A tunnel fin is a type of

surfboard 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 trajecto ...

used on

surfboard A surfboard is a narrow plank used in surfing. Surfboards are relatively light, but are strong enough to support an individual standing on them while riding an ocean wave. They were invented in ancient Hawaii, where they were known as ''papa he' ...

s, especially heavy longboards and longboard guns. The weight and length of these boards make it easier to control the fore and aft angle of the tunnel. As the name suggests, it is shaped like a tunnel. Tunnel fins were first used by Richard Deese and Bob Bolen (also known as the Greek) in California during the late sixties and continue to be developed by shapers today.

Background

The Tunnel fin is based on the principle of the annular wing with the lowest

drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street adj ...

and highest

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

fin configuration possible. It has no drag inducing fin tips, this is important as it means that rail to rail turning movements are drag-free and effortless. Tunnel fins have the ability to enhance hydrodynamics by slightly twisting the water flow into a

vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in t ...

as it passes through the tunnel, preventing random

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

in its wake. The tunnel fin has a large amount of horizontal lift utilizing the hydrofoil principle providing better

lift-to-drag ratio In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under ...

. The horizontal area in the tunnel fin also acts as a means of transferring rider energy into forward

thrust Thrust is a reaction force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can al ...

through board flex (similar to a dolphin tail). As the board pumps up and down it drives the 'fluke' at the tail. In other words, the speed of the tunnel finned board can be increased by weighting and unweighting (pumping). This is slightly different from the side to side movement used by thruster riders. The ability of the tunnel to organize

volumetric flow rate In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes ). I ...

means that (in the case where a central fin is used in front of the tunnel) any

coming off the

trailing edge The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge meets.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 521. Aviation Supplies & Academics, 199 ...

and base of the front fin is organized into a lower drag flow pattern.

References

{{reflist Surfing equipment