Subtitles section Play video Print subtitles 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.