Modern Surf Foil Design
Understanding today's best surf foils and where to go foiling
(Updated June 2020)
Whether you are just starting out or a seasoned charger, knowing how surf foils work will unlock more stoke. In this article we will look at how shape, size and materials impact performance, price and rideable conditions. Foiling has changed significantly from the DynaFlite "water-ski foils" invented back in the 1960's. The first attempts at self powered foil surfing used some OG foils MacGyvered onto SUP boards. After several rounds of improvements, foils became more surfable by using lighter materials and higher efficiency wings. Our article “History of hydrofoil surfing” goes into more detail about this evolution.
Determining what foil will work best for you depends on your skill, budget, and where you want to use it. Foils mostly differ in price, maneuverability, speed, and ease of use.
Before we explore these topics lets review the basic components of a foil:
Foils are ridden in fresh and salt water so it is important they can withstand both conditions. Carbon fiber is preferred for its corrosion resistance and high strength to weight ratio. Titanium is frequently used because it is strong, corrosion resistant and can be shaped precisely. Aluminum is also common because it is lightweight and less expensive than titanium. However, aluminum is not as strong as titanium and if precautions are not taken aluminum will corrode in saltwater. Some manufacturers use G-10 composite because it can be less expensive, strong and corrosion resistant, but it is much heavier than the other materials.
The top of the line hydrofoils generally use carbon fiber and titanium, while less expensive foils use aluminum, and G-10 composite.
Boards built for surf foiling are often a sandwich of fiberglass and carbon fiber or simply an all carbon shell with foam core. Purpose built foil boards are designed very differently than traditional boards. They use volume differently and will typically have bottom contours designed to easily release from the surface. It is possible (and usually less expensive) to reuse a SUP or prone board by installing a box to mount the mast but there will be a noticeable reduction in performance and ease of use. A properly balanced board that releases easily and sheds water quickly is one of the best investments you can make.
Front wings need to be lightweight and durable so carbon fiber with foam core or slurry core is commonly used. G-10 composite was used for front wings on some of the early current generation foils but is not generally used today because of weight.
Rear wings are much smaller than front wings so weight is less of an issue. However its connection point with the fuselage experiences high stress so it must be strong. Solid carbon fiber is commonly used, however G-10 composite is also an option.
The fuselage connects the front wing, mast and rear wing. It needs to be durable and capable of making solid connections. Aluminum bar is often used because it can be easily milled. A lighter but more expensive option is to use carbon fiber with titanium reinforcements. This hybrid approach saves weight with carbon fiber while maintaining the strength and precision where it is needed with titanium. Another option is to use carbon fiber with aluminum reinforcement. However unless necessary steps are taken to prevent the aluminum from contacting the carbon fiber, it will cause galvanic corrosion, eventually leading to failure. The steps required to durably treat the aluminum can exceed the cost of using titanium, so it is usually assumed they are not taken if found in a hydrofoil on the market today.
The mast connects the board to the fuselage and supports the board and rider. Needless to say it must be strong. Hollow aluminum masts are very common in hydrofoils today. They are inexpensive and strong, however they can be susceptible to waterlogging, require more maintenance and are heavier than some carbon fiber options. Solid carbon fiber masts are also common, and are roughly the same weight as a hollow aluminum mast. They require less maintenance and are not prone to waterlogging, however they are more expensive and may be less stiff at lengths longer than 28 inches / 70 cm than aluminum. Some premium masts are carbon fiber wrap with core material such as wood. These masts depending on quality can be very light and stiff but for a price.
The best surf foils on the market use carbon fiber and titanium because they are durable, lightweight, stiff and require little maintenance.
The shape of a surf foil mostly comes down to the front and rear wing. They have the most surface area and therefore have the most impact on performance characteristics. There are a few important shapes of a wing to understand; aspect ratio, section thickness and hedral.
Aspect ratio is the ratio of the wing span to its mean chord. Long narrow wings have a higher aspect ratio, while short, wider wings have a lower aspect ratio.
Higher aspect ratio wings are more efficient at lifting, they produce more lift for the same drag when compared to a lower aspect wing. This means they require less energy for pumping and have a higher top speed than lower aspect wings. However high aspect wings have more roll stability and the wing tips breach before wings with a shorter span. This makes them less maneuverable and some would say less carvy than low aspect wings.
Section is the shape of the wing when viewed from the side, perpendicular to the fuselage. The section shape also plays an important role in the amount of lift generated by the wing. In general thicker (taller) section shapes generate more lift, especially at low speeds. This is why the ideal foil for beginners often have a thicker section shape. Thinner sections will generate less lift but also less drag. Thinner sections have a faster minimum take off speed, but they also have a faster top speed. This is why the fastest foils will have a thinner section shape. Some wings are designed with multiple section shapes at different locations along the wing span.
What is hedral?
Wings may or may not include a hedral. When viewed from the front, wings are either flat or angled. An angled wing has "hedral" whereas a flat wing does not. If the wing is angled upward, it is called a dihedral, if angled downward it is anhedral. Wings with multiple angles are known as polyhedral.
Dihedral wings are not commonly found on hydrofoils. Their “v” shape self-stabilizes in the water, which may seem appealing initially but in practice they are difficult to turn and inefficient to pump. Flat wings are gliders, extremely efficient in a straight line and are therefore the best foils for pumping, straight racing and down winders. Flat wings can be used for carving and surfing, but require more effort to turn than less stable wings. However as skill increases it will require less effort to carve on a flat wing. The most maneuverable wings though are designed like stunt planes, where instability implemented correctly becomes an asset.
The best foils for surfing use anhedral or polyhedral wings because they are designed to be usefully unstable. Anhedral wings are shaped like an upside down “v” giving them roll instability. This makes them easier and more efficient to turn which is why are generally best suited for surfing, tighter turns and carving. They can be used for connecting and down winders but will require more effort over long distances than a flat wing design. Because a flat wing has more roll stability they require more energy and skill to maneuver efficiently. A flat wing's tips will also breach earlier, preventing them from turning as tightly as down turned (anhedral) wing tips. That said, flat wings can certainly be surfed and are by many advanced foilers. Polyhedral wings can combine the benefits of more than one shape.
The correct aspect ratio is a similar trade off. Higher aspect ratio wings are more efficient to pump making them less tiring to connect multiple waves, lower aspect wings require less energy to turn, making them easier to surf. The ideal section shape is generally determined by the desired range of speed the foil will be ridden. A thicker section shape will generate more lift at lower speeds, but then top out more quickly as speed increases. Therefore thicker section shapes are better suited for smaller, slower waves where thinner sections are better for larger faster waves.
There are several important dimensions for foiling; lets review wing surface area, fuselage length, and mast length.
The surface area of a wing is a good indication of how much lift it will generate. The larger the surface area, the more lift is generated. Surface area is usually measured in centimeters squared and a good way to visualize it is the size of shadow a wing creates on the ground. The best foil for beginners often use a large front wing because it will lift rider at lower speeds and provide more stability. Also the best foil for small waves will be large enough to generate the required lift off from the limited amount of energy provided by the wave.
However at faster speeds a large wing becomes difficult to prevent from breaching because so much lift is generated. Likewise smaller front wings are easier to control at higher speeds but have a higher minimum speed. The downside is the if the rider slows down too much the foil will stall and the ride is over. The bigger the wave the faster it moves, so the best foil for large waves will be smaller to match the lift generated with the power source.
For example, lets say a 170 pound foiler on a large wing stays on foil from 7 - 14 miles per hour. At speeds under 7 miles per hour the wing does not generate enough lift to raise the rider. At over 14 miles per hour the wing generates so much lift it becomes uncontrollable. When the same rider changes to a smaller wing, they are able to stay on foil from 10 - 20 miles per hour. In this scenario the stall speed increased from 7 to 10 miles per hour, meaning more energy is required to get up and stay up, however the top speed increased from 14 to 20 miles per hour. As skill increases it becomes easier to maintain a higher minimum speed, so decreasing wing size is a way to unlock higher speeds and more maneuverability.
The fuselage length impacts pitch and yaw stability. A longer fuselage will be more stable in pitch and yaw, making pumping more efficient because each pump is longer and more stable. However a longer fuselage will also add weight and increase turning radius which decreases overall maneuverability. With a shorter fuselage turns become tighter and more efficient with the trade off of requiring more energy to pump and to maintain balance.
The mast length impacts pitch and roll stability. Mast length also determines the conditions a foil can be ridden in. A longer mast has more clearance and therefore can cut through higher chop. A longer mast will also keep the foil submerged at steeper angles of pitch found on larger waves. In short, bigger conditions require longer masts. A longer mast also makes pumping more efficient because more leverage is utilized over the axis of rotation. However longer masts are heavier and less stable in pitch and roll. Adding length not only makes the mast more flexible but also amplifies sensitivity to the riders movements. Both of these characteristics make longer masts more difficult to ride. In contrast, shorter masts are lighter, stiffer and more stable.
The best size surf foil depends on the size and skill of the rider, and conditions it will be used in. Heavier riders will need a larger wing to lift them out of the water. Beginners will also find a larger wing easier to learn on because they will be slower and more stable. Lighter riders need the wing to generate less lift so a smaller wing should be used. More advanced riders will find smaller wings more maneuverable and controllable in larger / faster conditions. This is because they create the required amount of lift at higher speeds, but not too much to lose control ability. Generally a shorter fuselage length is preferred for surfing because they are more maneuverable. However if connecting multiple waves is desired a longer fuselage can be used for increased pumping efficiency. but it will not be as maneuverable. In smaller conditions a short mast is almost always preferred. It is lighter and more stable. However longer mast will increase maneuverability even in smaller conditions. In larger conditions a larger mast is practically required. Steeper take offs and cutting through larger chop can only be done on longer mast lengths.
Where are the best places to go surf foiling? One of the most exciting things about this sport is the amount of places that can be foiled. New areas are being found all the time that are not able to be ridden by traditional boards. When first learning it is important to stay in smaller conditions. One to two foot (.5 meter) crumbly white wash waves are ideal. Protected bays with long rolling swells are also great for both beginners and advanced riders. Once your skills advance, glassier conditions with waist high surf are many riders favorite. Surf foiling was invented on Kauai so naturally there are many excellent foil breaks that have been discovered there. Other places in Hawaii, such as the north shore of Oahu a few nice breaks. Because many different types of waves can be foiled, even places not previously associated with surfing are finding riders. South Padre Texas is one example, Thailand also has a growing foil community. The best places to go foiling have probably not been discovered yet.
The best place is where you and a few other foilers can go out and share the stoke. Just be sure to keep as safe distance from others and always wear a leash.