Basic terms in Fin science

Posted on May 19 2013

Some Helpful Surfboard-Fin Science Terms and Definitions:

Angle of attack

The angle between the direction of surfboard fin movement through the water and the fin’s chord line.

Aspect ratio

Aspect ratio is a measure of how long and slender a fin is from fin root to tip. The aspect ratio of the fin is defined as the square of the span divided by the fin area. Typically high-aspect-ratio fins have long spans and aspect ratios of 2:1 or greater, while low-aspect-ratio fins have short spans and lower aspect ratios. Higher aspect-ratio fins have lower drag and higher lift than lower aspect-ratio fins.

Boundary layer

The layer of water molecules near the surface of the fin whose velocities are changed that by movement of the fin through the water. Boundary layer flow may be either laminar or turbulent.

Chord

The distance between the leading edge of the fin and the fin’s trailing edge.

Chord line

The line between the fin’s leading and trailing edges.

Downwash

A fin with an angle of attack other than zero creates lift and has a difference in water pressure on the two sides of the fin. Near the fin tip, water is free to move from the region of high pressure to the region of low pressure, creating a circular water flow from one side to the other, which creates a vortex or helix because of the fin’s movement through the water. Larger circular flows result in larger vortices, greater drag, and lift. The presence of winglets at or near the tip of the fin inhibits this circular flow, reduces vortex size, decreases drag and increases lift.

Drag

Drag is the hydrodynamic force that opposes any watercraft’s motion through the water, and is a vector quantity along and opposed to the watercraft ‘s path of travel through the water. Drag is directly proportional to the area of the fin, and also is affected by fin shape, foil shape, fin thickness, and fin aspect ratio.

Fin root

That portion of the fin that constitutes the base of the fin when the fin is within the fin box, the lowest exposed portion of the fin when in use.

Fin base

The portion of the fin intended to fit snugly with a fin box to limit unintended movement, while providing a means of adjustability in the longitudinal direction.

Fin box

The channel within into which the fin base is placed, typically with a channel that allows longitudinal adjustment, while restricting side-to-side movement. The fin box is not claimed as an invention in this disclosure.

Foil

The cross-sectional profile shape of the fin.

Laminar flow

Layered or smooth-flowing water within the boundary layer, as opposed to turbulent or disordered flow within the boundary layer.

Lift

The vector-quantity force created by the movement or turning of water past a curved fin surface, which force acts perpendicular to the direction of water flow. Lift is directly proportional to the area of the fin.

Lineup

The spot outside the area of breaking surf at which surfers await waves to ride. The takeoff zone from which surfers must quickly accelerate from a standstill to a sufficient velocity in order to catch the approaching wave.

NACA

The National Advisory Committee on Aeronautics, the predecessor to NASA. NACA performed extensive testing on airfoil shapes to determine the lift and drag characteristics of various foil shapes.

Pitch

Pitch is the angle of deviation from the horizontal of the surfboard’s or other water sports board’s longitudinal axissurfinge.g., the nose of the board or watercraft is pointed somewhat upwardly or downwardly, as in airplanes when they take off and climb or descend.

Planform

The planar shape of the wing or foil, which for wings is typically the outline of the horizontal plane, and for rudders and fins, the outline of the vertical plane.

Roll

Roll is the angle of deviation from the horizontal of the surfboard’s or other waterspouts board’s side-to-side axissurfinge.g. the board is leaning somewhat on its right or on its left edge, as in airplanes when they bank their turns.

Stall

Loss of lift, as demonstrated by the turbulent flow of water past the fin. Differently shaped foils have different points or angles of attack at which they stall. A stalled fin moving through the water loses lift, but increases drag, thus acting as a brake.

Sweepback

The angle by which the one-quarter-chord line of the foil sections within the planform deviates from the perpendicular to the root chord. Some authorities refer to leading edge sweepback angle, which as the name implies refers to the angle away from the root chord perpendicular of the wing or fin’s leading edge.

Water sports board

A watercraft primarily used by a single rider, propelled by gravity, waves, wind or by towing, such as a surfboard, a kite-surfing board, a sailboard or windsurfer, a waterski, or a wakeboard.

Winglet

A planar, foil-sectioned projection substantially perpendicular to the fin plane, generally placed at or near a fin tip or wingtip to reduce tip vortices and consequent downwash and drag.

Yaw

The angle of deviation from straight forward in the path of travel to an orientation other than straight, a spinning about the vertical axis, as in airplanes landing in a strong crosswind that “crab” their way to a safe landing. Rudders that steer move though an angle of yaw, as do fins on a turning surfboard.

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