The Physics of Surfing

Every time you play or do a sport, there’s physics at work. Surfing is no exception! 
Surfing is a great example of how buoyancy, surface tension, mass, shape and hydrodynamic forces can act together to give surfers the perfect ride. Mass and shape relate to the body of the surfers themselves. But buoyancy and hydrodynamic forces relate to the surfboard. 

How has surfboard design changed over the years?

Before the 1920s, surfboards were heavy and huge. They could weigh about 45 kg (ca. 99 lb) and could be up to 5 m (ca. 16 ft) long! 

Surfboards used to be made of wood from Redwood trees. Redwood is not a very water-resistant material. As the surfer surfed, the surfboard would absorb (soak in) water. The surfboard would gradually get heavier.

But in 1926, an American surfer named Tom Blake had an idea. He drilled holes into his surfboard. This removed a lot of wood, which made these heavy boards much, much lighter. Blake then covered the top and bottom of the holes with a thin layer of wood veneer. This design was a huge hit. The world’s first mass-produced surfboard was born!

From then on, people began improving surfboards. In 1932, manufacturers began producing balsa wood surfboards. These surfboards weighed much less, usually about 14 kg (ca. 31 lb). A few years later, manufacturers began adding fixed tail fins. These fins made surfboards more stable. They also made the surfboards easier to steer. The fins helped surfers achieve the right balance of thrust and drag.
 

Surfboard technology stayed the same for decades. But in the 1950s, manufacturers began producing foam and fibreglass surfboards. These surfboards were lighter and stronger than their wooden ancestors.

These new materials were also more hydrodynamic. Hydrodynamics is the study of forces acting on, or exerted by, fluids. It also deals with solids immersed in fluids, like a surfboard immersed in water, for example.

The shape of both the surfboard and its fin change the way the water moves around it. They also affect the surfer’s ability to control the surfboard in the water. 

Varying the shape of the underside of the surfboard will give the surfer a different kind of ride. For example:

• Double-concave boards give a smoother ride
• V-bottom boards turn easily

Surfboard designers actually use a lot of the same principles as rocket science does. For example, the surfboard company Varial Surf was started by a former SpaceX engineer. Their surfboards are made from the same materials as rockets! The company has even created a new, more buoyant type of foam. This foam crystallizes, which forms tightly packed cells. This means more cells can be packed into the same area, making these surfboards lighter and stronger.

What are the physics of surfing?

When you surf, there are several forces at work. The forces of gravity and buoyancy are working against each other. Gravity is pulling the surfboard down. Buoyancy is pushing it up. Which force is greater? It depends on the density of the surfboard’s materials. As you can guess, the materials used in surfboards today are much lighter and less dense than the wooden materials used in the past. Surfboards are also less dense overall than water. This is what allows them to float in the first place!

Of course, the body of the surfer adds weight. Weight is equal to mass times the force of gravity (9.8 m/s²). If a surfer sat perfectly still on a surfboard in the water, that surfboard would likely begin to sink. Keeping a low center of gravity is one of the most important techniques to be a good surfer. To remain balanced and stable, surfers crouch, keeping their center of gravity low. Surfers can also change their position on the board. If a surfer stands in the middle of the board where buoyancy and weight are balanced, the board will be flat. If the surfer moves back, the nose of the board will go up, making it easier to steer.

Meanwhile, drag from the water creates friction in the opposite direction a surfer is moving in. This helps the surfer control the surfboard’s movement.

So how do you use physics to catch a wave? When you are lying on a surfboard, waiting to catch a wave, you are at rest. But when you see a wave approaching, you begin to paddle your arms so that you can move towards it. You accelerate until your velocity is the same as the velocity of the wave. At this point, gravity, buoyancy, thrust and drag are in perfect balance.