Fin
Encyclopedia
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
and thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
or to steer while traveling in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, air, or other fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
media, (in other words, a foil (fluid mechanics)
Foil (fluid mechanics)
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag...
). The first use of the word was for the limbs of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, but has been extended to include other animal limbs and man-made devices. Fins, as with other foils, operate in fluids such as water or air.
Swimming water animals such as fish and cetaceans actively use pectoral fins for maneuvering, and dorsal fins contribute stability as the animal swims, propelling and maneuvering with its tail, itself recognizable as a fin.
The fin on fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
is known as a vertical stabilizer
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...
. Fins are also seen used as e.g., fletching
Fletching
Fletching is the aerodynamic stabilization of arrows or darts with materials such as feathers, each piece of which is referred to as a fletch. The word is related to the French word flèche, meaning "arrow," via Old French; the ultimate root is Frankish fliukka...
on arrows and at the rear of some bombs, missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
s, rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
s, and self-propelled torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
es. These are typically "planar" (shaped like small wings), although grid fin
Grid fin
Grid fins are a type of flight control surface used on missiles and bombs in place of more conventional control surfaces, such as planar fins...
s are sometimes used in specialized cases.
Examples of fins include:
- PropellerPropellerA propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
s usually have a number of fins that work to translate torquing force to lateral thrust, thus propelling a shipShipSince the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
. These are also called blades. In the case of high power application it is important to avoid cavitationCavitationCavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...
, caused by excessive negative pressure, as this can cause noise, a loss of power, and damage to the propeller.
- For scubaScuba divingScuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
divers' fins, see swimfinSwimfinSwimfins, swim fins, fins or flippers are worn on the foot or leg and made from finlike rubber or plastic, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, kneeboarding, riverboarding, and various types of underwater diving.Scuba divers use...
.
Surfboard fins
In surfingSurfing
Surfing' is a surface water sport in which the surfer rides a surfboard on the crest and face of a wave which is carrying the surfer towards the shore...
, there are two major types of (typically stationary) surfboard fins
Surfboard fins
A surfboard fin is a foil , specifically, a hydrofoil that is mounted near the tail of a surfboard such that it is more or less perpendicular to the bottom of the board...
(hydrofoils), and a host of illustrative issues.
Both a skeg
Skeg
A skeg is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard...
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 (physics)) - 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
Skeg
A skeg is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard...
" (an upright, streamlined, often raked keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
) typically denotes one centrally-mounted stabilizer foil mounted perpendicularly to the riding surface, at the rear of the surfboard
Surfboard
A surfboard is an elongated platform used in the sport of surfing. Surfboards are relatively light, but are strong enough to support an individual standing on them while riding a breaking wave...
.
Smaller surfboard fins
Surfboard fins
A surfboard fin is a foil , specifically, a hydrofoil that is mounted near the tail of a surfboard such that it is more or less perpendicular to the bottom of the board...
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
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
, for greater lift
Lift
Lift may mean:* Lift , a mechanical force generated by an object moving through a fluid* Lift , rising air used by soaring birds and glider, hang glider and paraglider pilots for soaring flight...
.
A fin configuration with fins near the edge of the board stabilizes and contributes lift
Lift
Lift may mean:* Lift , a mechanical force generated by an object moving through a fluid* Lift , rising air used by soaring birds and glider, hang glider and paraglider pilots for soaring flight...
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
Thruster
A thruster is a small propulsive device used by spacecraft and watercraft for station keeping, attitude control, in the reaction control system, or long duration low thrust acceleration.-Spacecraft thrusters:...
"), 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
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
is increased, as is its lift-induced drag
Lift-induced drag
In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is a drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or a lifting body redirecting air to cause lift and also in cars...
. Rail fins also add lift (known as "drive") in trim
Trim
Trim may refer to:* Cutting small pieces off something** Book trimming, a stage of the publishing process** Editing*** Editing a posting style in online discourse** Pruning, trimming as a form of pruning often used on trees-Places:...
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
Surfboard
A surfboard is an elongated platform used in the sport of surfing. Surfboards are relatively light, but are strong enough to support an individual standing on them while riding a breaking wave...
, which decreases the angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
in trim
Trim
Trim may refer to:* Cutting small pieces off something** Book trimming, a stage of the publishing process** Editing*** Editing a posting style in online discourse** Pruning, trimming as a form of pruning often used on trees-Places:...
, which makes it easier to initiate turns. "Toeing in" rail fins also adds drag on the "outside" fin, as its angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
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
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
, 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
Planform
In aviation, a planform is the shape and layout of a fixed-wing aircraft's fuselage and wing. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military...
.
Rail fins evolved into being and surged into popularity as riders (Simon Anderson
Simon Anderson
Simon Anderson is an Australian surfer and surfboard shaper, noted for creating the Thruster design of three equal-sized fins on a surfboard, a design now used on most shortboards.-Early life:...
, 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
Stall
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
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 WindsurfingWindsurfingWindsurfing or sailboarding is a surface water sport that combines elements of surfing and sailing. It consists of a board usually two to four metres long, powered by the orthogonal effect of the wind on a sail. The rig is connected to the board by a free-rotating universal joint and comprises a...
, a derivative of traditional surfing, skegs are also often used as a central stabilizing fin (hydrofoilHydrofoilA hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...
) 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 sailSailA sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...
(itself an irfoil]) 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 liftLiftLift may mean:* Lift , a mechanical force generated by an object moving through a fluid* Lift , rising air used by soaring birds and glider, hang glider and paraglider pilots for soaring flight...
, drag (physics)Drag (physics)In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
, ventilationVentilationVentilation is movement of air in and out of an enclosed space, including a body. It is used in the following contexts:* Ventilation * Ventilation * Ventilation * Ventilation * Ventilation...
and stall (flight)Stall (flight)In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
.