I never understood the theory that fins need to have a foil like an airplane wing. It makes complete sense for an airplane, but not a surfboard. The reasons I have heard are that it pulls the board into the wave. I just don’t see that happening with any significants, nor do I see why that would be important. I’m thinking that fins should not be designed to produce lift, but instead to have the least amount of drag possible. In my opionion, fins should be made as thin as possible and with a symettrical foil on both sides. What do you guys think?
It should offer some lateral stability, though I was thinking something along the same lines… inspired by a post earlier on using the Center FCS plugs on the rails of a concave board, I thought about putting center fins on the rails. Unfortunatly the surfs been crap, hopefully I’ll give it a shot this weekend.
Asymmetrically foiled fins give more power, grab, dig, bite,lift whatever you want to call it when placed on the rail than symmetrically foiled ones because the foiled surface creates a lower pressure over it as the water is moving faster there than over the flat surface. Subtle changes in assymetrically foiled fins create more or less lift and thus make the board accelerate as the surfer drives off of them. Without this type of effect modern shortboards wouldn’t work like they do. No Worries, Rich
Single foiled side fins create lift away from the stringer. Which means the lift is pulling the rail down into the wave face. The result is more holding power. Even if the side fins were foiled with a symetrically foiled sides, the toe of the fin is going to present the angle of attack of the fin to work as a lifting foil. It just won’t have as much lift.
I have never really questioned the toe in of the side fins in the past, just accepted it as dogma. However if your going right on a wave, lets say your triming, then the left hand side fin must be almost out of the water and so not really doing much, and the right hand fin with the toe in towards the nose of the board is shorely pushing the board to the left, or at least stalling, generating drag, to pull the board along the wave and right hand rail along the wave the angle of attack of the right hand fin would need the fin pointing out to the edge, another way to think of it is that if a aeroplane wings were pointing to wards the ground then it would have a hard time taking off? Also I would question the lift of such small fins with large rakes, and short cords. With a 80kg boke on a board I reckon the lift effect would be negligeable especially considering the surfer is generally not going straight down the line and going up and down and varying the true speed of the board with respect to the water, generally going quite slow and using the power of the wave. It would be good to see some tank testing done on fins. Now if we are talking about higher speeds like those that sail board get up to then the lift effect is a real issue, but they are doing probably about 30knts accross the water. If the fins are too large the boards start to lift.
If you put Center fins in all 3 spots you will have a slower board with tail vibration, The reason is the outside fins push water out the center splits it off the sides if you have it splitting of the side fins it will distort the flow of water to the center fin causing chatter and a slowing, Plus it will ride like SHIToday ill go ride my twin fin. instaid of say having an even flow it would be like having rapids beneath your feet
What l found with single sided foils when they first came in, was that they where a bit faster and released the tail easier when you wanted it to but l also liked the smoothness of a double sided foil for turns, so l would start with a single sided foil and then do a 10% foil on the inside of the front fins balancing out the two better points by giving speed and better turning capabilities.
The camber of the outboard thrusters creates lift in the opposite direction of the turn. So, fin camber would have no benefit to the turn, but would provide a distinct anti-cavitation benefit. The toe and cantilever angles work to increase the tuning power of the “deep” fin and decrease the turning power of the “sallow” fin (deep is the inside of the turn and shallow being the outside). As for drag factors, thin may increase drag – the “tear drop” shape typically minimizes drag.
Shine wrote: “As for drag factors, thin may increase drag – the “tear drop” shape typically minimizes drag.” Is this really true? I’m puzzled by the difference in foils on a wing on an aircraft vs. the foil of the fins on a thruster. But then again I think about the difference in the media they are used in, water being much denser than air. I’ve only seen natural tear drop shapes happen in air. I’m not even sure it’s the absolutly fastest shape in air. Maybe the thinner foils create less drag in water at the speed surfboards are traveling. regards, Håvard
Andy, The toe of a thruster fin cluster was never designed to optimize speed. It was designed as a destablizing component. If the side fins were set parallel to the flow of water, the board would be very difficult to turn. Thrusters were never intended to optimize speed. They were designed to be ridden in the pocket of the wave while constantly turning. There are fin characteristics which will maximze this and the are others that minimize this characteristic. You’ll have to look somewhere other than a thruster for minimzed drag and ultimate speed. But, I don’t think it will turn very well.
I’m no expert but here’s a quote from a PJ fin record in the photo archive that has greatly impacted my ideas of how foil effects fins…(single fins at least). Q: I notice that they are quite thick at the leading edge…what is the theory behind that design? A: In PURE STOKE by John Grissim >>> Re: Greenough >>>“…he had perfected the high-aspect, laminar flow fin, a development which made him litterally the fastest man in the water.The key lay in constructing a fin which could harness the wave’s energy without generating drag-inducing turbulence along the trailing edge of the fin as it moved through the water…Laminar flow basically is over a very narrow range. Water only goes back an inch and a quarter or so before breaking into turbulent flow. So the fin must be very narrow. Look at any high-performance fish and you’ll see my fin is basically the same - it’s the same plan and foil shape as a large tuna…you’ll notice their tail is very narrow and quite high. With that kind of fin by the time the turbulence shows up in the water the fin has already left it behind. Hence no turbulence to affect performance.”
“shaped like a large tuna” . . . good point. Nature took millions of years to come up with a hydrodynamic foil that surely performs.
Foil shape is highly dependent on speed. Few surfboard fins go as fast as a tuna, they can get away with a lot more before creating turbulence. I recall reading something from some Aussies doing wave tank testing on fin foils where they seemed to claim that variations in foil shapes would make a difference for tow-in surfers, but only minimally impacted small wave surfers, because of speed differences. http://www.blakestah.com/surf/
I have made carbon fins that were thin (1/8’').They worked great as long as you were going fast down the line.If you slowed down ie:cuttbacks off the lips, they were hard to get back up to speed.Fuller fins work better on surfboards because they operate mostly at slow speeds.Airplanes work the same way.A Piper work best at slow speeds and a F-16 is best at high speeds.Your shaper is trying to make you a board that has the best balance for your riding.Sorry you can’t get both.
Just a quick one on lamilar flows, I recall reading in a dingy sailing book, this guy covered his foils ( centre board and rudder) in a sticky jelly like substance and then took a hose and poured water over the foils from the leading edge, the spots where the jelly stayed is where he had lamilar flow , ie no drag and the spots where is was washed away then he needed to adjust the cord of the foil. The main difference being that the foils on a boat are a few feet long and not 6". But you can still use this principle to test your new fin designs.
interesting idea, can you be more specific as to what type of jelly did he use and how long did he pour the water over it?
Cant remember but it was something sticky that you could see, you could probably go with vaso and some food colour in it, or maybe start out with something simple like a thick jam or peanut butter, sounds like I am taking the piss but if you experiment you should be able to find something that doesnt wash off too quickly, so that you can see the effect. As for the water you could just hold the fin under a tap the stuff will either wash off in a few seconds or it will not. The idea is to see where it washes off, the curvature of the foil at this point is incorrect to get lamilar flow of the water, might be too fat or to this, basically the friction of the water flow washes the goo off. Start on a low water pressure and work up. Also this allows you to alter the angle of attach of the fin to the flow of water to the point where the fin stalls and a hollow forms on the bottom of the fin ( if water is poured from the top). I used this method to form some convex side fins in an attempt to make the water flow around the fin longer, my theory was that this would reduce drag. Formed the fins from balsa but lost motivation, too many distractions, will sort them out this summer. But as I said before, this technique is used for jumbos with deep cords to reduce drag and also on boat foils, for a 6" board fin the efficiency gain would be marginal, better to work out how to get the water to flow cleaner in the fin. Those classic underwater sholts of a guy going down the line and the fins airrating ( not cavitating) amaze me, definate drag there to be eliminated. Andy