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• I'm going to try to explain the physics involved in surfboard fins within four minutes.

• That's approximately 500 words if we average normal speech... and go!

• Single fin, twin fin, thruster, quad fin,

• lots of configurations to choose from but first imagine riding a finless board.

• Slippery slidy and challenging to control and maneuver. Fins have two functions that

• contribute to control and maneuverability lacking in the finless board.

• Function one: stability. By adding a fin underneath the board we have a

• resistant surface area that will help to stop the board from sliding sideways

• when in trim across the wave. It also creates a pivot point that the rider can

• push off when attempting to rotate the board during a turn without the risk of

• the tail sliding away. Function two: lift. As outlined in previous episodes,

• the engagement of the rail is imperative to holding our board in trim on the wave face.

• It contributes to speed generation and our grip, reducing the chance of it

• slipping down the wave face and losing both speed and position. That's where

• the side fins particularly shine because as well as providing a resistant surface,

• they are foiled like an aeroplane wing, and so generate lift in a similar way.

• You'll notice the side fins and aeroplane wing are curved on one side

• and flat on the other. This produces lift in a specific direction and for fins

• that direction is towards the wave face. There are two explanations as to how the

• fins generate lift to help engage the rail in

• the wave face: Bernoulli's principle and Newton's third law.

• It is yet to be exactly determined how much

• influence either theory has but we know for sure that the result is lift

• generated towards the wave face, holding the rider in trim. The newest principle

• is explained by differing the distance the water travels across both sides of

• the fin which creates a pressure difference. As the pressures attempt to

• equalize, the high pressure on the flat side pushes the fin and the attached

• surfboard further into the wave face. Newton's third law however is explained

• by redirecting the flow of water traveling around the fin away from the

• wave face and as every action has an equal and opposite reaction must then

• direct the sir bored towards the wave face.

• Before we move on, I'll mention that all fins are there for function one: stability.

• But not all fins encourage function two: generation of lift. It's also worth

• noting that the byproduct of both functions is an increase in drag that

• can slow the board down compared to our finless example from earlier.

• So let's go through a few configurations. Single fins and center fins only offer function 1.

• They allow the rider to pivot and push off the fin while giving some extra

• stability and trim. Typically, you will find a single fin on longer boards that

• don't need the help from function 2 as they already have a long rail available

• for engagement in the wave face. Those long rails could be hard to turn without

• the big pivot point at the back. Twin fins, two large foiled side fins so lots

• of rail engagement. This explains why we see twin fins on shorter boards, or very

• rounded outline boards with a reduced rail length in the wave face.

• All that lift is compensating for the lack of effective rail length. Twin fins can also

• be very fast in a straight line since all the drag from the center fin has been removed.

• Thrusters. Well, kind of best of both functions. The center fin creates

• a pivot point offering a rider more confidence to push hard during a turn,

• while still benefiting from the rail engagement the two side fins offer.

• And quad fins. As per the physics quad fins function like a twin fin; two foiled fins

• each side amounting to four in total offering strong rail engagement.

• Like twin fins, quads function well on short rail boards but also excel in

• unforgiving situations that require extra speed and hold like big waves or

• barreling waves for example. Quads also allow for an easier pivot point than

• their twin fin cousins by creating a gap between the two side fins allowing water

• flow to release between then during pivoting, reducing the resistance to the turn.

• Well that was 701 words so thank you for

• sticking with me and until the next episode, thanks for watching.

I'm going to try to explain the physics involved in surfboard fins within four minutes.

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# Surfing Explained: Ep5 The Physics of Surfboard Fins.

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joey joey posted on 2021/08/07
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