Reading about the rusty flex fin last nite----some lod stuff resurfaced in my mind----back in the wilderness/liddle/john arnold egg daze i did some messing around with flex fins…being a physics teacher-formaer pilot i went off in a tangent—very thin foils. the starting point for a bunch of these was the FU polycarbonate inpected fins of the 70’s-awhole range of sizes of these were sold. When i did was remove about half of the total thickness from top to bottom and then return them to a true foil by carefull sanding them to a F-104 wing profile…they came out very flexy —my idea was that flex and rebound was really needed only when the board was moving at it’s fastest speed and then the thin foil was most efficent , grabbing on and holding into the water as it flexed and snapped back straight, firing the board back up the face of the wave…recently the trend seems to be toward thicker foils----NACA/NASA ----found that thin foils were faster and lower drag and didn’t stall at speed while thicker foils worked better at lower speed but stalled at higher speeds. Considering the Rynolds numbers of the water flow as the surfboard maybe it’s time to look at thinner fins again?
Hi Chuck, Because I build a few fins from scratch, I’d like to make a few comments. Bringing a scientist’s approach to things is invaluable. Clearly Thinner’s faster but it doesn’t perform as well in some situations, as you have pointed out. How the leading edge of a fin is shaped is real important I think. The cross section of the foil is just as important. It all depends on how much drive you want out of a fin or how neutral you want it to be. I know that wetsuits are being made for competitive swimmers with little channels in them to imitate shark-skin which isn’t smooth but is coursed with a serious of tiny channels. I think the idea is to contain the friction on the wetted surface as the residents of the sea do and thus create a faster surface. I wonder what the surface of an albacore’s tail and fins look like under a microscope. Now when it comes to flex fins I think that the flex that tends to torque in the direction of the turn is valuable. But fins that lay way over during turns seem to me to be an effective way to release power rather than developing it. How much drive you get out of a recoiling fin is questionable as far as I’m concerned. Some very experienced surfers prefer rigid fins some flexible ones. It seems to me the more flexible they are the more one must depend on the bottom and rails of the board for drive. It’ll always be an interesting topic to discuss. Though it may be more about styles and what feels good than anything else. Where fins are placed on a board, how big they are and how they are configured changes performance enormously. A well shaped board can be tuned for many conditions just with the fin set-up IMHO. The experiments continue, Rich
I have to agree 120% with HALCYON on how much fin size and positioning affect performance, even tiny changes in size, position, foil, tow etc…affect performance alot. IMO I think that this is one of the most overlooked aspects of surfboard design.
The “little shark skin things” are usually known as vortex generators. You can find them on any airplane wing (usually just aft of the thickest part of the leading edge). They work by creating controlled vortices over the foil which increases laminar flow over and behind the foil. On an aircraft this reduces drag and increases lift. Also the stall speed is reduced which allows for slower approaches. A good overview: http://www.avweb.com/news/reviews/182564-1.html On a surfboard fin they would need to be tiny. Aeroplane wing vortex generators usually penetrate 45% into the laminar flow. I forget the NASA numbers for the normal height but it’s not so much. Glueing actual fish scales (just a few) on the upper third of both sides of a fin with an even spacing may give the desired result. Or a stinky board. Or tiny pieces of Scotch tape may be better. In either case the effect may not be noticable. (VGs have been on surfboards for a while though: Herb’s Superchargers may have this effect; smaller leading fins on single twins; and MVGs (micro vortex generators) that Greg Loehr has some affiliation with are all good examples of VGs that aren’t attached to the fin). Another, more recent, wing modification that many aircraft are adopting is winglets or wing-tip extensions. This works by effectively increasing the aspect ratio, therefore increasing lift, and reducing the induced drag caused by tip vortices. I believe these are more effective on a surfboard (than wing-based VGs)if you dont have kelp or seaweed to deal with. From personal experience I also think they may add a lot more stress on the fin box. However the speed increase is very noticable. You can buy these from Cheyne Horan. However, I do think it would be interesting to try winglets on a raked fin. Sorry for my rant. I’ve been reading a lot. -Rob Olliges http://www.aviation-industry.com/atem/Acrobat/at54winglet.pdf
That’s hardly a rant – very interesting stuff. It’s not clear to me whether winglets (finlets?) would be of any use on a center fin, since those have a symmetrical foil. In the world of airplanes I don’t recall having seen them yet on symmetrical wings that generate lift by angle of attack. Do significant tip vortices exist at the end of symmetrical foils? Unfortunately there’s a lot of kelp where I surf most, which would makes experimenting with finlets pretty sticky…
Keith, I have a design all ready to go. I just can justify taking the time to build it because I only surf at sand bottom breaks 5 percent of the time. I guess I’ll get around to it but it’ll have to be a request from someone who wants something way out of the box. One thing is sure – when you put those winglets out on the tip of a center fin. The fin becomes extremely positive. If you want to surf off the fin they add a lot of holding power. I would think adding them to the center in a thruster set-up would tend to nulify the accelerating, lifting affect of rail fins however unless off course the profile of the center was minimal. A twin set-up with winglets at the tips might be very interesting. Mahalo, Rich
I have to imagine it would take a lot of experimenting to come up with a finlet that really works well. Here’s a couple of excerpts re winglets: “Despite the fact that winglets have been in use for many years, optimization of winglet design had been difficult because the complex airflow patterns around these wing-mounted airfoils could not be understood through wind tunnel testing. The Reynolds number has a big effect on the flow structure; flight tests always showed larger drag reductions than the ones from wind tunnel data.” "Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky-they have a tendency to flutter, for example. “We find that it really doesn’t matter what kind of wingtip device you use-they’re all like span,” he says. “The devil is in the details.” That’s not to say it isn’t worth trying, though.
I think the main problem with aeroplane winglets is that the wing is a very delicate object and any weight added to the end can be dangerous. It’s better to have two wings than one. Due to safty reasons the winglets on planes are designed to break off if they need to. You also mentioned angle of attact generating lift. This concept works with flying in air as well as water. Winglets do work very well on a double foiled center fin. I suspect any tip will generate a lot of induced drag. Winglets simple minimize that drag and increase the aspect ratio. More SPEED!!! -Rob Olliges
Airplane winglets and yacht hull winglets typically are non-planar (not in the same plane as the wing/keel/lifting surface). They do seem effecting in reducing induced drag from what I’ve read. However, adding non-planar winglets (or finlets) at 90 degree angles (or thereabouts) to surfboard fins seems destined to catch kelp, present sharp edges where feet might kick them, and may also produce potentially unwanted lift effects (we want the board to go faster, not have the tail subjected to vertical forces). BUT winglets/finlets don’t have to be non-planar. Below (I hope)is a link to a planar winglet (airplane)system that is very interesting… now, if one could build a fin that would reduce induced drag and thus make the board go through the water faster, without having horizontally protruding surfaces to catch kelp, etc, that would be worth having, eh?.. http://www.winggrid.ch/whatisit.htm
SEEEEEE! now THIS is a Design Forum!!! neat stuff----i dug out one of the old fins today and my son stuck it into his 6’6" egg(new style) and had a blast ! the main flex in this particular fin is in the bottom one-third and today the surf was 2 - 4 and offshore —perfect conditions for that kind of experimentation… basicly —back then we would swap diferent flex and thickness fin in and out of boards till we found what worked for that board and person…very fun…beat the hell out of getting shot at in SE asia! a great way to get back into the surf scene … anyway, Re textured surfaces-------at our speed range—the shark skin or riblets should have a positive effect----the numbers for the moving fluid at least say so Re tiplets-------the America’s cup boats went in that direction years ago–and they have a whole lot more development $$$$$ than we do! But those boats make long radis turns and lots of straight line run----not what we do at all… nite nite
OK, perhaps sailboats don’t turn as often as surfboards, or as radically… it still seems of great benefit to minimize induced drag, regardless of whether you’re talking airplane wings, sailboat keels, or surfboard fins. Here’s a link to a guy who’s working on planar winglets for yacht keels. I think it’s some pretty interesting stuff, albeit on the technical side. I have seen fins with a vertical slot, but usually parallel to the trailing edge (which is sort of similar to this guy’s multi-foil keel but that’s a different design topic). Anybody ever tried fins with vertical, foiled slots on the tip that you know of? I can picture that sort of arrangement on side fins (analogous to airplane wings) but the center fin would have to be different, more like this “condor keel” tip (single slot, widest at tip of fin)… http://www.esotec.co.nz/condorkeel/Html/condor_F.html
Keith, I used to have a high wind sail board with a fin set up somewhat like you described. It was a raked fin with a verticle slot near the trailing edge. I don’t remember the physics involved but I belive I was told that it was designed to relieve pressure on the fin in high speed turns so it would not cavitate.Seems to have worked well enough, but I doubt I could have truley felt a difference. Don’t know who made them though that was a while back (80’s).
In reading up on this subject it appears that there are several ways to skin a cat – in airplanes designers have a very strong cost incentive to minimize drag because that directly results in fuel savings (equal to millions of dollars for a fleet of passenger jets over their lifetime). Those designers have tried to reduce induced drag (not parasitic drag) by use of winglets of various types, as well as wingtip shapes especially designed to reduce tip vortices. Interestingly enough, one of these wingtip shapes, designed for the Boeing 767-400, looks in some ways like the end of a hatchet or paddle fin (although not exactly, the wingtip is 5 sided). It could be that surfboard fin design has accidentally/experimentally come up with fin planshapes that reduce (but probably not minimize) tip vortex-induced drag… it would be very interesting to see some computer modeling or tank-testing of various fin tip shapes and/or finlets to see just how much better the drag reduction could be.
Interesting thoughts here, especially since I’m back working for the company which by the end of the year will be making 100% of the leading edges for Boeing Wichita engine nacelles. For personal reference, wasn’t achieving the “shark skin effect” to make a microscopically smooth board surface the idea behind sanding the bottom of boards back in the 70’s/80’s when gloss coats were pretty much the norm?