Fin

A surfboard fin or skeg is a hydrofoil mounted at the tail of a surfboard or similar board to improve directional stability and control through foot-steering. Fins can provide lateral lift opposed to the water and stabilize the board's trajectory, allowing the surfer to control direction by varying their side-to-side weight distribution. The introduction of fins in the 1930s revolutionized surfing and board design. Surfboard fins may be arrayed in various number and configuration, and many different shapes, sizes, and materials are and have been made and used.

History

Historically, surfboards had no fins; some amount of control was achieved through convex hulls and the surfer dragging a foot in the water. A common problem with these early boards was "sliding ass", in which the tail would slip sideways, usually causing a wipeout.

The first fixed fin was introduced by surfing pioneer Tom Blake in 1935. In Waikiki, Blake attached a 12in long, 4in deep metal keel from an abandoned speedboat to his surfboard, and was immediately impressed with the results. Around 1936, Woody Brown independently added a fixed fin to his second surfboard design in San Diego, which further popularized the feature.

George Greenough made the fin flexible and took inspiration from the fins of fish (allegedly tuna) and dolphins. He altered the design from a wide-based, cumbersome keel to a more powerful and efficient dolphin fin-like foil.

The stability and control fins allowed revolutionized the sport, though many surfers avoided them for several years; evidently they were thought dangerous.

Bob Simmons was apparently the first to use more than one fin on a surfboard. He introduced parallel double fins in the 1940s, which became periodically popular. Simmons used a twin-keel design on innovative wooden boards of his own design and manufacture in the 9'-10' range, which he rode in large surf at Windansea in California. The feature grew more common in the mid-1940s and became the universal industry standard in the 1950s. Experimentation with fin design and configuration increased after 1966 with the popularization of shortboards.

In the 1970s, multi-fin systems became much more widely used, in competition and by average surfers, as top professionals like Larry Bertlemann and Mark Richards enjoyed competitive success maneuvering shorter boards with twin fins in smaller surf and tighter radius turns.

It wasn't until 1981 that Simon Anderson invented the popular thruster set-up (three fins - two on the rail 10-12 inches from the tail end, one center fin 3-5 inches up from the tail) which stabilized the board compared to the twin-fin set-up, and provided more control and lifting surfaces in an effective configuration. The design was an immediate competitive success for Anderson, inasmuch as he immediately won 2 very famous surf contests using "thrusters," and the entire surfing world quickly followed his lead. The thruster is the dominant fin configuration to this day, in both recreational and competitive surfing.

Configurations

In surfing, there are two major types of (typically stationary) surfboard fins (hydrofoils), and a host of illustrative issues.

Both a skeg and "rail fins" stabilize the motion of the surfboard. They also contribute to the desired effect of converting the (kinetic energy) push of the sloped wave face combined with the rider's mass on the sloped wave face (potential energy) into redirected energy - lift (lift) - the surfer deflects his surfboard and fins off the water of the wave face (and/or vice-versa) to make forward progress across the wave face, or "down the line," that is, parallel to the wave crest and beach - riding parallel to the crest (perpendicular to the pull of gravity down the wave's slope) in this way is known as "trimming." Lift (aka "drive") from the board and its fin(s) is what enables all maneuvers in surfing.

A "skeg" (an upright, streamlined, often raked keel) typically denotes one centrally-mounted stabilizer foil mounted perpendicularly to the riding surface, at the rear of the surfboard.

Smaller surfboard fins mounted near the edge (or "rail") of the surfboard are known as "rail fins" and are seen in multi-fin arrangements (often in combination with a similarly-sized central fin further back on the board). Rail fins enable high-performance surfing, and are most often "single-foiled," with one flat side and one "foiled" side, as seen on an airfoil, for greater lift.

A fin configuration with fins near the edge of the board stabilizes and contributes lift during turning maneuvers, which contributes to the board's ability to "hold" during turning maneuvers. Rail fins are often seen in addition to a central fin, but can be used without a central fin as well. Some of the most popular multi-fin configurations use two rail fins (a "twin-fin"), two rail fins plus a similar-sized central fin mounted further back (e.g. a "Thruster"), or four fins (a "quad"). Rail fins are more or less engaged by the rider's heel and toes as they lean in the desired direction of their turn. As the rider does so, an "inside" rail fin sinks deeper and its angle of attack is increased, as is its lift-induced drag. Rail fins also add lift (known as "drive") in trim and with greater holding ability, enable steeper wave faces to be ridden and higher speed "down the line."

Rail fins are typically "toed-in," that is, the leading edge of the fins are oriented toward the centerline of the surfboard, which decreases the angle of attack in trim, which makes it easier to initiate turns. "Toeing in" rail fins also adds drag on the "outside" fin, as its angle of attack is negative during trim or in a turn. These combined factors of toed-in rail fins cause several issues: drag on a toed-in outside rail fin can slow the board down in trim, but it can also give a braking effect during turns that is useful. The inside rail fin (and the board itself) can be "pumped," attacked and re-attacked, by swerving up and down the face, causing acceleration down the line, or similarly pumped to achieve a desired trajectory through a multi-stage turn. At higher speeds, the drag off toed-in rail fins can cause surfboards to oscillate and become unstable - a phenomenon known as "speed wobbles".

Most surfboards intended for larger waves are longer (to increase hull speed for paddling, wave-catching, and surfing), and as most shapers orient the rail fins toward the nose of the board, a longer board inherently results in reduced toe-in of rail fins, therefore less negative angle of attack, less oscillation, greater stability, and higher speeds. Rail fins also typically have some degree of "cant," that is, are tilted out toward the rail they are adjacent to. This is a significant additional factor in lift at various attitudes, drag, and performance, as are the variables of other foils - including flexibility, thickness, and planform.

Rail fins evolved into being and surged into popularity as riders (Simon Anderson, most famously) sought a solution to two major performance issues of a central "single" fin - both related to engagement of the foil: For one, a centrally-mounted fin is tilted up out of the water as the board is leaned over, and thus it loses more and more of its lift as the lean angle increases - if the lean angle is acute enough, the fin's tip can be the only area left in the water; the tip may then rapidly stall and, having lost its lift, become disengaged from the water, leaving the board's bottom as the only control surface still operating. Before rail fins became (extremely) popular, this tendency of "single fins" led to riders "nursing" turns - this tendency was a significant limiting factor on performance. The enhanced hold offered by rail fins during turning led to more types of maneuvers being possible. The other major issue leading to rail fins' use is the fact that a rider can use the lift near the rail to increase speed and performance on smaller waves due to the above effects and abilities of these foils.

In windsurfing, a derivative of traditional surfing, skegs are also often used as a central stabilizing fin (hydrofoil) located at the rear of the board. A windsurfer's skeg also has the effect of producing lift, which allows the rider to direct the craft laterally against the lift the sail (itself an airfoil) produces. The skeg has undergone numerous phases of development and, as with other foils, its design is determined by the balance of the pressures it experiences in use, including lift, drag, ventilation and stall.

Terms and Measurements

Toe

The angle of the fin (box) in relation to the center stringer is known as a fin's toe. Most side fins can be described as toe-in, meaning that the front of the fin is turned in closer to the stringer. Toe-in causes the water to pressure the outside of the fins, which in turn makes the board more responsive to rider input.

Cant

The cant of a fin is the angle it makes in relation to the bottom of the surfboard. A fin that sticks straight up, perfectly perpendicular to the board's base contour, is said to have a no cant. Canted fins point outwards, toward the rails of the board. Increasing the fins' cant leads to a more responsive board through turns, while decreasing the cant (bringing it closer to 90°) makes the board faster, especially when traveling in a straight line.

Foil

When you look at a surf fin, you'll notice that it is shaped in an aerodynamic fashion from its front edge to the backside. Most often, the thickest portion of the fin is the in the middle, while the thinnest part is the outer edges. This shape is known as the fin's foil, and it has a big impact on the way the water flows under the board. Some fins are flat on one side and foiled on the other (usually side fins), while others are foiled on either side (single/center fins). The idea is to create lift under the surfboard and help propel it most effectively in different wave conditions. The more pronounced the foil, the more lift it will provide. Unfortunately, this also causes more drag on the board, which slows it down.

Rake

The rake measures how far back the fin curves in relation to its base. To find a fin's rake, imagine a flat line continuing out from the base of the fin; next imagine a line that extends from the back of the fin base to the very tip of the fin. The angle that these two lines form where they intersect at the back of the fin base is the rake. The smaller the angle, the farther back the fin tip reaches (a larger offset from the base). Fins with a small rake/large offset will propel the board faster and remain fairly stable, but there is a sacrifice in turnability. Fins with a large rake/small offset give the surfboard a tighter turning radius, but don't offer as much stability. As usual, you'll need to strike an effective balance determined by the type of waves you ride and the style of board you have.

Flex

The flex or stiffness of a fin plays a big part in the way a surfboard will ultimately handle on the water. If you are a beginner, stiff fins are more forgiving and will give you the stability you need, so you might start there. Their lack of flex makes it hard to make sharp turns, and the turns you do make will be wide and sweeping. However, a stiff fin has the tendency to revert quickly to its natural position, so the turns will be faster than with a flexy fin. Flexible fins add a level of feel to the board that is hard to match with their stiffer counterparts. They are slower to reach their maximum flex, meaning the board continues to respond to the rider's input throughout a turn. This can cause some problems for new surfers, as it makes the board more difficult to control.

Base Length

This measures what is usually the widest point of the fin's outline – the base. The base length impacts the way that a board will turn and drive (accelerate through turns). A longer base provides more surface area to push against the water, so turns are stronger and drive is increased. However, the larger base doesn't permit the board to turn sharply, so if that's what you're after, go for a fin with a shorter base length.

Height/Depth

The depth of a fin is basically how far it sticks into the water, measured from the bottom of the surfboard to the tallest point on the fin. Depth affects the way a board will grip the water through turns, and it determines how well the board remains stable. The taller the fin, the more hold it will have in the water, providing the rider more control. While shorter fins don't grip the water as well, they do allow the board to slide-out a little, which some surfers enjoy.

Setups and Placement

	Single Fin The single fin setup is the original fin setup. Single fin setups are common on long boards, as well as on beginning surfboards. They are usually long and wider than other fins, which make the board controllable with only the one fin.The idea behind the single fin is to provide stability and control. Unfortunately, the increased control also means a sacrifice in performance, as the movements that a single fin allows are restricted to sweeping turns and straight-lined charges.
	Twin Fin The twin fin setup has two smaller fins mounted near the rail. Twin-fin boards are harder to control in large waves but offer good maneuverability in smaller conditions. They are usually found on shortboards and fish boards, because the two-fin setup encourages speed.
	Thruster The most common setup, the "thruster" is a tri-fin and is found on all kinds of boards. It performs well under most ocean conditions, lending a stable feel to a maneuverable board. All the fins are the same size. The outside fins are flat on the inside to increase drive, while the center fin is foiled normally. Additionally, the outer fins are toed-in to speed up the board and allow it to turn more easily. A thruster set-up is the go-to choice for most shapers.
	2+1 Setup The 2+1 denotes a larger center fin (for reference, larger than a thruster center fin) with 2 small to medium-small fins at a position close to thruster rail fin positions. The "sidebites" contribute some lift, control, and stability to the board when it is "on rail," arcing through turns. Typically, "sidebites" are removable, so the surfer can take them out for use in smaller waves, which gives less drag and freer turning. The 2+1 is a popular configuration for mid length to long boards.
	Quad Setup The quad setup is four fins, two on each side, in a similar position to the rail fins on a thruster. The fronts are typically larger than the rears but this is not always the case. The rears are nearly always inboard and aft of the fronts . The exact measurements and configuration of the quad set-up can vary pretty widely. This setup is often used in short boards and provides more lift and control surface near the rail. There is no center fin.
	Twinzer Setup The Twinzer is a design by Wil Jobson and similar to the Campbell brothers' "Bonzer," the fin set-up is held to be functionally integral and synergistic with the bottom contours of the board, specifically a "bat-tail" with an integral convex/double-channel. The fin set-up itself is four fins, two on each side, in a similar position to the rail fins on a thruster. The fronts are smaller than the rears, often roughly 1/3 the size, mounted ahead and outboard of the fronts, with ~8 degrees of outward cant, and notably, the fins' trailing edges are behind the leading edges of the main fins. The water coming off the trailing edge of the "canards" becomes part of the flow "behind" the main fins. This fact is held to enhance the lift and speed of the set-up, because of the "slot effect." The exact measurements and configuration of the twinzer are basically standardized by Jobson, but some variation is seen amongst different builders.
	Tunnel Fin See Tunnel fin.
	Bonzer The Bonzer is a 3- or 5- array invented by the Campbell brothers in Oxnard, California in the early 1970s for the powerful waves of a well-known wave near their home. The Bonzer array is an approximately 7" center fin aft and either two or four delta-shaped fins ("runners") mounted near the rails in somewhat similar fashion to other rail fins, but they are substantially lower aspect and aggressively canted outward. The Bonzer array is firmly held to be an integral part of the Campbell brothers' overall board design featuring double concave bottom contours out the tail.
	Diamond Quad The Diamond Quad is a design by John Coby of Noosa Australia. It was first test surfed on 16/7/11 at Teatree Point Noosa. The purpose of the design was to induce more looseness into longboards. The design features a stabiliser tail fin, two side fins, and a destabiliser fin in the centre, and anterior to, the side fins, called a 'dorsal fin'. The initial test, performed on a 9'6" Tolhurst Tuflite, with a retro-fitted slot for the dorsal fin, was extremely successful making the board feel at least 50% looser. After continuous development, the Diamond Quad has seen the tail fin become smaller and the side fins become bigger. The main drive in the Diamond Quad now comes from the side fins. The dorsal fin is small, 3.75" deep and usually somewhere between 600mm to 680mm forward of the tail (to trailing edge of fin), depending on the length and width of the board. The final 'sweet spot' of the dorsal fin can only be found by surfing and adjusting. Too far a forward placement induces excessive instability and makes the longboard too loose and difficult to control. The fin is progressively moved back in small increments, usually about 5mm at a time, until a suitable amount of stability and control is achieved. The Diamond Quad also allows a longboard to trim a couple of degrees tighter, ride higher on the wall and be more stable in white water turbulence.

Types

Glass on fins are fins that are permanently connected to the surfboard through fiberglass. This type of fin was mainly used on older model surfboards. Glass on fins are broken easily and are hard to repair. You rarely see these types of fins today because a different type of fin has replaced them.

Removable Fin Systems The most common types of fins used today, removable fins are surfboard fins that can be unscrewed from the surfboard and be replaced by different fins or be moved about the board for a different setup in maneuverability and stability. In the early 90’s three Australian surfers invented the fin control system (FCS). Since its global release in 1994 FCS has become the industry standard; providing elite athletes and everyday surfers an abundance of fin designs and a platform to change the performance of their surfboard by changing fins. The system also streamlined the surfboard manufacturing process by making it easier to install fins into boards and repair damaged fins. The leading competitor to FCS fins is Futures fins. Using a single larger fin box, the manufacture claims the fins provide a stronger connection and more closely approximate the feeling of a glass on fin.

Flexible fins fins are used on most rental boards because of liability. These fins are much safer than a hard fin because they cannot cut you. However it does lose some of the performance.

See also

• http://passyworldofmathematics.com/surfboard-geometry-and-design/

References

• "Fins History | SURFLINE.COM." SURFLINE.COM | Global Surf Reports and Forecasts, Live Surf Cams and Coastal Weather. Web. 20 January 2010. [1].
• "Glass On Fins |." The Surfing Handbook. Web. 20 January 2010. [2].
• Brandner PA and Walker GJ (2004) Hydrodynamic Performance of a Surfboard Fin 15th Australasian Fluid Mechanics Conference, Sydney.
• "Removable Fin Systems |." The Surfing Handbook. Web. 20 January 2010. [3].
• "Surfboard Fin History |." The Surfing Handbook. Web. 20 January 2010. [4].
• "Surfboard fins with flexible edges - Patent Search." Google. Web. 20 January 2010. [5].
• Unknown, Author. "HISTORY OF THE SURFBOARD." Wetsuits, surfboards, snowboards, videos and boardblog. Web. 6 February 2010. [6].
• Unknown, Author. "Surf Fins." Surf Fins. Web. [7].
• Unknown, Author. "Surf design." [8]
• Dimpled Fins Technology | 3DFINS.COM [9]
• http://www.tactics.com/info/guide-to-surfboard-fins
• http://blog.surfride.com/surfboard-fin-guide/