What is the effect of reverse foiling fins? I picked up some old books from an antique shop. One is naval architecture, the other is all about yacht racing and design. The later has a lot of intersting data compiled from tests done on sail types and configurations, keels, and hull design. Also some some data that seems loosely related to the shark skin fin that Mr. Thrailkill is working on; its about the surface of a duck shot from different angle with light shining form different directions to show the dimpling. Pretty cool stuff.
Anyway my mind is reeling so I thought I’d ask you guys rather than beginning experiements.
I have no experience with them, but my thought would be that it wouldn’t work so great in surfing… i know that, at least with aerodynamics, having a sharp leading edge into the wind creates a sudden cut in the air. Because the chord is positioned in the back, this means that the leading edge would be more flexible and create turbulent air due to said leading edge “fluttering” in the force of the wind. Also, in hydrodynamics water sticks to convex surfaces, this would mean that rather than “releasing” off the back of the fin, the water would be trying to grab at the thicker section.
I’m pretty sure that this is why a surfboard naturally wants to turn around when riding backwards. I’m sure you could overcome the tendancy for the board to want to spin by placing the fins at the tail, but i still think it will create more turbulent water than you want… not to mention it would make the leading edge the sharpest surface of the fin. You might as well surf around with samurai swords on your board
to clarify I meant just moving the thick point back. The two previous responses seemed appropriate to answer, thanks gents.
Me too surffoils. After riding the same board with and without properly foiled and sized fins I have come to realize the importance of fin design; and its also the easiest and cheapest aspect of board design to tinker with and get dramatic result variations.
While we’re on the topic of things being reversed, what about fins that are swept forward instead of backward? Like if you put you fins in with the leading edge facing the tail, trailing edge facing the nose. Foiled normally though, wide part in the front, thin and sharp in the back.
While we’re on the topic of things being reversed, what about fins that are swept forward instead of backward? Like if you put you fins in with the leading edge facing the tail, trailing edge facing the nose. Foiled normally though, wide part in the front, thin and sharp in the back.
interesting thought zoidberg… I’m not sure if that would work aith todays modern, agressively swept shortboard fins it wouldnt work because the swept point adds flex just like the thin part of the fin does… that said though, People like Takayama and other longboard legends have been using “Reverse D” keels for years with great result
If we’re talking about real swept forward fins. What will happen is a small deviation in angle of attack will create a lift force that will increase the angle of attack even more due to flexing. This will create a big torsional force on the fin.
It is also likely that the fin will start stalling and suddenly lose lift, at that moment the fin will spring back, the flow will reattach and the story will start all over again. This results in a vibrating fin.
Depending on the dampening of the material, the fin can even switch to the other side and start generating lift in the other direction, resulting in a greater amplitude of vibrations.
backward swept fins do the opposite and can postpone stalling by flexing and lowering the ange of attack.
This is a theoretical thought experiment. I never tried foward swept fins, and I probably never will. But hopefully somebody else tried it and can share his findings.
I’m talking like this. But of course, that’s a fighter jet and we’re riding surfboards, so the conditions are a bit different. They also use the trapazoidal looking fins, which I think people on here have tried. Did they offer less hold or something? Cause if so, I was thinking if you used one in the middle of a twin and trailer you’d get more twin fin slidey feel, but maybe keep the drive and projection boost from using any center fin. Or is rake needed for that? Or maybe I’ve seen parallelogram fins on here before. I don’t remember. One of these two.
I wonder if I got the biggest cheap plastic fin for futures I could find and cut it up to be forward swept. It would be really little and have to be re foiled a bit.
Swept forward fins are going to be problematic both from safety and structural reasons.
Since fins are generally not safe I will ignore that first concern.
The structural problem with forward sweep is something called 'aeroelastic divergence' or 'hydroelastic divergence' in the case of surf fins.
This phenomena occurs when lift at the fin causes the tip to move sideways and then twist so as to increase its angle of attack even further. The consequence is fluttering vibration of the fin from side to side, followed by eventual catastrophic failure. Otherwise known as a snapped fin.
The research aircraft shown above, was designed with a carefully engineered composite wing that resisted the twisting caused by flying with a forward swept wing.
Used one of these for a time back in the 80’s on Oahu. Worked well, but if the leash got underneath the board, it would get caught against the base of the fin and cause really strange, hyrdrofoil type effects. Anyway, forward swept, but still some rake towards the tail. Bobby Owens’ brainchild as the story was told to me.
Theres a fin thread deep in sways with some anecotal stories about a guy on Hawaii (i think) that used fins patterned exactly like those forward swept wings shown on the french jet earlier in this thread. Even has a picture of th fin. Hell, I even made a template and foiled a plywood mockup, but never glassed it on. The original guy used it in a thruster setup. If I remember correctly, the anecdote had them going great and the guy rode them for years. I’ll see if I can findd the thread or pic again
