High Aspect and Low Aspect Fin: Definition?

A rookie question for anyone with the time to respond: I’m confused over what “high aspect” means in regards to fins. And on the converse, I don’t know what low aspect means either. I have a bunch of fin questions, and this is the one that has got me most confused. Thanks, -John

The aspect of any foil is the ratio of base to height. A high aspect fin is a deep narrow based fin. A low aspect fin would be a wide based shallow fin.

Thanks, tom. If you’re still checking in, I have another question: How far back does the fat part of the fin start, usually? Is it about a third of the way back? I think that’s called the foil, but if there’s another name for it, let me know. And if I push that fat part back, how does it affect the performance of the fin? I also have some questions about thruster fins but I need to think about it first.

Basic rule of thumb is about 1/3 the way back. If you are into such things 30 percent of the chord length, back from the leading edge will give you the maximum camber thickness. That’s a good starting point. I’ve seen test fins with max thickness as far back as 60 percent. If you need profiles of foils, Compufoil is a good software program with a large range/registry of various foils. If you are doing calculations, be sure to use the correct Reynolds number for the class of foil. Water is a bit more dense than air, and most of the foils (not all) are prescribed for optimum in-air performance. Good Luck…

Plusoneshaper gave you some good information. The only thing I might add is that the fuller foils designed for higher lift (like thruster side fins) may have the apex of the foil at 20% or less. Finer foils will tend to push that appex point further back into the 40% range. But, 30% was a good middle ground.

I have to disagree with some of the common beliefs put forward about foils for fins. This is the 21st Century. The best cambered section profiles for fins are likely to be the NACA 4- series. These section profiles have a maximum thickness at about 40% of chord. Section thicknesses generally need to be at least 12% of chord. These foils have high maximum lift co-efficients at the Reynolds Numbers typical of surf fins, being in the range of Re = 100,000 to 500,000. Refer in particular to figures 28, 32 and 33 of Naca Technical Report 586, and figure 97 of NACA Technical Report 460. A fairly good starting point is the NACA 4412 section. Its about time that thin foil sections for fins were consigned to history, as they are fairly poor performers. Thin foils are also a safety issue which can be improved by thicker foils with their more rounded nose profiles (lawyers read this!). The challenge for fin designers is to work with a more rigid fin implied by the thicker sections. References. 1. NACA Report 586, 1937 Airfoil section characteristics as affected by variations of the Reynolds number. http://naca.larc.nasa.gov/reports/1937/naca-report-586/ 2. NACA Report 460 , 1933 The characteristics of 78 related airfoil sections from tests in the variable-density wind tunnel. http://naca.larc.nasa.gov/reports/1933/naca-report-460/

Please read this for the second paragraph: The best cambered section profiles for fins are likely to be the NACA 4- series. These section profiles have a maximum CAMBER of 4% of chord. Section thicknesses generally need to be at least 12% of chord. Thanks.

Hey Mark, I notice many of the quotes are seria early 19th century and are concern with air foils not aqua foils. The stuff is different. Viscosity does play its role along with the other variables of wind and wave and surface conditions. The correlation is simply not so simple and direct as the theorist may think. Experience has taught me that thick foils do work better on slow moving single fins (Long boards) and that holding the thickness in the vertical cord forward make the fin drive better and gives it more sensitivity. As you move the veritcal cord back into the fin it becomes more neutral possibly a little faster but drive is certainly sacrificed. Adressing assymentrical fins (rail fins) and how changing the foil in them affects the lift they produce is a far more complex issue than simply where the thickness in the verical cord occurs and the relative thickness of the fin I can assure you. So tell why an albacore’s pectoral fins and tail are so damn high aspect, the thickness in the vertical cord is about 20 percent back off the leading edge and their so damn thin? Hell, wouldn’t a couple of nice little stubby low aspect thickly foiled fins and cod fish tail work so much better? Of course he’s down in the water and things are a little a little thicker there than they are for NACA’s airy theories that have little to do with surf-craft performance. Hmmm… http://www.surfingheritage.com/reg30.html ??? What No fin yet? http://www.futuresfins.com/noflash.html ??? Things have progressed a little haven’t they??? Mahalo, Rich P.S. We reveal what we don’t know by what we say.

Any comments on comparative lift and drag coeffiecients of the NACA - 4 series and NACA - 6 series ? What is the primary distinction between these two series of airfoils ? - chord ? - thickness at chord ? - other elements of the foil ? Any comments on thickness at chord affecting these coefficients in the denser medium of water ? Does the NACA data apply without significant variation to the medium of water ?

John, The aspect ratio can be defined in two ways, a geometric way, and a hydrodynamic way. The aspect ratio is a fundamental property of a fin. The formula for the aspect ratio is general, and the aspect ratio can be determined for any fin. 1. Geometric way The geometric aspect ratio (GAR) is defined as: The span of the fin divided by the mean chord of the fin. That is, the height divided by the average width. It is not generally correct to say the aspect ratio is the height divided by the base. This would only be correct for the case of a rectangular planform. 2. Hydrodynamical way In the water, the fin effectively performs like half of a symmetrical wing. It is like the fin is joined to a mirror image of itself at the board. The hydrodynamic aspect ratio for the fin is twice the geometric aspect ratio. That is, the HAR = 2 x GAR. The HAR is really the true aspect ratio in fluid mechanics terminology. The HAR is used to calculate important hydrodynamic properties of the fin, such as the “lift curve slope”, and the “induced drag co-efficient”. This is an important concept as it allows wing theory and practice to be readily applied to fins. Note: I didn’t really answer your question John. I will consider the question about high vs low aspect ratios. My apologies for diverting from the gist of your thread previously. Mark.

Mark, Technically you are correct. A foil would have to be rectangular for the base vs. depth messurment to hold true. But, the industry standard for describing fins for surfboards has always been base vs. depth. So, that is the way I described it for a surfboard design forum. It’s kind of like the industry standard for positioning fins has always been measured from the trailing edge. But, effectively it should about the moment of effort of the foil. Unfortunately, it’s just much easier to identify the trailing edge and that became the standard. Which brings up another point. Report 586 describes foils that are 5" X 30" rectangular templates. Since fin templates have a curved and swept leading and trailing edges, the natural cord length would be measured perpendicular to the leading edge. While the effective cord length should be measured parallel to the flow of water. And since there is a laminar flow boundary layer which varies in thickness relative to the amount of board in the water and the speed at which it travels, how do you determine the effective average cord length? It’s constantly changing. Initially when we were considering how to develop the Red X fin foils, we considered generating foils based upon the NACA-4 models. But, we descided that since we had access to the master templates of numerous world famous shapers, we took advantage of their experience. Our foils have all been derived from the master templates of Maurice Cole, Al Merrick, Tim Patterson, Matt Biolas and Casey McCrystal. That’s not to say that we are not open to developing foils from alternative methods. It’s just how we started the program. But, before we start cutting steel, we would want a model that describes at least a majority of all the dynamic forces that surfboard fins are encountering while riding a wave. Short of that we just get back to shaping something and trying it out.

Mark Vin … , Diverting from the high and low aspect thread - if you have the time I’d be pleased to hear your comments on the NACA 4 and 6 series airfoils. I’ve been taking a close look at the NACA Reports - the airfoil profiles, the graphs of characteristic of airfoil sections, and the discussions. The airfoil profiles that seem the most interesting to me are 4406, 4506, 4606, 4409, and 4509 - 6406, 6506, 6409, and 6509. What is your interpretation of the differences in the two series and the advantages and disadvantages of the two when applied to surfboard fins. I’m particularly intrigued with efforts to maximize lift and minimize drag, with “Optimum Lift” being the goal. Variation in chord, thickness, profiles, leading edge, and trailing edge seem to be the components that would yield different results. Constant change in the angle of attack (with boards turning) seems like a very important consideration in designing foils for surfboard fins. Templates, flex, and surface tension must have an affect on those coefficients as well, and although those components are an integral part of the surfboard fin, they could be considered a constant (in this discussion) with the components of foil being the variable. (Unfortunately, many fin makers seem trapped in the battle between covering costs and making a profit on an item that wholesales and retails at an affordable price (not unlike surfboards) and making fins of true technical merit. Thus I see the market flooded with hastily hand and machine foiled or molded fins with significant flaws in design and execution of design.) Best regards, Steve Coletta

Swaylockians, Steve Says, “I’m particularly intrigued with efforts to maximize lift and minimize drag, with “Optimum Lift” being the goal. Variation in chord, thickness, profiles, leading edge, and trailing edge seem to be the components that would yield different results.” Tom Says, “But, before we start cutting steel, we would want a model that describes at least a majority of all the dynamic forces that surfboard fins are encountering while riding a wave. Short of that we just get back to shaping something and trying it out.” Take note! These men have invested many many years into understanding what makes a surf fin perform. We should listen carfully to what they have to say if we want to make the next successful developmental step. I’m sure Curtis Hasslegrave would have some very interesting things to say on this subject. It takes a tremendous amount of knowledge and technique to produce a surfboard that approaches the edge of performance. It takes the same stuff to make a good fin. I am just beginning and have come to realize that because fins are smaller and seem a rather to some a secondary force on a board. Hidden under the water as they are they can be passed over easily at times. In acuality they are just as primary to performance in a wave as the board itself. Just an eigth of an inch difference in fin base length or depth in a thruster set or a single will make a very noticable difference in performance. When we start adding in the manifold foil factors one begins to see that, though they may be small, how we configure and place them will enable us leave a section in out wake or leave us in the soup and gobbled up by the wave. Putting the ultimate fin set-up on a board (if one really exists) takes an understanding of hydrodynamic theory and a willingness to experiment. Creating something new that is worthy obviously ain’t easy. Aerodynamic theory always seems to find its way into the mix but water and air, though both are fluids, have very different properties. Surface tension, Cavitation, & Ventilation are contrary things to manange well and thus a very important part of what is going on under and around a surfboard. Yes there are similarities between airplane wings and surfboard fins, but one would struggle flying an airplane with wings configured like a surfboard thruster fin. Some singles might work but they would have to the rather vertical high aspect ones it seems to me. This avenue has been expored before and no doubt will be again. Some sailboard fins are very like airplane wings but we’re not talking sailboards here. I think the closest thing to a wing on a surfboard are Cheyne Horan’s Starfin and Geoff McCoy’s Gull wing. Certainly there is a correlation but we must draw a meandering line somewhere between the two to make their separate functions merge. Maybe or friend the pelagic flying fish may have a trick or two under his fin we need to look at. It takes many angles, arcs, passes of the hand and serious viewing of the contours produces in process to get something that’s worth putting in the water. And then you have to be fortunate enought to know someone who’s willing to try something new and is a good enough surfer to give things a proper go. Then you have to trust you’ll get some feed back that will serve the learning curve. Looks like some weather on the way for NorCal. Off to work, Rich

Tom, Please excuse my terse commentary and read between the lines where necessary. With swept fins, the chord is still measured parallel to the symmetry plane (the base of the board). Likewise, the span is measured perpendicular to the plane of symmetry (the base of the board). The aspect ratio is still calculated in the same way. The boundary layer underneath the board will be thin because of the falling pressure gradient which accelerates the water from the spray root to the rail. Sweep From aerodynamic theory, the fin can be represented by a lifting line located at one-quarter of the local chord length from the leading edge. The sweep angle of the fin is the angle made between the perpendicular and this one-quarter chord line. The Independence Principle The “independence principle” states only the component of flow velocity perpendicular to the quarter chord line makes a contribution to the lift of the fin. Conversely, the component of flow velocity parallel to the quarter chord line makes no contribution to the lift. Note : For this reason the standard section profiles for the swept fin need to be oriented at right-angles to the quarter-chord line. In practice, the lift-curve slope of a swept wing is reduced by the cosine of the sweep angle as compared to the unswept wing. I am unaware of the formula for the effective equivalent sweep of a fin with a compound sweep. The sweep angle effect effectively “gears down” the lift-curve slope for the fin. However, the main problem with swept fins is premature stalling. This is as a result of the boundary layer being pulled sideways toward the tip and separating from the fin. Elastic bending of the trailing edge of the fin may delay or prevent stalling but it further lowers the lift produced by the fin. Other Comments High-performance fins (whatever that may be) need to be rationally designed and precisely made for best performance. The only way to make such a fin profile is with the application of a computer controlled machining process. Thanks, Mark.

Steve, I hear you. I didn’t realise this discussion forum would be so intensive. I think I can only put out one article a day, if that. I will answer you soon … Another day or two in 50 years won’t make that much difference. Here is something to ponder. Comment. I am surprised in the United States that with so much expertise available in aerodynamics and hydrodynamics, it seems none of these professionals have come forward and offered technical advice to any significant degree about designing or making surfboards. It’s another thing of course whether that advice would be accepted. I think it is a disgrace to the engineering profession. California is a classic example of the existence of world class aerospace expertise in close proximity to the beaches there. I cannot believe the lack of cross-pollination. You can bet if the Russians designed surfboards then things would have been different … Mr Reagan. Even now, a lot of aerospace and military technology research being developed for small unmanned aerial vehicles. Will any of the Low Reynolds Number aerodynamic lessons learned from designing these vehicles be applied to surfing-craft or other watercraft. Not much chance. Thanks, Mark.

Knowledge and expertise of aerodynamics and hydrodynamics applied to surfboards and surfboard fins should be a significant contribution to the ongoing evolution of design for surfing. The intuition of surfers and shapers is also a great design resource. Mark. Thanks for your input. It’s great to discover relevant information from a fresh prespective. I’m looking forward to your comments. Reply when you’re able. I’m in no hurry and I believe the best discussion and analysis happens in it’s own time.

So Mark, Here’s your oppurtunity to set things straight. I’ve got a Fadal 4020 VMC. I’ve got Mastercam 8.11 CAD/CAM. So, the tools are available to machine a theoretically perfect foil. If you provide the geometry, I’ll provide the capacity to make the reality. I’m not talking overnight. Because we got to keep paying jobs going to keep the doors open. If you don’t have one ideal foil in mind and need to test some rough models, I’ve even got access to a rapid proto-typing machine. It won’t produce the same physicals. But, steroe lithography will produce the shapes with a course texture. What I don’t have is a wind tunnel ,tow tank or finite element analysis program. Although the program is on my wish list. E-mail if you are at all interested in progressing the science of surfboard fins.

According to Mark’s final statement the computers have it and it’s the only way to proceed with the research and developmental process. Tom is willing to give it a go. That’s great. Sounds like we might have the possiblilty of something very progressive happening. But I say that talk is cheap and quoting the theories and quipping about how one can’t believe that space age boys and the surfers haven’t gotten together is bunk. According to Mark, all these great shapers & foilers we have around the world just fell off the turnip truck. Sorry buddy, There a lot of theory and a lot of art going on you don’t see yet. Go educate you eye a little and then tell me the two haven’t spent some time in bed. So if the computers are up to it, just plug all the details in of what the perfect fin is according to the theories are on what will give maximum lift and speed and you’ll get an ultimate fin and foil and I won’t need my grinders & sanders any more. Great, go for it guys. But in the interum I’ll be the caveman with tools in hand and continue to make my promordial mistakes. In about a year or so bring 'em on down! The challenge is on – you bring your best set-up and I’ll bring mine. We’ll put them on the same board in the same conditions under the same surfer and see which set of fins out does the other. No wind tunnels or drag tanks just some good 8 foot sectioning surf like Sunset. It’ll be sort of like John Henry revisited. If the mechanized hammer outperforms the the hand held one fine. The only difference will be that it’ll be fun and nobody will have to die in the process. Remember now boys no cheating with hands on work. We’ll both use the theories we think best apply of course but you have to produce you fins with you computures. Seaya next Thanksgiving, good luck (-; Rich P.S. We reveal what we don’t know by what we say.

Rich, Can’t be done by CNC alone. You know how CNC shaped blanks still have to be hand finished to get the tool marks out. We’ll the same thing occurs when cutting metal. Sure we use tiny cutters and tiny step overs that run all night to get the tool marks as small as possible. But, you still end up with scallops that need to be hand blended out. And, while steroe lithography will make a physical model, only the really expensive machines will make parts with durable materials. And even those, are laying down little stacked strings of plastic with physical properties that come know where near a molded part. So, the best way to approach this would be with your collaboration rather than in competition. Mark could provide the geometry in the form of a parasolid program. I could have a model generated. You could foil up a by hand generated model for testing purposes. And, if we get some favorable results from testing. I could have some injection or compression molds made to make them publicly available. Now we all know that there won’t be one perfect set of fins. Like tires there will just be certain models that will preform better in certain specific applications. But, we can pick a set of parameters that we want to optimize towards and work from there.

This is nice work dudes. The future of performance is a hand in hand commitment between board and fin makers. The board is the fuselage, the fins are the wings.