The Engineering Behind Surfing

Surfing is a sport that has long been associated with the identity of California. Along almost every stretch of coastline, a casual observer may notice surfers bobbing around in the water. Surfing is also one of the few sports with a direct connection to nature, for the beauty and power of a wave is at the heart of the sport. Once a sport reserved for Hawaiian chieftains, surfing has evolved into a billion dollar industry that is popular around the world. Technological advances have drastically improved the design of surfboards, and this has made the sport more accessible to the public. Yet, when admiring the skill and grace of a surfer riding a wave, few people consider the physics behind catching that wave or the engineering design that produces the board.

The modern surfboard is the product of a hundred years of material and design evolution, but surfing actually has its roots hundreds of years ago in Hawaii, where the boards were essentially long planks of wood. Early California surfers used boards made of redwood. Then, in the 1920s, the lighter, more functional balsa wood became the predominate material used. In the years following World War II, board design changed radically, due to improvements in materials technology developed during the war [1]. During these years, polyurethane foam and fiberglass became the dominant surfboard materials. Then, in the 1960s, the fiberglass and foam technology improved, as did the manufacturing of fins. By the 1970s and 1980s, board design had evolved into the state that it is today, with the advent of the option of number of fins and the creation of various types of polyurethane and fiberglass materials. During this time the epoxy resins were also brought into their current state [2].

Modern surfboard developments include using computer-aided design programs that are synchronized with polyurethane molds in order to create a computer-shaped core. Injection molding may even turn out mass-produced cores in the near future. Research into plastics that could potentially be injection molded into a final product board that would not need to be shaped or coated with fiberglass is currently being conducted. These both have the potential to greatly reduce the cost of a surfboard; however, there is debate over whether the public will accept these mass produced, plastic surfboards, which are known inside the industry as “boards without souls” [1].

Although surfers often dream about the “perfect wave,” in reality, most would be surprised to learn that there is more physics than mysticism involved in riding a wave. Consider the principle of the wave itself: the optimal waves for surfers are generated by offshore storms. Powerful wind currents blow on the water, and the transmitted energy results in waves. These waves are initially large and turbulent. However, over time and hundreds of miles, the waves combine with one another, and the turbulent regions are eventually smoothed out [3]. “As the ocean waves propagate into shallow water, they slow down, and the orbits squash to ellipses. As the waves slow down, the wave-length decreases, and this, coupled with decreasing depth, concentrates the energy of the wave. The wave height increases until the wave becomes unstable and breaks” [4]. Thus, through wave interference patterns and drag, the coastline waves are formed.

A second, vital physical principle behind surfing is the concept of buoyancy. Archimedes’ Principle of Buoyancy is what keeps the board floating and, ultimately, allows the surfer to ride the wave. Archimedes’ Principle states that “a body wholly or partially submerged in a fluid is buoyed up by a force equal to the weight of the displaced fluid” [5]. It is, therefore, the force of buoyancy that counterbalances the weight of both the surfboard and the surfer and prevents both from sinking. Since the weight of the surfer is distributed evenly by the surfboard and counterbalanced by the board’s buoyancy, the surfer can, literally, stand – atop the board – on the top of the water [6].

Once the physical principles of waves and buoyancy are considered, the physics behind riding an actual wave can be better understood. The weight of the surfer on the board produces a force that is straight down. At the same time, buoyancy produces a force that acts perpendicular to the board on the wave: up and at the same angle as the incline. This force, coupled with hydrodynamic forces, push the surfer forward. The sum of all these forces (gravity, buoyancy, and hydrodynamic) results in a forward force that propels the floating surfer in the direction of the wave [3].

While surfers cannot control the physics of wave generation, they can control the buoyancy of the board and its ride. This is done through the changing of both the materials and the design. The surfer must consider the materials used in each of the components of the surfboard: the core, the stringer, and the laminates. In addition, the surfer cannot afford to ignore the design of the board.

The core of a surfboard is made of polyurethane foam. This material makes an excellent core because it is easy to shape, buoyant, closed cell (so it does not absorb water), and relatively inexpensive; however, even a polyurethane core can have varying properties. A smaller cell polyurethane is very dense, having the strength and rigidity needed for large, high-speed waves. On the other hand, a larger cell polyurethane would be less dense and, thus, easier to ride but more likely break [1].

Another vital component to a board is its stringer. The stringer is a wooden center strip that runs the entire length of the board and is usually made of bass wood. Stringers may also be made of balsa, redwood, cedar, evergreen, or mahogany, depending on wood availability and aesthetics. The stringer is embedded into the polyurethane core, and this serves as the board’s “I-Beam,” both stiffening the board and keeping it from breaking in half in the surf [7].

Finally, a surfboard is coated in fiberglass and sealed in resin. The fiberglass is applied in a cloth format and is spread like a sheet over the entire board. Then it is sealed with laminating resin. Both the thickness of the fiberglass and the type of resin can be changed, depending on what characteristics a surfer desires. Typically, a thicker fiberglass cloth is used for the top than the bottom because more stress is applied to the board on the top than on the bottom, and adding the thicker cloth makes that section more durable [8].

The shape of a board is vital to its function. The most obvious component in the shape of a board is its length. There are three main lengths for surfboards. The term “longboard” is applied to a surfboard that is typically above nine feet long. A “hybrid” (or “fun board”) is usually between seven and eight feet long, and a “shortboard” is under seven feet long. The longer the board, the easier it is for the surfer to paddle the board through the water and, correspondingly, the easier it is for the surfer to catch a wave. This longer board works well for smaller waves and for beginners. Shorter boards have increased turning capacity and improved “riding” performance on larger waves.

Even subtle changes in other aspects of a board’s shape can radically affect its performance. Changes in width, thickness, nose width, tail width, nose kick, tail kick, and type of fin or fin placement all create a difference in the interactive forces between the surfer and wave. Although surfboards are usually between 18 and 22 inches wide, small changes in width further affect performance. For instance, a wider board contains more foam, and, therefore, it is more buoyant. This, again, makes the board easier to paddle and, initially, to catch waves. However, a smaller board will be more maneuverable because it has less surface area on the water and, thus, less drag. Furthermore, when riding larger waves, wider boards tend to lose control because the increased speeds result in the edges of the board drifting on the face of the wave. In a similar manner, the thickness of the board affects performance as thicker boards are more buoyant and easier for beginning surfers to use, but offer less control to the surfer.

Nose and tail width are typically defined as the change in width of the first and last foot of the board. A wider nose will prevent the tip of the surfboard from sinking when the surfer moves forward on the board while catching a wave. However, surfers sacrifice performance once they are riding a wave because a wider nose like a wider board cannot significantly cut into waves. Wider tails tend to be more maneuverable in small waves because the increased surface area harnesses more of the smaller, weaker wave’s energy. Smaller tails are superior on larger waves, where “there is an abundance of energy and control is the primary objective” [8].

The nose kick, tail kick, and fin choices further the complexity of board design. The nose kick is the distance that the front of the board is raised. Nose kick plays a very significant part in board design because the nose has to be raised in order to prevent the board tip from going under the water when catching a wave. However, if too much nose kick is added, then the surfboard literally pushes the water as the board moves through it, which generates significant drag and makes it harder for the surfer to paddle the board and catch waves. In addition, the tail kick plays a vital role in board design because a slight increase in the upward curve of the tail also helps the surfer keep the board’s nose out of the water, since a board with tail kick acts as a better fulcrum than a board with a flat tail. Tail kick is limited by the fin placement because the kick cannot rise so much as to inhibit the fin placement in the water [8].

Fin choice adds the final touch to basic board design. Fins vary significantly in design and performance. The larger the fin, the more stable the ride will be, but large fins also make it difficult for the surfer to turn the board. In contrast, smaller fins provide more ability to turn, but there is the potential that the board could drift from side to side and cause the surfer to lose control. Typical surfboard designs offer either one large fin, for beginners, or three smaller ones, for more advanced surfers [1].

Human beings have been riding the ocean’s waves for hundreds of years. Like the surfers of old, modern surfers cannot control the physical forces of the waves themselves. Furthermore, few people – even those who surf – think about either the physics of catching a wave or the engineering of a surfboard. Today’s surfboard designers not only understand the physical principles of buoyancy; they also spend considerable time analyzing and testing new materials and designs. This has enabled record numbers of surfers, with varying skill levels, to enjoy the sport around the world – as well as in the birthplace of modern surfing: California.