By the way…
The Surfers Journal vol.15 #6 has compiled around 8 interviews with some industry leaders of fins. worth check out.
The primary purpose of a surfboard fin is to provide directional control. You notice I did not say stability. A secondary purpose that has become popular for thrusters is kinetic propulsion or “pumping”. The size and shape of the waves, the style, size and strength of the rider and the design of the board all factor into the Flow Rate and Angle of Attack that a Fin Foil should be optimized for and balanced with Plane Form. At slower rates of flow water can remain attached to rounder curved foils over a greater range of angle of attack. But, as the flow rate rises those rounder wider foils cause the fluid flowing around them to separate, become turbulent, create drag and ultimately stall as the rate gets higher. So, for higher flow rates shallower less curvy foils prolong the flow attachment and less turbulence. But, it does so at the cost of a narrower range of angle of attack. Continuous clean curves are vital to keeping flow attachment and optimizing lift while minimizing drag. If the foil is optimized for medium to higher flow rates, you can get more directional control with less plan form area and consequently much less drag. Future Fins had already focused in on the low to medium flow rate market with their Vector Fins which to optimize kinetic propulsion or pumping in the same way a swimmer cups his hands to get out of the hole. But, they did so at the expense of higher flow rate turbulence. I chose to focus on optimizing for medium to high flow rates at the expense of low flow rate pumping. By starting with our most popular template the X-2 (which is a medium small plane form) I used the outline to control the cord length of an NACA curvature formula to create the foiled surface from base to tip without any angular disruptions. Or, the perfect foil. I matched the outside foil to a shallower inside foil to minimize the leading edge turbulence that is created when an outside surface abruptly terminates to a flat inside surface. At medium flow rates this leading edge turbulence helps with direction changing at the expense of needing a larger template to over come the inefficiency that turbulence creates. So, the traditional flat sided fins work fine in the medium range. But, the Next Generation X-2 work well in the medium range due to their smaller more upright plane form and best in the steep and deep when hauling ass!
Yo Tom!
Now there’s a grand literal discription of how fins work!
What you say is so essential and fundamental to understanding fin dynamics. I know a few will read it with care and learn much.
I would like to note that there is a place for undercambered fins in bigger surf. I have seen fins foiled by Curtis for big wave surfing that are undercambered that work beautifully. The camber as you have so clearly addressed must me more subtle as waves get bigger because of increased board speed.
Regarding Carbon fiber in fins: I have been using it for about 3 years I think. I’m not keeping track of time ~ it just goes too damn fast to bother. I have done lay ups with one layer of 5.7oz and 3 layers of the same strategically places in the panel. Fins with 3 layers of carbon fiber and twenty five layers of 6 oz E glass are very very stiff. I have had no problems whatever in foiling these panels. If truth be told epoxy/glass/carbon panels foil more easily thay polyester/glass panels at least with the methods that I use. At present I using just one panel of carbon in my rail fin layups for mid size waves and the flex pattern is grand as long as the fin is tapered nicely. Locating the carbon in the panel and sorting out how it’ll fit in the foil conformation is the trick. That’s much more demanding on undercambered fins than on symmetrically foiled ones.
Off to the fin shop, Rich
This link might be useful. It’s a resource for the NACA foils that surfboard fin designs evolved from.
Yup love that stuff Tom! Simple and to the point.
But, what constitutes low, medium and high flow rates for typical three fin surfboard riding?
Low flow rate scenarios are small wave groveling or when you’ve just completed a big round house cut back and as you S backout you find yourself behind the section.
Medium flow rate would be right in the comfort zone where your not groveling and your not driving for dear life.
So, take a guess at what I consider high flow rate.
Tom,
has anyone (I seem to remember seeing this somewhere but maybe not) ever used anything like notches, or ah, “zits” of some kind (“micro turbulators,” vortex stimulators) to stimulate the leading side boundary layer into mainstream integration so as to delay separation (on outers or concaved inners) until the trailing edge?
I was wondering because you were talking about optimizing the foils and the compromises you had to do with the foils to not get turbulence, and I’d been looking at the turbulators airplanes use to keep the flow even over the wing and ailerons.
Thanks for the content–great to read
The topic has come up.
Oh. Okay.
has anyone (I seem to remember seeing this somewhere but maybe not) ever used anything like notches, or ah, “zits” of some kind (“micro turbulators,” vortex stimulators) to stimulate the leading side boundary layer into mainstream integration so as to delay separation (on outers or concaved inners) until the trailing edge?
May work if AOA are small, like on airplane wings. AOA on fins are very large, easily exceeding the most demanding angles on airfoils. AOA of 11-14 degrees will stall wings; while turning/railing a surfboard, fins “operate” at much higher angles; they behave more like boat sails, and a lot less like lifting wings. So while turbulators are intriguing, they are not much help while railing a modern surfboard, but might help slower, large turning arc surfboards.
Newtonian lift, like AOA deflection off flow – but I think that if you add a (perhaps very) light sanded finish to the foils you delay separations which has to help
Just thought I’d get it out there
yup I dont surf unsanded fins.
O’Sin,
Just pickup the mag and read the article. Thanks for the heads up!
Interesting some of the GG comments about fin/board flex relationship…I started a thread a long while back and posted an update recently. Originally, it was thought that stiff fins and flex board is preferred. But after making some really flexible boards and trying some things myself, I dont agree, esp on this coast. Gotta have fin flex, just not too much.
The real key seems to be the tuning/matching of the variables: wave power/speed, surfer style/power, board design/flex, fin flex etc etc. Its getting that last 5-10% out of the most advanced equipment.
Its difficult tho. The challenge lies in the variability within those variables: waves and styles change constantly, even withing sessions. Probably would explain why some days Im just on fire and some days Im so so.
Re fin panels. I have found that the base-to-tip thickness flow and the outer glass has a huge impact on flex charac’s. One approach I want to pursue is getting a 1/4" thick panel of G10, cut the outline, shape the base to tip thickness flow (no foil shape) then adding sw core foam (or plywood) on each side, shaping that (and G10) for foil and glass at 45/45 with flex epoxy. I reckon there would be flex and rebound from that const.
Lots of juicy stuff in that article…Kudos to Jim R (lokbox) for his contribution on 4 finners…cant wait to finish mine!