I took the fin out of my board and I’ve been looking at it for a while. Why is the fin so thick in the front. Wouldn’t it make more sense to have it be more knifey in the front and have the thick spot in the middle then foil out to knifey again? How do different foils effect a fin?
Very sharp or pinched leading edges on any foil tend to make them hyper sensitive to direction change input. That would create a fin that is always trying to change the direction that it is moving in. It wouldn’t feel very smooth or forgiving.
Austim and Skip, Hey Austin & Skip, Before this thread gets filled with misinformation. You guys need to go have a look at some scientific information about fin theory. There are so many variations on a foil that making simplified statements about very complex issues is not only grossly misleading but naïve. There have been many volumes and studies writing on fin theory and how a given foil produces lift and drag. I don’t mean to be condescending but if you want to know why things work the way they do you can’t expect the answers to just drop in your lap. The internet is a huge resource. If you want to learn about fin theory use it. A fin foil with a sharp leading could widen quickly into a vertical cord close to the leading edge of the fin. A rounded one could fill back quite slowly into a vertical cord, which is a line marking where the thickest portion of the fin runs from base to tip two fifths of the way into the profile of the fin template. All other things being equal moving a vertical cord closer to the leading edge of the fin is what makes a fin more sensitive to directional changes. A very sharp leading edge doesn’t serve smooth directional change and tends to create excessive turbulence. Finally asking a question like “How do different foils effect a fin?” isn’t one that will get you many answers. I know what you mean, but saying what you mean is damn important if you care about communicating your ideas. Good Surfin’, Rich p.s. One reveals what he doesn’t know by what he says.
You are absolutely correct. My simplified answer was based only upon my years of sail plane airfoil construction and the effects on craft stability that various airfoils will produce. NACA and Selig-Donovan low speed airfoil tests fill volumes are readily available for research. Programs to produce templates used to produce these foils are also inexpensive, and could be of benefit in surfboard fin design. I have yet to go to the trouble of applying these templates myself. Commercially available fins have worked well for me. My comments were made solely upon this experience. If anyone can point to similar research and published results in the area of fin design, this would be a terrific resource.
Good show Skip! Ultimately how fast you’re going to go has a more influence on what kind of fin or fin set-up will work the best. I would like to excange a few emails with you on fin concepts and share some things I’m working on. Drop in anytime at – I’m sure I can learn a few things from you and hopefully I can enlighten you as well. Mahalo, Rich
Its radical how much you guys are into fin foil and use science to work out what it can do, great stuff, l am no where near being a scientist but know exactly what you are talking about through riding, adjusting and testing my own fins and foils, when you talk about the effect of having a sharp or blunt front edge or the amount of tip or base or rake or single sided or double sided foils or thickness or upright foil and so on, we learnt these effects by being surfers and going on what we feel is right, slowly changing variations to feel and understand what we have done, maybe the scientists will come up with a completely different theory on how to get a board to drive and turn and not even have to feel what we beleive is so special. Good on em if they do, but for now there are to many factors that make a good board and l will rely on what l have learnt and felt through out my surfing life rather than take someones word that has the theory but no prac. KR http://groups.msn.com/MyKRSurf/krcomweb.msnw
Fascinating stuff- wonder if anyone has done any experimentation to see if some of these airfoils will give analogous performance in water, also lift/drag numbers and such. Something as simple as a stream and an arrangement similar to what the Wright Brothers developed could prove useful. I’d imagine hydrofoils ( see http://www.foils.org/index.html ) might have some useful data as well… doc… http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Cheilopogon&speciesname=melanurus
KR & Doc, What I wrote about the affect of the vertical cord on fin sensitivity may sound somewhat scientific, but I have to be honest. I didn’t get it out of a book. It came from that same school of hard knocks that you come from KR; that is by making fins and seeing what they do on the bottom of a surfboard. In the end it’s the only way to find out what the hell is going on. When you cook all the scientific jargon down, which by the way helps identify what it is we’re talking about, what you get is a foil or mold of resin & fiber that change how a board performs. Understanding a little science can’t hurt IMHO, because it seems to me that the main elements that drive the design progress are the natural world, intuition, experience and science. After you get through with sculpting what you think will be really hot you often find out that the balance is off and you have to start all over again. Occasionally there’s happy accident, which can be more progressive that what you’ve spend hours working on. It’s all good but those unexpected surprises help the journey continue. Fins are important no doubt but the heart of the future of surfing will always be in that little cubical where boards are shaped. Mahalo, Rich
Its funny how the industry has changed, you used to have to work at having an edge above everybody else, a little bit of a secret that you found out through your own research. Fins and their positioning were one of those things that you had to be able to feel and play with and if you got it right you had a definate advantage over your compeditors. Over the years l have taught many guys how to shape but only the basics, you dont tell them all that you know because it teaches them more to learn it for themselves and also you lose your edge. Now that l am leaving the industry and dont need my edge anymore l am more willing to give out a bit more info on what l have learnt for hope that the art of making boards for yourself and your friends does’nt fade away. Your right Rich the only way we are going to progress is from being in that room and learning for yourself and feeling the ride, after all, we are mainly trying to make ones self surf better. KR http://groups.msn.com/MyKRSurf/krcomweb.msnw
You have to give credit to nature. There is no shortage of things to learn out there. The more you learn, the more you are aware of how little you know. Here is an interesting aerofoil/wing software site. They don’t do swept fins … yet. Click below. http://www.hanleyinnovations.com
I’d really like to see the C5/supercharger set up modeled with this if you have access to the software referenced in the site. There’s so much talk about how the fins interact and “why” they work, but I’ve never seen any flow tank data to support the explainations. Of course, as referenced in this thread, If it works, that’s what matters. Thanks - Skip
BINGO!!! Skip, you’ve hit one of my pet peeves dead center. Fin makers ( and assorted other parts of the industry ) make all sorts of technical/engineering/scientific claims with little or nothing to back it up.Zero testing beyond handing a board to one of their stable of pro surfers and letting them loose with it. As you’re familiar with aircraft, you might think of the analogous stage of aircraft design - someplace around 1860 with guys jumping off cliffs with imitation bird wings strapped to them. As Rich mentioned, sometimes you have a happy accident…and often you don’t. Intuition is a powerful tool…but an informed intuition is far more powerful. The thing is, we’re not talking about NASA laboratories here. More along the lines of the Wright Brothers, home-made gizmos to do a little measuring and maybe to see what the flow patterns were doing. Ah well…rant mode off. Maybe one of these days I’ll have the time to do something about that. But not this week doc…