Tabs and poles are really a suboptimal solution they are not needed if you leave the outline attached just until the last pass.
just giving the leading and trailing edge a small and consisten sanding is much more accurate and convenient than having to work out the tabs and poles one by one risking ending up with dimples in the surface.
If you contact me with your machine parameters, I can generate you some sample gcode so you can try it out.
I think I finally understand what you are saying about leaving the outline attached. I am not sure exactly how I can make it work in aspire, but I’m also not that experienced with it yet. I’ll send you an email about machine parameters and gcode.
just bought some twin fins from powerdrive fins (mark thomson). curious. the foil on the twin fins i recieved is quite similar to the ag10 with the max thickness at about 25% chord. just without that camber. super interesting read. i imagine the flex profile of the fin gives it a similar ‘camber profile’ when pressure is applied in turns (and while pumping) in theory reducing drag and increasing lift through turns and getting more out of your pumps. though it may not be a proper camber, as it would cause the trailing edge to have a ‘curved taper’ as you described, creating drag, and probably change the chord line in the process. but maybe still closer to what we want than mass marketed twin fins? so would one be able to properly foil a fin to have the flex for an ever changing camber profile that adjusts to the waterflow through turns, while keeping the trailing edge from creating a ‘tapered curve’ during the flex? or would a fin that flexes that much be too thin and cause problems? maybe use a different material that flexes more while keeping thickness for the front half of the fin , and a stiffer material so a straighter trailing edge can flex freely on that joint without curving near the rear. do you think mark thomson had a similar aim? or is it another discussion of whether or not flex provides propulsion? anyway… have not ridden them yet.
im really curious to know how those goe 265 fins go, glassed at a -5 toe…
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This seems to be an appropriate place to post these pictures without starting a new thread. A collection of pictures from my journey trying to cut fins on a cnc router. I’ve had some help from Hans and Mr. Mik as far as process and models.
The first picture is a test. It was actually an airplane wing stl that I slapped a fin base on the bottom of.
The next couple pictures was a “high aspect ratio” fin I made in finfoil with a NACA0015 foil. It was cut on the cnc as two halves, glued together with poly resin and glass, molded into a finbase using bondo hair and a split mold, and finally glassed and sanded to fit. It ended up kinda junky so I never bothered to make it nice looking, but it surfs extremely well.
The fifth picture is one of Mr. Mik’s tubercle fins cut out as a test. The base has a wood tab so I could mold a fin base similar to the last fin. Unfortunately a pupper got a hold of it and chewed it up. I plan on cutting another out of nicer wood and giving it a surf.
The last two are a friends broken longboard fin and the test one I cut to replace it. I’m trying to recreate it for him. It seems to be relatively high aspect and shark dorsal looking. After cutting it and comparing it to the original it is pretty close.
Hi Phillip, I see quite some glass sticking out the leading edge, didn’t that change the leading edge shape?
This is why I do the resin halo around the fin, it doesn’t matter that you sand through the fibres as resin is underneath, it allows to keep the leading and trailing edges true to the shape.
Yeah, I don’t think I am going to put glass in the center anymore. I hadn’t figured out how to do the two sided cutting when I made it. It actually wasn’t too difficult to glass, but I did change the foil slightly while sanding.
One of the things I wan to try is cutting a fin slightly undersized, glassing it relatively thick, and then repeating a finishing cut at full size, hopefully just skinning the glass. The thought being that the machine would get the final shape fairly perfect and be mostly ready to surf.
Oh I did not know you had glass panel sandwiched in between.
I had the impression the fin got extended in front, with the panel I assume you were able to keep the foil profile pretty accurate.
You’ll have a hard time repositioning on the machine.
Just use a vacuum lamination, this gives consistent thickness and has many other advantages.
One of these days I’ll quit being lazy and actually put together a vacuum rig. I work with all types of water, hydraulic, and pneumatic piping systems with pressures ranging from 4500 psi to ~30 in hg in my day job. Building a vacuum set up is the easy part, but it always seems like work so I have yet to build one. I even have an old refrigerant compressor to use as a vacuum pump. You can bet whenever I build one I’ll be asking for tips and tricks on vacuum laminating.
Same here!
Thank you Scott Jarrett for writing this up, it took me well over an hour to read the first post, opening all the links and playing with the settings. Will do that again for sure.
I intend to devote more time to 3D-printed fin design again, soon. The G-Whale-7FRR-Open Source is just working really well now and I have released a few ‘into the wild’.
Scott, have you experimented with various grades of fin surface roughness?
Or do you have a theoretical understanding that could answer the question?
I am wondering how to balance manual surface finishing vs the inadvertent imperfections caused to the foil by manual sanding and polishing.
Is a shiny fin surface required or even desirable?
I’m always worried that I damage the foil when I try to sand a fin.
The foils are Eppler 168 and Eppler 169. Or at least that’s what I was aiming for. The thinner E168 at the peaks and the thicker E169 at the troughs.
But the surface is actually concave at the peaks, so it’s not really an Eppler foil any longer, only maybe at the peaks.
Anyway, sanding it is tricky, the more raised areas always get sanded more than the recessed ones. And the higher parts end up polished while the troughs are still matte.
Sorta yeah, sorta no. After learning about tripping the boundary layer, I realized that I noticed that many of my boards worked better after smoothing out leading edge nicks with rough sandpaper. That was nearly 40 years ago. The Eppler series of airfoils are notorious for developing laminar separation bubbles at our Rn - and the fix that Dieter Althaus* demonstrated in the 80’s was to put a strip of Scotch Tape on the upper surface at 02c of the E193. Knowing that, I just sanded everything to be rough. Since everything off the shelf had faceted foils through the 90’s, I foiled my own fins. I always left the surface rough with 60 grit or so. Then came Futures Fins, and I got out of the habit of doing that. However, after the research that went into my initial writeup for this thread, I went back and put zig-zag tape or tape strips on EVERYTHING. Then came the coronavirus… and nothing got into the water to be tested. With that herringbone leading edge, you have significant 3D flow effects going on, so it is not possible to say just what different surface roughnesses will do to the performance. You know, Makani makes the Piranha Pure Wave series of fins that feature herringbone leading edges…
What is 02c ? I suppose it means a point along the chord line.
I suppose the tape is roughing up the surface due to the tape’s surface structure? Or is it the sudden transition where the ‘leading edge’ and ‘trailing edge’ of the tape causes a disturbance?
If the sudden steps at the tape edges cause the effect, then the ridges created by imperfections in the FFD 3D printing process might be the equivalent of hundreds of tapes applied to the surface. Like speedbumps on a road every few meters.
The way I print the fins causes multiple ‘speedbumps’ perpendicular to the water flow.
Is there an evidence based answer to the question about how much a fin should flex?
Or have you been able to try out fins with equal shape, but different flex yourself and found obvious results?
That’s exactly what I was thinking about today when I rode a board with a stiffer fin than a different board has. The templates are identical, the foils are similar, but the one I rode today was made from very stiff solid G10 glass. The waves were too mushy to make any determination whatsoever. T’was just a ‘get in the water at all costs’ sorf of day.
Yeah, 0.2 of the chord, or 20% of the chord. Looks like I dropped the decimal point in an edit.
Yes, yes and again yes… it forces our low Rn sticky laminar flow boundary layer to transition to a turbulent boundary layer that’ll behave better and stay stuck to the shape of the foil.
My opinion on flex: Flex is just a patch to make badly foiled fins work.
Flex softens the stall characteristics of a fin, by twisting under load, the base stalls first while the tip remains effective. Which gives you more control when heavily loaded, without spinning out.
This only works if the fin is swept (raked), otherwise the twist will not change the AOA of the tip. With forward swept fins, the effect is even opposite.
In my experience the same benefits can be achieved by selecting a foil profile that stalls more softly allowing a stiffer more upright fin that makes your board more reactive and efficient.
Another great option is the “thrailkill twin”, it’s biplane effect softens the stall even more. And also note these fins are very upright and stiff, they don’t need the rake and flex.
Please note that my experiences are mostly on sailboards, but I’m convinced they apply to surfing too.
Spent some time at the computer and workshop today. Using Hans finfoil and his tips for cutting on a cnc I made/remade a fin for my buddy, complete with his name engraved on it. You can barely see his name because I didn’t change bits like I should have. The second fin is Mr. Mik’s whale fin. These fins are straight off the machine so some sanding is required. Better plywood would also help.
Very nice fins People!
If you could please, post and add pictures over in the ‘big’ home-made fin thread as well for posterity:
https://www.swaylocks.com/forum/45240/show-us-your-home-made-fins?page=55#comment-563438
