the fluid motion inside the dimples that leads to reduced drag [4].

The 2006 paper also corrected an earlier guess. Bearman and Harvey had suggested that dimples shed distinct little vortices that energize the layer. The Seoul team looked for such vortex pairs near the surface with a smoke wire and did not find them in their setup, and they noted that the earlier studies had different dimple sizes and different incoming flow [1]. The overall picture holds up, but details of how a particular dimple shape works depend on the conditions.

This explains why the dents help. It does not yet say when they help, and that turns out to matter.

Dimples Only Help Above a Certain Speed, and Can Hurt Above Another

At the slowest wind in the Seoul experiment, 5 meters per second (about 11 miles per hour), the dimples did not reduce drag at all. The air separated and reattached inside a dimple, but the reattached layer did not have enough energy to resist the pressure at the back, so the main separation did not move and the drag stayed high. By about 9 meters per second (about 20 miles per hour) on the paper’s speed scale, the drag had dropped sharply [1]. A golf ball is hit fast enough to be well into the range where it helps.

Soccer shows that it is also not true that dents always win. In 2017, researchers at the University of Tsukuba in Japan made ten kinds of soccer ball prototypes and tested them in a wind tunnel. At medium speeds the dimpled prototypes had lower drag than smooth ones, but at the higher speeds they tested, the smooth prototypes had lower drag [5].

The simulation of a strong kick (30 meters per second, about 67 miles per hour, launched at 30 degrees) for a 12-panel prototype gave 55.1 meters (about 60 yards) for the smooth ball and 49.0 meters (about 54 yards) for the dimpled one. At a gentler 15 meters per second and 25 degrees, the conventional 32-panel ball with dimples traveled 17.9 meters against 17.4 meters for the smooth one [5]. These were research prototypes, not balls from a store, so this is not a verdict on any real soccer ball.

So the dimples are not magic. They move the speed at which a ball’s drag falls, and for a golf ball that speed has been moved to where golfers actually hit.

Backspin Gives the Ball Lift, and the Rulebook Keeps the Ball Fair

Drag is only half of a golf ball’s flight. Golf balls are hit with backspin, and spin bends the airflow around the ball, producing an upward force called lift (the Magnus effect) that extends the flight [4]. A golf ball’s design aims to reduce drag and to maximize the lift from backspin [4].

Because the details matter so much, golf has rules. The R&A and the USGA, the bodies that write the rules of golf, say a ball must weigh no more than 1.620 ounces (45.93 grams) and be no smaller than 1.680 inches (42.67 millimeters) across. It also must not be designed or modified to behave differently from a perfectly round, symmetrical ball [6].

Try It at Home: Watch the Wake Behind a Spoon in Water

You cannot see air, but you can see a wake in water. This is a classic classroom-style demonstration, not something from the sources below, so treat it as your own experiment. It shows a wake behind a blunt object. It does not show what dimples do.

1. Fill a sink or a large bowl with water and let it go still. Sprinkle a pinch of dried herbs or a few tea leaves from a bag on the surface so you can see the water move.
2. Hold a big spoon with its round back facing forward. Push it slowly through the water and watch the leaves behind it.
3. Try again, a little faster. Is the swirling area behind the spoon wider or narrower? What changes if you turn the spoon sideways?

Then look at the balls around your home. A golf ball has its dimples. A tennis ball is covered in fuzz and a soccer ball has panel seams. Which of them looks smooth, which looks rough, and how fast does each one usually fly?

If you want to read more, the 2006 Seoul paper (only four pages long) and the 2017 Tsukuba paper are both free to read online, and you can compare their graphs with what this note says.

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