I found this program applet that simulates forces on an airfoil - you’re invited to try it.
The information given may stifle a lot of the hot air (hot water?) that’s passed here about lift, drag, and so on, relating to surfboard fins.
Where was this program when I did my senior high school science project about the effects of dimples on airfoils (wings, helicopter blades, etc)? 
Man that would have been useful.
aloha
Bryan
Thanks for the reference and link. However, in reading through the technical support paper for the program, my impression is that while this can be a valuable educational tool, it is too simplified to be of substantial use or application to surfboard fins. The reasons?
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It appears to be a 2-D program that addresses only airfoils of infinite aspect ratio (i.e. infinite span). In contrast, surfboard fins tend to be of quite low aspect ratio. The (3-D)cross-flows present in low aspect ratio foils/fins substantially alter the relationship between the lift and the angle-of-attack, as well as the induced drag.
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Since it doesn’t address foils of finite span, it also doesn’t address fin/wing/foil planform effects. Surfboard fins are substantially swept and of fairly low aspect ratio. So their aerodynamics are closer to that of a delta wing than a conventional wing. There are special 3-D aerodynamic effects that are present with a delta wing that substantially affect the lift as a function of the angle of attack (e.g. see: Aerodynamics, Aeronautics, and Flight Mechanics. Barnes W. McCormick. John Wiley & Sons, NYC. 1979).
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The bottom of the surfboard acts somewhat like an end-plate for the surfboard fin. However, the bottom of the board is at a finite angle-of-attack with respect to the free stream velocity of the water flowing past it (this angle of attack is necessary to create the lift necessary to support the rider and board). This also adds new 3-D effects to the flow over (and around) the wing that can be significant with respect to boundary layer separation (e.g. stalling), etc.–but which will not be simulated by this program.
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The program treats an airfoil operating in an infinite (unbounded) volume of air. A surfboard fin operates in water, and in close to an air/fluid interface (the sea surface). In this case, the low pressure side of the foil can, in some circumstances, suck in air (“ventilation” of the foil), causing the fin to act more like a planing surface than a wing or hydrofoil (and resulting in approximately a two-fold reduction in the lift and a large increase in the induced drag).
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Water is a fluid. But this fluid will vaporize to a gas (water vapor) at sufficiently low pressures or at high temperatures (boiling). At high speeds the pressures in the low pressure areas of the flow around a foil can drop to below the vapor pressure of the water. This causes the water to ‘boil’ and create a gaseous cavity over the low pressure areas (“cavitation”) of the foil. This is similar to a ventilated foil, but now the cavity is filled with water vapor instead of air. As the vapor moves into an area of high pressure, it can again transform back into a fluid. This can cause very high pressusre gradients that can erode the foil/fin. However cavitation is unlikely to occur at typical surfboard speeds–although it might become important for surfboards ridden on very large waves.
MTB
Good idea though Honolulu…
Hicksy
Ooops! I’m having a little bit of humble pie for supper tonight. I guess I should have read the instructions before the technical support document since it seems the latter is lagging behind the program development. It turns out the program does have a limited 3-D capability. However, that capability is limited to a few specific planforms, none of which are very good approximations to a surfboard fin (for the reasons I listed previously). Still a good educational tool, and with the 3-D capability, at least one can explore the effects of aspect ratio and span loading (e.g. elliptical planform vs rectangular).
Sorry about that.
MTB
Good points MTB…
But (here we go) you got anything better?
I posted for several reasons
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I was hoping not only to provide a useful tool for any of us to see what happens when a parameter is varied.
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There’s too much smoke being blown about how this does that, such-and-so is ultimate, and I wanted a reputable authority for explaining some of the basic (and not so basic) fluid mechanics behind flow. An airplane wing does indeed have many similarities to a surfboard fin, and has been studied by people whose training and knowledge, not to mention their research budgets, put damn near all of us surfies to shame.
We’re stuck with limited span. So be it. It is suggested that circulation is minimized by a wing of elliptical shape, and this was why the WWII Spitfire and Hurricane planes did so well for Britain. Little we can do but learn from that.
I don’t think the angle of attack in your point 3 matters to the flow - it just increases the pressure regime, not the streamlines.
Your use of “ventilation” is correct - surfboard fins do have to deal with entrained air passing the foil. I seriously doubt that cavitation you mention in item 5 is ever significant because it don’t think a foiled fin goes fast enough to create the necessary pressure gradient or low enough pressure. Maybe a poorly foiled fin, I know I’ve had enough of those and fin hum resulting from poor/dull trailing edges. But that’s a issue arising from vortex shedding, not cavitation.
Quote:
Good points MTB…
But (here we go) you got anything better? I posted for several reasons
I was hoping not only to provide a useful tool for any of us to see what happens when a parameter is varied. * As I hope I had indicated, I think it’s great that you posted the reference to this simulation program and I think it’s a fine tool to get a basic introduction to airfoils. The part that I took issue with is that I don’t think it will do a very good job of predicting the characteristics of typical fins on a surfboard, and generalizations generated from use of the program may lead to misunderstandings or erroneous conceptual views of the characteristics of surfboard fins. My recommendation to someone who is interested in learning about foils would be to use the program and explore the effects of changing the variables that are available to the user in the program. Then get a more advanced book that includes discussions on induced drag, the characteristics of swept and delta wings at Mach numbers well below unity, lift at low Reynolds number, etc. I would also recommend reading some references on practical hydrofoil design (e.g. the use of section profiles that minimize low pressure spikes to avoid ventilation and cavitation, methods to hasten bubble shedding if ventilation occurs, etc.).
There’s too much smoke being blown about how this does that, such-and-so is ultimate, and I wanted a reputable authority for explaining some of the basic (and not so basic) fluid mechanics behind flow. An airplane wing does indeed have many similarities to a surfboard fin, and has been studied by people whose training and knowledge, not to mention their research budgets, put damn near all of us surfies to shame. * Yes, I agree that the computational power required to adequately simulate a surfboard fin (or worse, a set of surfboard fins) requires, or would require, a large research budget and substantial knowledge about the use of CFD programs. But I suspect that even a surf enthusiast with a background in CFD and access to such facilities would be able to carry out adequate simulations for all but the most simple (and frequently the least interesting) conditions as I suspect that either no CFD program capable of carrying out these simulations presently exists, and if one could be adapted to address the surfboard fin(s) problem, it is unlikely that a person with both an interest in the problem, and the skill and time to explore the problem, would happen to be in the same research facility.
We’re stuck with limited span. So be it. It is suggested that circulation is minimized by a wing of elliptical shape, and this was why the WWII Spitfire and Hurricane planes did so well for Britain. Little we can do but learn from that. * Yes, theory indicates that induced drag is minimized by a rigid wing with an unswept elliptical planform–but that’s not very similar to a surfboard fin. As I mentioned, most fins have swept leading edges, and the fins probably act more like a delta wing (or a swept wing) than an unswept elliptical planform. Since the tip of the fin commonly sweeps aft, any flexibility in the fin also tends to introduce washout into the wing, altering the load span-wise load distribution on the wing from that of an elliptical planform and reducing the efficiency. There’s also the question about if one would even want the characteristics of an unswept elliptical planform. For one thing, it will catch a lot more kelp. Secondly although it will be very “tight” (i.e. produce a large change in lift force per unit increase in the angle of attack), when it gets to the stall angle of attack the whole wing/fin will tend to stall simultaneously. A rectangular planform stalls more gradually, and a delta wing hardly stalls at all (the drag just increases rapidly)–i.e. there frequently is a trade-off between control and performance.
I don’t think the angle of attack in your point 3 matters to the flow - it just increases the pressure regime, not the streamlines.
- The point I made was in regard to the separation of flow. Where and when separation occurs, and over what area, depends on the pressure gradients. A positive angle of attack for the hull modifies the distribution of pressure in the area of the flow over the fin from that of a uniform free stream (no gradients). Hence the stall characteristics of the fin can be expected to change in comparison with a fin on a hull that does not have a positive angle of attack–especially near the root of the fin and the point of initial contact of the hull with the sea surface.
Your use of “ventilation” is correct - surfboard fins do have to deal with entrained air passing the foil. I seriously doubt that cavitation you mention in item 5 is ever significant because it don’t think a foiled fin goes fast enough to create the necessary pressure gradient or low enough pressure. * Or entrained air that becomes “attached” to the low pressure side fo the foil and dependent on bubble shedding to return to the idealized flow. In regard to cavitation, I did qualify my comment toward boards traveling exceptionally fast (i.e. a tow-board ridden on very large waves). Although the thickness/chord ratios of surfboard fins tend to be small, that’s not true of all of them, and I seem to recall posts to the message board at the international hydrofoil society web site indicating that for relatively thick foils (15-20 percent of chord?) of a cross-sectional shape that tends to produce low pressure spikes in the along-chord distribution of pressure, it is possible for cavitation to occur at speeds as low as 25 mph. Maybe a poorly foiled fin, I know I’ve had enough of those and fin hum resulting from poor/dull trailing edges. But that’s a issue arising from vortex shedding, not cavitation. * Yes…plus a shedding frequency that is in resonance with a vibrational mode of the fin. MTB
Great site for education on foils…thanks…