Yes I remove the fillet with a chisel, or a square file. It deosn’t take much time so I don’t think it’s worth changing the bit and have an extra machining operation. In some cases, like my curved fins, the tab is not parallel to the worktable (to use the thinnest stock the fin will fit in) so it would be tricky to get a right angle between tab and fin bottom.
Not sure I explained properly so the image should make it simple. I want the channels created by the sine and also the foil section to be oriented with the (unperturbed) flow. Obviously the flow IS perturbed. The layer closer to the board sticks to the bottom and goes up with the tail rocker. But a bit deeper the lower pressure created by the tail rocker is filled with water coming from the sides. You can see this if you look at your wake when trimming. And a bit deeper again the flow is coming straight. So I feel that for the most part, the fin sees water coming straight at it, not going up with the rocker. There is probably no perfect orientation because the board’s pitch changes with weight shifts but I feel that aligning the fin with the horizontal flow makes more sense when you have a horizontal component on the fin. Same thing for the Wavegrinder’s winglet.
We had the impression that if we let horizontal components aligned with the rocker, the tail was more draggy and even could feel like it’s being pulled down. This also happened with tubercles. It could be an advantage, if it’s not too much, for nose riding.
So, for now, I align my fin’s foil with a theoretical horizontal flow. Even for tubercles or simple fins. Just to make sure I’m making sense, if you were to saw a fin to see the foil section, you would cut it at some point parallel with base. If you cut it at an angle not exactly parallel with the base, then you will see a different foil profile right ? So may be I’m splitting hair but since I spend hours designing a foil for performance, I want to make sure that the water sees the foil I intended.
It might be unnecessary for normal fins, but for fins with 3d or horizontal parts, and even I think, tubercles, it does have a noticeable effect.
Oh and the board project is paused until the country gets back to normal. Done with the shaping but too complicated to get resin and plugs right now. Keeping busy on the week-ends with fins, studies and other projects instead.
Here are the front quads, lower frequency but same 5 degrees leading edge oscillation. Made these for my groveller board in progress. (Actually the project is paused until the world is done ending, who would have thought that the end of the world would take so long and be so boring ?)
I also wanted a central fin to have a thruster set, but this oscillating LE is inherently assymetric. So instead of varying the AoA, I designed some center fins alternating 2 foils of different thicknesses, progressively morphing from 8% chord to 14% chord. I tried 2 methods. One is to morph the foils continuously, and the other is to let each foil plateau for a bit then blend quickly into the next.
Surface pressure analysis shows accelerated flow in the valleys. The goal here is to smoothen the stall as well, by having the thicker foil stalling later, and possibly the difference in pressure between foils is creating interactive effects like the tubercles vortices. A horizontal component is also created so I align it with the flow, but it is less pronounced than with the varying AoA concept. Frequency and amplitude will have to be experimented with.
Working on a longboard single with this concept as well.
It doesn’t show well on pics so I’ll post some 3d renderings too.
Impressive stuff ! Any chance you’re located somewhere close to San Diego? I’d like to have an opportunity for an eyeball to eyeball discussion with you. Perhaps commission a project, if you were interested.
I’m only 10 000 miles from San Diego, I live in Pondicherry, India, btw I am a Frenchman, my name is Alex.
I would love to hear what you have in mind. I would be so stoked to collaborate with you on a fin design, and if it was to end up on one of your boards I’ll be boasting about it for years
Hi Alex, in the case these fins wouldn’t “work” or bring any benefit, they could be sold as “Art” as they are eye catching and really really beautiful !!
Thank you Pierre, well that’s good news because Art sells for a higher price than a set of fins.
More seriously, I think there must be some configurations that could bring some benefits in terms of delayed stall, so better behavior in turns. If I can make some sets that my friends and I can enjoy, that will be a good start.
Well, Padauk and Asian Mahogany are the woods in which I have large offcuts, that’s why I use them for trying ideas, and I really like them too. But I can’t wait to get some G10. I don’t feel like laying my own panels yet because I’d rather save time and materials and do as many crazy fins as I can instead. When I’ll have enough feedback to know which ideas are worth pursuing I’ll invest more time and/or money.
Actually planshapes are the most mysterious part of fins at the moment, for me that is. You can’t just “science the sh*t” out of them. Just about any of the planshapes that have been succesful on the long run make no sense in terms of aerodynamics (in surfing, not so in wind surfing or sailing). I mean you’d never fly (efficiently) with a surf fin shaped wing. So I suspect that the “inneficient” wing shapes we see in fins work because we actually need drag for control, and also because the foils are so thin (in the fin industry) that we rely more on vortex lift than foil lift (hence the high rake wich make them half delta wings, high drag for control and lift at high AoAs).
So for now I am exploring foils and other means of softening stall and creating a more useful drag. Then I’ll have a good think at the planshape.
If you think of the planshape as an idealised wing you end up either with an elliptic spitfire shape or the wavegrinder’s tapered rectangle (which approximates an elliptic lift distribution when the tip is around 40% the width of the base). These 2 shapes are the best shapes if you want to fly, but if they were the best to surf we’d know already. There is really something about controlled drag for surfing and I think that if we find better ways to soften the stall and induce just the right amount of drag, at the right time in turns, instead of using inneficient planshapes, then we’ll have progressed.
Right now I think that the best planshapes for fins that incorporate passive flow control methods, like tubercles or the ideas in this thread (and Bill Thrailkill’s revisitation of biplanes, which is also a stall delay mechanism), will end up somewhere between a perfect wing and a high rake shape with a long chord at the tip.
Just sharing where I’m at, take this with a cup of sea water.
