if fins and plane wings have much of the same concepts, then why don’t we see thruster shaped wings stuck on planes and wing shaped fins on the bottom of surfboards? Why are fins more soft and curvy, and plane wings more edgy and straight? it seems like they would be more similar then just foil…
Quote:
if fins and plane wings have much of the same concepts, then why don’t we see thruster shaped wings stuck on planes and wing shaped fins on the bottom of surfboards? Why are fins more soft and curvy, and plane wings more edgy and straight? it seems like they would be more similar then just foil…
‘DEATHFROG’ [I LOVE that name !! … it gives me a [very] mental picture of a big cane toad / bull frog. Dressed in black leather. SS armbands. Machine gun slung over one shoulder. Handgrenade in one hand, pin in mouth. “Gnarly” , snarly face … there you go, speedneedle…your next board’s spray idea !!]
Anyways , “deathy”, welcome to 1981 !
… Simon Anderson’s early thruster fins…
Closemindedness in the surf industry at the time [gee, has ANYTHING changed ?!] may have meant the death of this design , at THAT time.
[ I made these ‘replicas’ a year ago. They’re fun !]
“chip”
In shortboard surfing, your board is continually turned, going from one rail to the other, with only a shortperiod steep straight drop in.
An airplane is mostly judged by it’s straight line performance.
An airplane has rudders and ailerons to help initiate turning.
A surfboard does not!
Good question that exposes many of the myths and misconceptions surrounding the two. (Note: See the ‘Myth of the Bernoulli Principle’ thread on this site for more insight on this controversy.)
The better comparison between the two is probably to sailboat keels (rather than airplane wings).
Both surfboard fins and sailboat keels are operating in the same medium (i.e., water) so both will be subjected to EXACTLY the same forces (believe it or not, there are some differences, albeit minor, between how water and air behave in this regard) and both perform much the same functions (i.e., providing thrust and directional control).
This then, of course, brings up the interesting question, ‘why don’t we see thruster keel set-ups on sailboats?’.
In fluid dynamics the most efficient planform is an ellipse. Aircraft flying at subsonic speeds try to compromise between an elliptical and a tapered wing because the latter is easier to manufactured while loosing a very small part of the lift distribution. A sweptback wing starts getting its efficiency as it approaches mach 0.6 which means the fluid, air, is going though some compressibility effect. Surfboard fins started looking like rudders and through out practical and “fashional” evolution has become to what we have today. Early surfboard builders didnt put a fin under a water tunnel and figure out what conditions make the fin stall and how much drag it develops…they just follow the “it looks like it should work” method. Recently, FCS has been investing a lot of money on fin research using computer simulations and water flow tunnels. They are finding that the original concept of elliptical planforms still is the most efficient and with additions of undercamber (inside concave) the size of the fin can be minimized and in turn lower the amount of drag. Eventually the goal is to have boards with the smallest fins without sacrificing performance. If we look at sailboat keel and rudder evolution its easily seen the trend towards elliptical planforms, why? Because research is done using theoretical and empirical methods. Sadly, surfboards have evolved out of pure trial an error which is not the best method but at least its done out of the love for the sport. Can anyone imagine if engineers get together to optimize surfboard design year after year like they do on jet fighters, transport aircraft, cars, etc…? Sadly, the biggest factor for something like that to ever happen is money…so unless more companies are willing to invest on real R&D we will continue to see snail pace improvements on our sticks.
“Sadly, surfboards have evolved out of pure trial an error which is not the best method but at least its done out of the love for the sport.” - Cappio777
True, but much the same could be said of airplanes, sailboats, etc.
Humans have always looked to Nature for inspiration as well as rely on their own trial and error to come up with with rather remarkable, innovative, and successful designs, whether for airplanes, surfboards, or sailboats.
For example, Marshall Islanders were building sailing canoes with asymmetrical hulls centuries if not millenia before racing yacht designers in the 20th century came up with asymmetrical hull designs of their own. The Marshallese didn’t need any fancy technology to ‘prove’ the validity of the asymmetrical hull (i.e., that it was the fastest hull design possible and allowed them to sail against the wind without a keel).
so technicly the wing shaped fins should work best, but scince most of how things sell is by who endorses them and asthetic appearence we have thruster fins? I think im going to try making my own fins now… Does anyone know where to get plastic that I could sand down into fcs? I don’t like wood, it takes a while, then you have to glass it, but I might have to glass plastic too… but…I want to try out some of the concepts discussed here…
It might be easier to suggest why plan shapes have evolved the way they have.
As pointed out above the most efficient plan shapes give an elliptical loading, the shape itself doesn’t have to elliptical though. Sweep in a foil (air/hydro) helps to increase the stall angle; a heavily swept plan shape will lose lift at higher angle of attack than a straight, and low aspect will go further than high aspect. Stunt planes have short stumpy swept wings for manoeuvres; gliders have long straight wings for efficiency. If you sweep an ellipse you get something like something like the new FCS H-2.
So why the curve though? The next bit is an educated guess; the pressures on the tip of the fin cause the fin to twist more inline with the direction of flow further increasing the stall angle. This flex may also allow the fin to align itself across the flow directions next to the board skin and the unaffected wave (boundary layer with torsion as well as sheer, can’t remember the proper terms for these).
Generally very curvy plan shapes are less efficient under ideal conditions, but they can be more efficient across a wide range of less than ideal conditions.
As the arcs on fins and boards straighten or get to the tight the resultant foil becomes less versatile. More elongated shapes are faster. Rounder shapes are more responsive. One marries these things together to get the complex compound arcs that produce a fine surfcraft and high performance fins. Though similar, what goes on around a surfboard on a wave is a much different dynamic activity than what happens in an airplane. Consider that a surfcraft can only develope as much energy as there is in the wave it’s on except for tow in or sailboarding where you load energy from an external source. (jetski or wind) A plane, unless it’s a glider, can power up or down as the pilot deams necessary. A Sailboard is much more like an aircraft and the boards and fins reflect this closer relationship.
However with paddle in surfcraft what we end up with is a huge variety of equipment (boards & fins) and each variation will shine in different condtions.
My take is that round is better – too much of it is, of course, counter productive.
Mahalo, Rich
I agree with simon on the curvy sweep. Airplane wings with “washout” (twist) behave with better manners near stall (separation). Some rake in the fin allows some washout in either direction, depending on whether you are going left or right. Also, think of it from a stability point of view. Rake back is stable; rake forward is unstable.
My impression is that surfboard fins have to handle a much wider range of speed, angle-of-attack, and transient flow conditions than most lifting surfaces. I suspect the current shapes are pretty jack-of-all-trades and inoffensive for that reason.
Yeah, tip washout is a clear benefit of swept tips.
There’s another clear reason for rake in surfboard fins, and that is stability in turning. Upright fins are pivot-y, swept fins turn more slowly and predictably with respect to center of mass. Rake is real important in any system that can at times rely on only one fin (which includes singles, thrusters, and twinnies, but not quads).
Quads often use a much more upright trailing fin, because two rail fins can give you the stability you would otherwise seek in one rail fin on a thruster or twin.
The best way to answer the question, though, would be to ride some fins shaped like airplane wings (a la Chip).
Planes fly, surfboards don’t. Worst, they live between two worlds. But, leaving this philosophical discussion aside, we could say that planes generate speed to keep their flight in a denseless environment. Surfboards practically slide on the water. Rails do the main job, but fins do the dirty work. If they work imersed in water, they dont belong to planes’ world. Have you ever seen a fly fish? Did you notice if their wings look like planes’ wing? I don’t understand why these questions are worrying you. Try to compare jet planes’ speed with those performed by surfboards. Surfboards don’t need more speed. They need drive! Sharks and dolphins have more performancein the water than us. They are in a evolutionary stage more developed than us. So…
hey chip,
I used this shape a lot as singles, this ones a side fin.
So just to add to this discussion, I can understand why people don’t want to accept physics and maths to help design a better fin.
The aeronautical industry spend millions on development because they stand to make millions on commercial aircraft. To use what they have refined was my pleasure.
Right on jeffshaper! I was waitin’ for somebody to mention the ‘ultra-refined’ dolphin. I think that we have yet to find all the answers even in the high-powered aerospace industry. I worked in the U.S. space program for a number of years and one of the projects I was assignd-to was: fins.
You would think the older engineers I was working with would be aware of some of the things in nature and with payloads reaching past the $1 Billion mark, that they would at least spend a decent amount of time on the subject. Well, to be honest, they don’t. The designers have a bunch of old drawings to go by and they are almost all straight lined. They had (slide) rulers and the behavior of curves was too hard to calculate.
When I got to re-design a mission-critical fin and we had budget to push the envelope, and we had access to the first feasible computational aerodynamics programs, and we had access to composites that loved curves and also could go into space (remember, bubbles in the epoxy can potentially explode up there) I derived a fin with curves. Calcs showed that we could put something like 40 extra pounds into orbit at about $10,000 per pound which is more than twice what it cost to make the change (this was also to be amortized over a 5 year, 10 vehicle program).
With all that work done, I came to a personal conclusion; only relatively recently have we been able to analyze complex shapes in complex media and the objects being the most efficient for reasons not always initially obvious tend to be like those you see in nature. It also turns out that riding a wave is the combination of two media (air and water) and one of the media is curvelinear and accelerated (the pitching wave face) which makes it in some ways more complex to approach the design than things like spacecraft.
Surfboard builders are the best. They take the design risks. It is quite possible for someone untethered by the pressures of industry who is in their own home-workshop, to reflect from nature and a “seat of the pants” reckoning to come up with the next significant discovery in wave riding. The variables (as of now) are just too cumbersome to calculate. I hate to say it but the best violins ever like Stradavarius designs where not calculated, they were built a lot like surfboards; they have yet to be matched 300 years later! It is of little wonder that the best fins out there look like they belong on some sort of animal. I was really stoked to see someone mention this…
I’m as keen as anyone to see a little bit maths and physics applied to all aspects of surfboard design. So here’s a place to start: Reynolds numbers and Froude numbers, Rn for fins, and Fn for hulls. Learn a bit about these find out what they are for different things. You’ll find that fins are at the other end of the scale to a jet fighter, down there with model aircraft (and insects), and hulls are closest to the fastest powerboats on earth.
Fins are pretty damn good as they are, maths and physics can improve things but probably not much.
Here are two interesting concepts that are used in sailboats that I’m justing throwing out there to you guys:
Weighted keels…weight at the bottom helps keep stability by keeping the keel in the water and i would think it would create more drive becasue its harder to move…
The Keel depth, the deeper it penetrates the water, generally the more stabl it is, of course there are other factors such as foil and shape…
simonc,
If you check the archives I'm confident that you'll find we have discussed Reynold's numbers for fins in general. And, if you take note of what Dr. Nick Lavery and his grad students are doing over at the University of Whales at Swansea they are beginning to study lift/drag coefficients for specific fin designs. Empirical study is a tremendous tool that has been under utilized in the refinement surfboard development. But, getting out there and intuitively tetsing lots of different concepts has been one of the greatest assets to the gross development of surfboard and fin design.
Turbojets,
The purpose of weight at the bottom of a keel is couteract the heeling moment generated by the sails, putting weight on a fin would make a board harder to turn.
Stability improves performance in a yacht because hull shapes and sailplans are generally more efficient at low angles of heel. The deeper a keel the greater the stbility. I guess you meant effeciency, in that a high aspect foil is better than low.
Tom, I know but I just can’t help myself!
Someone once told me there is no such thing as a stupid question…So here goes; What exactly is the purpose of the fin and is it’s purpose really known?
I ask because it seems to me that in designing something you must begin with it’s purpose. I have always, without really thinking about it, thought that fins direct the board/track the board. But to be honest I am not sure at all what is going on down there. Further up in this thread someone mentioned that ideally we would have no fins at all, which makes fins a “necessary evil”, so to speak. If that is true, then that also seems a starting place for fin design. i.e. what are the negative and positive features of a fin. (Would some one who knows list them, cause I don’t honestly know).
(Look at the advancement in fin design that could possibly take place right here. Numerous minds and willing parties scattered across the globe in a concentrated R&D effort. I love Swaylock’s!)
Fins
-
Provide, along with rails, traction in a turn. That is, they lift toward the inside of the turn.
-
They keep the board oriented roughly along the tangent to it’s path at any given moment.
And other things that I will not be the one to articulate.