We at Swaylock’s…Just kidding Rich!!
Seriuosly though, as far as fins go… how thin is to thin? If you go to thin it seems it would be difficult to foil. But it also seems you’d have less drag. The reason I ask is that I was wondering about drag and thinking of a super thin fin, i.e. .080-.100 titanium. What say ye fin men?
As the camber is reduced to almost nothing, the lift will come more from dynamic lift of the high pressure side rather than hydrodynamic foil lift from the low pressure side. Sure, it will work, but with efficiency being very low. My guess is that without much flex, the “thin plate” fin will be sensitive to increased angles of attack, basically you will spin out.
If there is some flex, then you might not spin out but the twist in the fin will probably produce significant drag…
…with that said there are certain fin outlines that will still work but they would have a lot of sweep-back/delta. Their leading edge produces quite a bit of lift and the high pressure side still does its’ thing.
I guess I am a fin guy as I have a whole bunch! Seems to the fin guys that I know, 3/8 stock fins are not fat enough. The customized fins that are 1/2 seem to work better. Drag has not been a problem and in many cases helpful given the flat rocker that I prefer. We are looking into having a 1/2 inch panel made up so we can foil em as we wish. A bit expensive but, we will get what we want!!
Roger
what if the thin “plate” was formed so that it had a convex foil on the one side and was concave on the other but was still made of a very thin material?
Now your heading into RichHalcyon’s territory. Did you see his pics with that same concept?
The convex side will produce lift according to how the curve runs when under various loads.
The concave side will likewise produce more lift than a flat plate, again this varies according to shape.
Turbine blades are kind of like this, they are very thin, some are not foiled and are regarded as vanes.
(Of course much of this flow regime is trans-sonic and has very different parameters (Mass Flow Rate, Medium Density, Reynold’s Numbers, etc.).)
Some fins are used to “fence-in” flow to keep the flow under the board effective. Bonzer fins are like this. Carefully foiled and outlined fins are at the other end of the spectrum. These maximize lift for a given size (efficiency) of which Rich Halcyon’s fins are striving towards.
…" fins … how thin is to thin? If you go too thin it seems it would be difficult to foil. But it also seems you’d have less drag. The reason I ask is that I was wondering about drag and thinking of a super thin fin, i.e. .080-.100 titanium. What say ye fin men?
Hi Aqua !
Firstly …which fins are we talking …side fins ? back fins ? The whole set ? 3 or 5 finners ? 4 finner ?
Secondly …is this going to be for the “stingray” , or another board ?
If it’s for the “Stingray” … why ?
Won’t you need a bit of thickness, or depth, or base length, to hold the wide area (where the wings are [on the “stingray”] ) into the face ?
…that’s assuming you are intending these thin fins for the “stingray” , of course . [you have so many jobs on the go at once, it’s hard to keep up with you sometimes, Gary ! ]
What sort of base , depth , rake , placement on the board in question were you thinking ? [these factors will I think all play a part ]
ben
p.s./ disclaimer … - even though I make quite a few fins , I don’t by any means consider myself a “fin guy” , or expert, or anything , by any means …
Hopefully Rich Halcyon and others will ‘chip’ [haha] in with their thoughts / findings on this . I’m interested to hear on this subject , too , as the last F.C.S. tabbed tinted fin I posted a shot of recently is the thinnest one I’ve ever made ! [haven’t ridden the pair yet , unfortunately , so I can’t really share any findings there , sorry !]
Proneman,
You are on the right track.
thin = stagered jerky turns , earlier spin out , cant handle high angles of attack , drag kicks in earlier at lower angles of attack …
thick = smoother feel , more hold , handles higher angles of attack , can be pushed harder and later into a turn and maintain more speed at higher angles of attack …
my best fins are at almost 5/8 …
thin fins are great if you dont need to turn , and your doing mach speed …
what seems logical isnt always practical …
regards
BERT
We at Swaylocks? Hmmm. Are looking for new ideas, answers and inspiration? Oh, and a good joke too.
How big the fin and how sensitive to turning you want it to be determines how thick you make it. There some very accurate ratios in the NACA publications that discussion optimum thickness for maximizing performance.
Bert’s right, thinner goes real fast but without some foil you don’t get any lift and changing directions can be a very abrupt and out of control experience unless you have riding a highly specialized board in very special conditions.
Using and applying the complex arcs in a fin foil intellegently are what make for better performance IMHO.
I make tow-in single fins and they are very thin for their size and have a 55/45 foil on them so they are very neutral and create a minimual amount of drag. When you get going over 25 knots on 50 foot + faces on a single fin you better be getting the lift need much to turn from your board if you’re riding a high aspect fin. The thing is already very tender because of how sensitive it is the A.O.A. so it functions much like the wing of an airplane. The other end of the scale is the single on a 11’6" traditional long board. If it doesn’t have a nice thick foil on it will resist changing direction. On a board that long you want some sensitivity. Hell you’re not going to go that fast anyway. So there thicker is better in moderation of course.
Rounded leading edges on fins also help them change direction more smoothly and easily IMHO.
With that said George Greenough built some Stainless Steel fins some time ago and said they were tremendously fast. I don’t know any more details than that on his metal fin creations. Metal just seems like the wrong material to me. When it comes to surfboards and fins lighter is often better.
Good Surfin, Rich
Thin fins CAN generate lift, even a flat plate can generate lift with some angle of attack, lift is not uniquely dependent on foil. When you consider angle of attack, planshape, and foil, the smallest effects come from foil. When you fix planshape and angle of attack, as is the case on most surfboards, foil is the only variable, and so has a large effect.
A thin fin has better lift:drag ratios, and its lift is concentrated at smaller angles of attack. So small turns generate more lift and more drag than fat fins. The thin fin will also stall at lower angles of attack, which becomes relevant if you have any clue of the angle of attack that fins actually see. The best choice for a responsive fin is the thinnest fin that will not stall.
Unless your board typically sets up fins at something other than zero AOA.
A critical value, and one often ignored, is how STIFFNESS depends on thickness. In a laminate, almost all the stiffness comes from the outer half of the layup, and it is dependent on the cube of the radius. So, if you try to make fins thin from fiberglass, you’ll rapidly get to a very flexy product. Even using carbon fiber you can only drop 20% or so of the thickness before the fin turns into a wilted piece of lettuce in the water.
0.080-0.100 titanium would be a little too thin, I don’t think you will find it stiff enough. I calculate you would need 0.20" on a single fin to maintain stiffness, and about 0.120" on a thruster.
As for all of the naysayers, there’s a little something to be learned in everything new you try. If you’ve made a fin that is JUST AS STIFF and a little thinner, you would have seen what I’ve seen. I don’t envy foiling the titanium, but save some of the dust and make a little sparkler…
“…save the dust and make a sparkler.” Ha ha!
I’m glad someone mentioned that a flat fin will still lift albiet not as well. About 10 years ago we put outlined fins without any foil (both leading and trailing edges were square) and the board still went okay. What it lacked in top speed it seemed to gain in direction change. A couple of riders figured out how to make it go fast and started getting stoked on them; how’s that??
I did two other boards like that but did not pursue it because it was not received well (just based upon aesthetics and style).
Oh yeah, after some more feedback, I foiled the rear edges with my grinder (glass-ons) right on the board and we could swear there was no substantial difference between regular thruster fins and the square leading edges! This is when I realized that we don’t know all the forces going on under a surfboard (but that would be another thread).
So, my take on things is that there is no true flat plate, since everything bends. When a flat fin bends it is producing a little bit of foil. Most of the lift is coming from the other side (this comment is coming from someone who has experience in the aerospace industry) kind of like when you stick your hand out of a car window at speed and feel the underside of your hand being pushed up. The vacuum feeling on the top of your hand (not the best foil) is there but empirically it is less. This is just dynamic lift. Most any object will lift given enough speed.
disclaimer
Remember: when that foil is done right, that vacuum becomes tremendous. I think that the foiling of fins is getting so good that the sizes should probably come down quite a bit It is just strange that a flat, unfoiled fin should work at all…
“It is just strange that a flat, unfoiled fin should work at all…”
LARGE AOA’s!!!
Thanks +1…this jives with what I’ve said here so many times. In rail to rail surfing, common AOAs are beyond most common fin foil stall angles, which generally, are very low, <<10-12 degrees. AOAs in modern surfing are significantly larger than that…although some disagree.
Nevertheless, foil does matter…and most of what has been said here makes sense. It would appear that a Bert style foil, thick with rounded leading edge, would handle large AOAs better…although proper base length is something I still grapple with…I made a thick set of fins but I think they were too long in the base (4.6") and I could feel them drag a bit more from toein.
yep Meecrafty,
it’s what I suspected for many years but have no test data to support it. The fin(s) seem to be acting in several different modes. The most commonly ‘designed-for’ mode seems to be fairly low AOA’s and almost lab-like conditions. Wind tunnel data, static flow models, etc. apply directly to this design criteria. Trouble is, I suspected there was a lot more going on. I think your estimates of Angles Of Attack 10-12 may still be conservative.
I think there are flow regimes of some fins approaching 30 degrees or more! A good example of flow visualization is the spray coming off the rail. This flow is happening all around the tail area, it comes from the supporting pressure we derive when we go skimming over the water. That flow is fanning out off the tail and has a lateral component. Combine this with the lateral movement of a surfer pumping his board and we get some fins subjected to more translation side-to-side than we realize. Yet the surfer still needs “something to push against” like the bottom/rail and fin(s) to get some forward drive or momentum.
So, unlike most aircraft designs, (high-stall X-29 excepted), it seems that surfboard builders are unknowingly ahead of the available data; most of these advances have come by trial-and-error and seat-of-the-pants reckoning. There are four major lift components: foil like in aircraft, dynamic lift like the underside of a wing adding to lift, flex (or any related variable geometry) and outline such as elliptical, swept-back, or delta shapes.
I am starting to think that we are developing a thrust-generating fin more than a basic lifting fin.
I am thinking of two examples. Propellor technology like on subs (though the flow regime isn’t the same) and of fast-swimming fish like the tuna. There is information available on both topics. I think a tuna robot was built at MIT…
The tuna fish swims very fast, uses rapid, lateral burst movements that develop forward thrust. The fish’s tail controls a segment of turbulent flow and kind of “pushes” off of it. The tuna’s fins are a membrane in thickness except at the leading edge, kind of like a flagpole and a flag. Somehow, by outline and membrane tension the flow does not migrate much spanwise, but I believe this flow across the membrane is turbulent or vortex-like (ala Magnus Effect) in intitial bursts to get up to speed. Wouldn’t it be cool if someone developed a membrane-like fin! The geometry should be such that the harder one pushes, the tighter the membrane gets.
Once at speed, the tuna probably uses its’ body for steering as much as the fins. Tail fins also act as “wind vanes” to keep the animal pointed straight at low AOA to exploit the momentum. Surfers use the hull of their boards in a similar glide mode. When not under a lateral load at higher speeds, a membrane fin would ruffle and create too much drag. Somehow if someone could design a fin to retract (or a hull that would cover the fins) when not under significant load then the surfer too could exploit this momentum.
Basically, I just think there is a lot yet to be explored.
If you use a leading edge thats thick like 5/8 and then foil it down to very thin at the trailing how would this ride in theory? This seems to be the most logical for normal situations to me.
I wish I understood what the heck you guys are talking about. I can’t be sure if you are all in agreement or if you are to polite to disagree with eachother.
Let me ask this, When you say “lift” are you talking UP? or are you talking BACK AND FORTH. For instance. UP being lifting the deck, and BACK AND FORTH being the fin reaching/pulling vertically or parrallel to the bottom of the board?
Lift for a fin usually means a force to the side. In hydrodynamic technically correct terms lift is a force perpendicular to flow.
I say try it and see how it turns out. Just beware that if you make a fin too floppy the flex will dominate the feel of the fin, and you might not like it.
I think there are flow regimes of some fins approaching 30 degrees or more!
Yup and I’ve tested this…crude but effective
http://www.swaylocks.com/forum/gforum.cgi?post=157948#157948
My statement was that common air foils used to design fin foils stall very early…AOAs at 10-12 deg max.
Thin fins with sharp leading edges stall much sooner, proly less than 7 degrees.
Hey Aqua, fin conversations can get convoluted at times but one can analyze things very simply…a foil is primarily designed to provide lift (wings) and reduce drag by reducing frontal pressure and air flow turbulence (streamlining)…common air foils are typically designed to provide lift and not designed to handle large AOAs which is what fins deal with in modern rail to rail surfing.
Designing a perfect fin foil for a surfboard is very tricky if not impossible - build a Bert style fin riden by an average surfer and fin pressure drag shoots up boggin a slow curvy bottom board…its not a coincidence Bert boards have fast rockers. Thin fins have their place too. Too many compromises.
Nevertheless, pressure drag is the speed killer IMO…too much rocker or too big a fins…try surfing without fins on your board…interesting experience.
TurboJets,
I’m thinking maybe like you describe. I’m also thinking a fiberglass frame like a harp, open at the rear edge, and strung with something real strong like Kevlar with a flexible membrane stuck to it… Seems to me that as the fiberglass leading edge bent under load, the strings (depending on the pattern they were strung) would pull tighter. This would meet one of the parameters…
…trouble is in the no-load scenario; flutter becomes the killer. Significant drag. Resonant destruction.
I’m still troubled by the fact the flat plate fins I put into some boards way back, worked so well. They shouldn’t have worked. The guys were ripping on them.
Also, racing sailboats utilize a keel to resist a lateral load with horizontal lift. How is the forward momentum generated? Some of the winged-keels seem to actually generate thrust. Vortices of turbulence are generated where the main keel meets one of the wings and the bobbing action of the hull is the kick against this vortex to propel the hull forward a bit. This is kind of like a fish flicking its’ tail, but staying rigid while doing it. A fin that moves could be part of the equation (variable geometry). How this VG fin flexes and “cups” water (for lack of a better term) will determine effectiveness.
I’m still troubled by the fact the flat plate fins I put into some boards way back, worked so well. They shouldn’t have worked. The guys were ripping on them.
Also, racing sailboats utilize a keel to resist a lateral load with horizontal lift. How is the forward momentum generated? Some of the winged-keels seem to actually generate thrust. Vortices of turbulence are generated where the main keel meets one of the wings and the bobbing action of the hull is the kick against this vortex to propel the hull forward a bit. This is kind of like a fish flicking its’ tail, but staying rigid while doing it. A fin that moves could be part of the equation (variable geometry). How this VG fin flexes and “cups” water (for lack of a better term) will determine effectiveness.
Keels DO generate thrust, they allow you to go upwind. It is critical that the sail-generated force vector and the keel-generated force vector are in slightly different directions. The boom leans to the leeward side of the hull, optimizing the boom angle with the angle of attack is one key to sailing well.
A fin that altered concavity/convexity like a sail would be a neat trick, if adequate stiffness could be built in…
Fins that move are cool…