A lot happened in the area of airfoil research, development and design that is applicable for use on surfboards long before shapers started carving surfboards out of foam and glassing them with fiberglass. Decades ago, the late fin guru Curtis Hesselgrave started producing windsurfing fins with 1940’s era NACA airfoils carved into G-10 glass with aerospace wind tunnel model accuracy. The performance improvements over mystery-foil fins were phenomenal and started a performance revolution. Later, Curtis applied his talents to surfboard fins; however, most surfers are unaware of the technology behind some of Futures Fins offerings.
Much has happened since the development of the NACA airfoils, with little to none of it migrating to surfboards. Surfboard fins operate in a transitional fluid dynamic regime where the molecules can have difficulty moving along the surface no matter how streamlined and precise that surface is manufactured. Computation tools to accurately predict these transitional effects were not available until fairly recently - the most notable being Xfoil developed by M.I.T. engineers. Xfoil analysis of foils commonly used on surfboard fins reveal problems, but ways to partially fix those problems have been known to the low Reynolds number aerodynamic community since surfboards were first carved out of foam and glassed with fiberglass. Since Xfoil accurately predicts the transitional effects, it is possible to design foils that work well in these conditions, and it is even easier to just find ones that already fit our criteria.
I’ll be linking my discussion to pages on AirfoilTools.com. All of the performance data are from Xfoil predictions. Information and links about Xfoil can be found there. When you go to the pages, uncheck the Rn 50,000 box and then click on the 'Update plots' box. Few surfers will ever get to Rn 1,000,000, but it should stay for reference. The angle-of-attack convention is the angle of the free stream flow relative to the chord line that extends from the trailing edge to the tip of the leading edge - that line is drawn on the airfoil plots.
Nearly everything here is about two dimensional flow. Surfboard fins, especially thruster, twin fins and quad fins, have low aspect ratios. Even our ‘high aspect ratio’ fins have relatively low aspect ratios. And they have a lot of rake/sweep. Both of these design characteristics result in significant three dimensional flow effects. I will not be addressing 3D effects here.
The symmetric foiled back thruster fin commonly has a foil that is or resembles a NACA008 or NACA009 airfoil. Taking a look at the lift coefficient (Cl) vs angle-of-attack (alpha) plot, you’ll see that the response at the lower Reynolds numbers is not linear - there is a partial dead-band on either side of zero/zero. This results in poor centering response when we are first getting to our feet and during slower maneuvers. Nor is it completely linear at our higher Reynolds numbers. Note how more thickness of the NACA0012 foil results in significantly higher maximum lift coefficients while widening the drag bucket - drag coefficient (Cd) vs. Alpha plot. A thicker foil can achieve lower drag at higher lift coefficients; however, don’t think that I’m suggesting we all start using fins with thicker foils - it’ll work well in some situations, not so well in other situations, and it has centering issues over a wider range of alpha.
Side fins with similar thickness distributions, but with the addition of camber, have the same problem. Looking at the results for the NACA2408, depending on the Reynolds number, the zero lift angle of attack ranges from a wee bit less than 1° at Rn 100,000 to about -2.5° at Rn 500,000 and above. If your fins are toed in properly for the higher Rn, then they are toed out too much at lower Rn. Do take note how adding camber to the foil increases the maximum achievable lift coefficient at all Reynolds numbers. Getting more grip out of our fins is a good thing!
This centering deficiency results in surfboard handling difficulties and the creation of drag. Throw in some manufacturing variations in the toe-in angles and the back fin alignment, and thrusters end up with three fins each going their own way a little bit different at different speeds. Twin fins can get downright ugly with odd interactions between the direction each fin wants to go and what the rail/rocker wants to do. Quads can have four fins going every which way at different speeds, and for Quads on narrow tails, the back fins move in and out of the slipstream and downwash of the front fins thus exacerbating the problem. These characteristics are so prevalent on all surfboard fins, boards are designed to allow for it and all surfers are used to it and think it is normal.
The fins commonly used decades ago had parallel sides and curved bevels on the leading and trailing edges. Those foils resulted in numerous locations where the flow would separate from the shape of the foil resulting in non-linear Cl vs. Alpha response, excessive drag, as well as premature stall at fairly low angles of attack. Those limitations undoubtedly drove both surfboard design and fin template design to compensate.
Those were all WWII era foils that are employed on a limited number of fins offered for sale. The foils on many fins are designed with a ’that looks about right’ process.
While trolling through the airfoil library of AirfoilTools.com, I came across several promising prospects for use on surfboard fins. The Drela AG10 airfoil looks like a good foil candidate for all things ’side fin,’ and the Drela HT08 airfoil looks like a good foil candidate for thruster back fins and even single fins. Both have good centering response and achieve decent maximum lift coefficients despite being very thin. They’re similar to what is already being used, so improvements should be easily achieved without impacting 3D effects. And I’d like to try the GOE 265 airfoil on a twin fin; it has good centering response, but that maximum lift coefficient of 1.7 or more has got to be tried. Being radically different to what we’re currently using, 3D effects may change, so this is a totally experimental venture - a lot of CNC carving could result in a total flop. With all that camber and a zero lift angle of attack of -5°, it won’t integrate into our boxes. I’d also like to try the NACA0012 on a high aspect ratio longboard single fin - though I already have a couple fins with what look like NACA0010 foils CNC carved in them that I’ll soon be trying out. I’ll see how they work first before thinking of going thicker. And the NACA010 and NACA012 will both require modifications to make the molecules in the boundary layer think they are at a higher Rn. Maybe trying a variations of parameter test with the HT08 and HT12 foils at 1.0x, 1.5x and 2.0x of the thickness ordinates to see how thicker foils with good pressure recovery work on single-fin setups.
Characteristics of the foils that work best are: thin, max thickness around 20% chord, and straight taper shortly after the max thickness. Do a survey of numerous production surfboard fins from several manufacturers, and you’ll find max thickness points as far back as 50% chord, and curved tapers into the trailing edge - such foils will have poor centering performance and reduced maximum lift coefficients due to separated flow. Separated flow is not cavitation, nor is it a stall (complete separation), it is where the flow detaches from the surface and can even reattach further down the foil creating a bubble in between. You’ll also find many fins that have the proper characteristics for good performance at our flow conditions. Albeit many have leading edges that are too sharp for 2D flow - Futures calls their flat bottom foiled fins ’speed controlling.’ The sharp leading edges will cause early chordwise flow separation. I presume the ‘speed controlling’ feature is because they’re easy to partially stall to slow down and stay in the tube. By far the most advanced side fin is Futures' Solus. I wish they offered it in a twin-fin size. I have one set that came with an AM1 back fin instead of the stock hatchet fin. The second most advanced side fins are Futures' V2 foiled fins that come in several template offerings. Some of the worst offerings from all manufacturers are side bites and the trailer fins in twin + trailer setups. The side bites operate at an Rn half of the center fin, and always are of poor design. The trailer fins operate at 2/3 the Rn of the twin fins, and always are of poor design. I have one with a foil that has the thickest part at 50% chord. The purpose of the trailer fin is to provide some centering for the twin fins, but such a design will do little for that task. Poorly foiled fins can be corrected to some extent with a sanding block. I’ve gone so far as to extend the trailing edge with plate glass, thus making a template change, filled it in with epoxy filler, and sanded it to make a new foil.
So, what do we need to do to take advantage of this foil technology? The same approach that Curtis Hesselgrave took decades ago: CNC machining of sheet glass or CNC machining of molds to make fins in. We need to maintain tolerances around 1/1000 inch to fully take advantage of the foil characteristics. Since Curtis started making fins, we now have more manufacturing tools, so 3D printing may be an option as well, either the fins themselves or molds to make fins in. Or, just buy fins with foil characteristics as close as possible to what the data says works best and break out the sanding block if need be.
My own fins are highly modified from what everybody else uses, but I start with fins that have the best fundamental foil and template design to begin with. I do have some fins that I plan to add plate glass and epoxy filler to the bottom of the leading edge to make them resemble a Drela AG08 airfoil - cause the rest of the foil looks nearly identical. On some flat bottom foiled fins I’ve put very aggressive dropped leading edges onto them and initial results shows I'm getting more grip out of them - to get them to perform something like the GOE 265. Another thing I plan to do is put an HT08 or HT12 foil onto a traditional longboard fin template to hide modern tech on something that looks like it’s from 1960.
Prototyping with a Drela airfoil is certainly Ok with them already being in the public domain, but production use might require some sort of licensing agreement. But Mark is easy enough to find since he teaches at M.I.T.. The GOE airfoil is out of Germany from the 1920’s or so.
As you’ve looked at many of these foils, the thought may have crossed your mind that a portion of the curve looks a lot like a surfboard rocker. And they do. And that is something we have to be careful about not doing - making rockers that are upside down airfoils that actually suck the nose of the board into the wave. But that is a different discussion.