Surfing Explained: Ep5 The Physics of Surfboard Fins.
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- čas přidán 30. 07. 2024
- In Episode 5 of Surf Simply's animated exploration of surf science, we explore the physics involved with surfboard fins, and which fin configurations work best in what conditions.
This episode was written and animated by Surf Simply Coach Will Forster.
Season 1 will cover 5 episodes over the next 5 weeks:
Ep1: Understanding Speed and Grip
Ep2: Generating Speed and Grip
Ep3: What is Surfboard Volume
Ep4: Using an Online Volume to Weight Calculator
Ep5: The Function and Physics of Fins
Transcript:
I’m going to try to explain the physics involved in surfboard fins, within 4 minutes. Thats 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 1: Stability.
By adding a fin under 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 2: 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 us slipping down the wave face and losing both speed and position.
That’s where the side fins shine, Because as well as providing a resistant surface, they are foiled like an airplane wing, and so generate lift in a similar way. You’ll notice side fins, and airplane wings, 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, Benoulli’s Principle, and Newtons 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 on the wave face.
Benoulli's 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 size pushes the fin and the attached surfboard further into the wave face.
Newtons 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 surfboard towards the wave face.
Before we move on i’ll mention that all fins are there for function 1, stability. But not all fins encourage function 2; generation of lift. It’s also worth noting that the bi-product of both functions is an increase in drag, that can slow the board down compared to our finless example from earlier.
So let’s name 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 in trim. Typically you will find a single fin on longer boards that don’t need help from function 2 as they already have a long rail available for engagement in the wave face, but 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 centre fin has been removed.
Thrusters. Well. Kind of a best of both functions scenario. The center fin creates a pivot point, offering a rider more confidence to push hard during a turn, whilst still benefitting from the rail engagement the two side fins offer.
Quad fins. As per the physics, quads function like a twin fin; 2 foiled fins each side amounting to four in total offering strong rail engagement. Like Twin Fins, Quads function well on short railed 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 than their twin fin cousins by creating a gap between the two side fins allowing water flow to release between them 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. - Věda a technologie
Great video given the short time however you got the physics partially wrong! You cannot assume flow on one side of the fin will meet the other side hence flow on one side goes faster than the other thus the pressure change. This is a very common mistake in fluid dynamics and many wrong CZcams videos. The real answer is that the curved side creates an angular acceleration that then creates the low-pressure differential. But the separate flows don't meet again at the trailing edge there is actually turbulence. It is true that velocity is faster atop but it is the pressure differential that causes this not vise-versa like you depict on the video.
How does angular acceralation create a low pressure differential?
I don’t think it got into the physics enough. Nice injection of rail length, but I think we missed many aspects of the fin itself. Such as: rake, base, depth. Then there’s cant which is more the result of the fin box but changes the fin dynamics. There’s also the outlines of fins, narrow tips vs fatter tips, and how much fun is cut away where base ends and rake ends. We also didn’t talk about what symmetrical fin foils do. I’d love a longer video that hit all aspects of fins, rather than one trying to fit a time constraint. I don’t have ADD, so I’d watch the whole explanation if you made it. 🤙🤙🤙
It would be great if you could do a video on fin placement and the effects of placing fins in different locations. Thanks for this video you provided
Bravo to whomever did these animations
Coach Will did! Thank you! That’s very kind
Love videos like this that explain complexities that even I can understand. Probably the most helpful video about fins I've seen to date.
Few people realize the importance of not just fin shape and size but small changes is placement as well as tow and cant...can make a board feel completely different
Sera l
Good stuff! I d like to discuss one point. I think the pivotal point of a thrust or quad setup is the point between the side and and the rear fins. Not just solely on the rear fin. Also where you position your rear foot and weight will move that point closer to either fin. And the further forward that point is, the snappier the board will turn. Further backward that point, the more drawn out the turn.
Stoked to have found your channel! Twin fins forever... Quads when it gets scary lol
Haha that’s a great outlook Krsna! Thanks for joining our channel. We have two videos ready for release today and tomorrow at 2pm!
So what would be the advantage of running twins instead of quads all the time. Too much speed from the quads?
@@matthewgray8334 opposite - the addition of the two inner fins combined with the existing outer fins creates more drag but also more stability. thus quads on bigger days when you don't necessarily need the speed, twins when you need all the speed you can get (think mushy waves)
It's crazy how little the entire industry and community really understands about how surfboards actually work!
This video is way better than most (as is the rest of this awesome series) yet even here there are assumptions that feel like they aren't quite right; the idea of a double foiled centre fin not generating a lift force towards the wave face, for example.
I've read this assertion in a million and one places but I cannot believe it could be correct.
The centre fin is still a foil and will still generate lift at any AoA greater than 0°
The instant you're on your feet and on trim the surface of the water is meeting the fin at some angle well above 0° and thus the fin will generate lift towards the wave face.
If this were not the case, the high aspect single fins pioneered by guys like greenough in the 60s/70s simply would not hold and would ride like an alaia.
Honestly I feel like as an culture/industry it's time to start being brave with fin experimentation like those old legends. We're getting well into an awesome phase of exploring new shapes: asyms, planing hulls, grovellers, mid lengths.......
Maybe the next step is reassessing preconceptions about fin design.
The fact is that there are companies doing the experimenting and making progress already!
Hanalei, finscience, Larry (ProBox) Alison, even Roy Stuart 🤣.
Maybe if these sorts of fins managed to find their way under more boards surfing tech progress would move a little faster 🤣
Absolutely correct about the single fin generating lift towards the waveface. This video should amended accordingly.
Trying to get my head around the use of the word "lift" where all I can understand is a reflective force similar to a rudder , when the flow of water is diagonally across the board. Comparing to the lift generated by a proper foil to lift 100kg out of the water , any Bernoulli lift would have to be 20-50x less than that
@nickchristie2221 If you look at photos from underneath boards, it's evident the flow of water is more aligned with the fins than you'd first imagine, especially with how the inside/engaged fin is toed inwards.
I always ride bigger fins than what is recommended. I feel like it makes me focus on technique rather than cheating my turns. Thank you for another amazing video!
Absolutely Head Coach Jessie! Bigger fin = more security to apply to progressing technique!
What in the Wild World of Sports ! That did it - I will now add 4 more fins on my custom made 8 footer nose rider that Ben Aipa made for me 44 years ago !
*Side fins on the thruster are angled the wrong way.
Ya, when I saw his diagram with em angled out I knew that had to be wrong. That would immediately throw you off the board when you tried to carve a turn. Having the thruster fins angled inward means that you go straight when both are fully engaged, but set an edge, and far fin becomes less engaged and the near pushes the rear of the board away from you a bit, decreasing the radius of the turn.
Fully! Immediately got distracted by that, brain shut off the narration while I questioned reality
yeah!
Love these short vids, thank you. Please add to the pipeline: body position/mechanics of turns, floaters, and cutbacks
Thanks Tony. I think we have a couple of those subjects included in some
Of the new Quick Tips series coming out in the next week or so!
Thank you for this, still new to surfing and good to understand the different fun configurations!
Glad you enjoyed it Dan!
I’ve been wondering about the effects of fin stiffness/rigidity. A more flexible fin will provide a restoring force which generates more speed out of a turn whereas a stiffer fin will create better hold in powerful turns. Any thoughts ?
Love this video style. Well done
Fantastic short video to explain the science behind fins
Thanks again for such a detailed and clear explanations!! Saludos!!!
It’s our pleasure The Deivyl. We’re glad you’re enjoying the series!
Come surf the river with us! Solid video as usual....Thank YOU!
Thanks Luke, we’re stoked you enjoyed it!
Cheers fella just had an epic seshon on a 5'4" assymetric split keel fin fish ...your explanation clicked in my brain somehow ...cheers
Fantastic! Glad to hear it Paul, thanks for watching and giving us feedback on your session!
Great content! Thanks and keep up the good job!
Great Junior, thank you!
This is so clear and concise. Thank you! My only confusion is about speed and drag with respect to single and twin fins. Sure, a twin fin has "less drag through the center of the board", but should have more drag overall when both fins are in the water, right? So why is a twin fin often described as being faster down the line than a single? Is that more a function of the bottom contour than the fins alone?
Thank you so much for this video!
Nice vid. And even a bit of the old Bernoulli v Newton debate re lift. The debate is pretty much resolved though if you're interested. It's a bit of both, best expressed through Navier Stokes equations which are tough for pretty much all of us. NASA has a great accessible website explaining lift if you're interested. The flow over the foiled side of the fin is faster both in Bernoulli theory and wind tunnel demos, although your animation showed it slower...
Thanks Sean! We are always looking to learn more about these subjects and feedback on our animations. Glad you enjoyed it :)
Even better than that, they are totally and strictly equivalent. I've always wondered from where the "debate" came from since any student in 1st year of fluid physics can demonstrate it by writing the equations down. Flow over the foiled side must indeed go faster though ;)
@@clementl3925 I think the debate continues because some of those first year teachers persist in teaching incorrect lift theory supposedly as an introductory or simplying assumption, and it seems to be in every pilots guide. I've had this debate many times and they respond that its analogous to the simplifying assumption of zero friction for beginning students. My response is that to misrepresent the basic behaviours of fluids is more analogous to an astro physicist using a simplifying assumption that the earth is flat...
Lol was not expecting a discussion on circulation theory of lift. Good explanation folks.
Recommend this video! Learned a lot. Great video! ❤
So nice to understand thanks a lot
Really awesome stuff and great animations. Can you talk about the physics of how waves break on certain reefs as well? Maybe even how different types of surfboard rails create different lines?
Hi Will. Thanks for the questions, we will try to cover everything that is interesting, in either this series or another we have planned!
I’ve tried surfing with a Quad setup before because I had thought that more fins = more speed but when I tried a quad fin setup I had a lot of drift on my board. Do quads and twins only work so the board doesn’t rock so much or what’s the big deal about them?
Much needed video
We’re glad you found it useful Revejmal!
Hilarious explanation! Thanks ❤
Nice work!
Great video! I've looked for but didn't found a specific video regarding Fin's materials. That would be awesome. Today we have lots of different materials from plastic through carbon to fiberglass fins etc. How much do they actually impact in everyday's surfer performance...are they really worth it? Speaking for myself: I'm a average surfer and I surf average waves. Nothing fancy. Would a carbon really change the way I surf compared to a plastic Futures fins?
Excellent thanks 🙏
Great video!
Thank you. We’re glad you enjoyed it!
I wish you guys would also talk about toe in and cant angle for fins
Hi Hanno, we might cover them in a future video!
Good stuff, as always! Makes me think maybe I don't need the side fins on my 10'6 yachty board?
Indeed Fritz. You would need to decide whether the small amount of extra rail engagement is more beneficial than the loss from drag caused by the fins. Ride it over a few sessions with and without them, to build some data to help make your decision!
awesome stuff, cheers
Thank you! Cheers!
Solid series, love how you explain it, best explanations I've heard so far! I don't like bernoulli's explanation much since the distance difference does not explain the pressure difference, in fact the low pressure side particles arrive at the trailing edge sooner than the high pressure side, there is no requirement for them to arrive at the same time. This is all very confusing so I often avoid explaining this theory at all and prefer Newton third law as a "popular" explanation. Great series, worth spreading 👌
How does physics affect the central fin, what forces does the water exert on the fin? And following these principles, the same force that pushes the fin to the face of the wave pushes the other one outwards. So why do we stay attached to the face of the wave?
@@AlfredoOrtolani this is exactly the confusion I mean. The theory of different distance is interpreted by many that both side fins pull to opposite sides, which is obviously untrue as it fully disregards the angle made by the fin in relation to the water flow. Toe-in is important to ensure fins do not pull opposite to eachother and function together instead of working against eachother.
But Bernoulli's Principle also acts on the other fin, which would pull the board down the wave face, cancelling out the effect
Hi Vlad, often we surf in a traverse where there is more weight on one side of the board than the other and so the outside fin is lifted away from the water flow, this makes the opposing force less than the inside and engaged fin.
@@SurfSimply sheeeeeesh
Nice to hear more of a scientific explanation for board and fin functionality. Remember that each side fin is producing lift similar to an airfoil or wing, that is opposing to the other side fin.
Would be nice to see more colored cartooned graphics of CFD related to boards/fins...
You’re right CW, the fins do generate opposing lift, however because we typically surf in a traverse with the outside fin always in minimal contact, the inside is allowed to dominate.
Have you seen the FireWire Ricardo Rossi videos on his CFD modelling? They produce some graphics to show areas of drag.
@@SurfSimply i have and i love that stuff. it's about time some the physics behind design starts to become common knowledge. will help people with better board choice and hopefully improve everyone's surfing
also, a lot of need for the laymen to understand the concepts behind buoyancy, surface tension, drag/types of materials, volume vs planing, etc.
Interested. I like the science of surfing. I'm such a nerd. Lol
Is there any measure to this sideways lift force? Comparing the profile and size of the fins to a real foil that lifts you out ofnthe water , id guess the sideways force would be kg or two max. Does anyone flip these to compare? When turning the flow of water across the board diagonally would put a deflective force on the outside of the fin,like that of a rudder, pushing the tail inwards and facilitate the turn
cool information about fins, u even didn't explain about the form of the fin, but all the rest what it does with the board. Nobody on the web talks about that before.
Great to hear you enjoyed it Tony, and I completely agree with you!
@@SurfSimply cool work !!! keep them coming
love the animation of the surfer, how was this done?
Hi Perry, I drew it frame by frame using Adobe Photoshop.
@@SurfSimply No way!? 😅Really how was surfer animation done?
Nice vid bro
Thank you Augusto. We’re stoked you enjoyed it!
Flex also is a big factor to the feel.
Absolutely Troy, though hard to measure without equipment so it’s difficult to determine how much of the flex one might be ‘feeling’ at any one time. It’d be great to get access to some equipment to measure flex and flow speed along with a scientist to decode the data!
Missed out drive, hold and release
What about boards with five fins?
Why the rear quad fins must be smaller that the side fins?
The pressure difference generated by Bernoulli is too small to push the fin. It is, however, an important force in displacing water toward the center of the board. This is where Newton’s 3rd law comes in. Look up Coanda.
Not buying this take. Surfboard fin foils, traveling at board planing speed, through water, are entirely capable of generating functionally significant lift.
If this were not the case the recent rise in high-camber, thin section, side fin foil (industry calling it inside foil [see NACA3305]) wouldn't feel different in the slightest; it's catching like wildfire because these foils do exactly what's needed for a side fin in surfing, high lift at low Reynolds numbers with soft stall characteristics.
The simple fact that fins stall and let the tail slide out when AoA is too high should be convincing enough.
@@IamtheWV17 yes, lift is generated. It’s just Coanda not Bernoulli though Bernoulli is involved in moving water towards the center of the board. Bernoulli does not push plane wings up and it does not push fins either.
@@christianharriot1578 Apologies, I misinterpreted your comment as refuting the suggestion that the lift generated (via more complex physics than Bernoulli, as you rightly point out) was in itself insignificant to the function of a fin.
PIVOT!!!!
Hahaha! There must be a couch surfer pun in there somewhere?
Still haven’t quite understood the benefits of having lateral lift generated by the side fins
More like “surfing complicatedly” jk great vid bro
And yet nobody ever discusses the thickness of fins. Why?
Kelly slater is sad you didn't mention the penta-fin
I want a board that has the entire bottom covered with right sided foiled fins that all pull to the right and smash me into the wave with incredible power to show the wave I'm a force to be reckoned with and teach it a lesson.
animation of Devon Howard