I was reading an old issue of Scientific American and an article about flexible wings caught my eye. These wings that are in development flex and change shape to maximize efficiency for a given condition/velocity/angle of attack. There are ones in development which are active systems and would obviously be difficult (to say the least) to practically apply to fins on a surfboard.
However, in a sidebar they highlighted something called an Ornithopter which has shearflexing wings which “flap” like a bird. Part of the problem of developing the ornithopter was designing a wing that could change its angle of attack as the it flexed so it could still generate lift.
One solution was to have a hollow airfoil with a slit cut in the trailing edge. The slit would allow the angle of attack to change as the wing moves because shearflexing allows the structure to twist around a rigid wing spar at the leading edge. I drew a crude diagram to illustrate the design.
If there were some way to anchor the rigid spar to the bottom of a board while still allowing the rest of the fin to flex, this could be an alternative method for constructing a flexi-fin, or even an alternative to Blakestah’s box installation, if someone wanted a tri-fin or twin with this passive system of flexible fins.
what if you made foam cored fins. flattened the leading edge about 3/8" of an inch in. laminated them. scraped out the foam and seperated the trailing edge. molded resin/roving mixture to form leading edge. sounds liek an expirement.
That’s how I figured the construction would go, but how to properly install the fin? It can only be attached by a relatively small area, which would lead to a relatively large amount of stress on the based compared to other fins. If there were a way around this, I think it has the potential to work very well.
That was kinda how I started. However, I rapidly discarded the idea of making a fin to work with a standard box, mainly because I wanted to be able to highly control the rotation - forces, end-limit. It’s a tough nut to crack, in my book.
Well - think about something like the fin you’re describing except with a cutaway base - like this, modified from a Fins Unlimited glass fin,
You could even use the original fin ( or another, wider fin ) as a mold for your flexing panels, sand down the fin and glass 'em on which’d give ya a nice foiled shape into the bargain - the cutaway allows for a nice sized base and you could 'tune ’ it relatively easily.
I like Blakestah's rotating fin.It makes alot of sense in that the fin stays rigid while turning. Flex fins bend and blow off speed occasionally.Is there a "steering dampner" to keep the fin from wobbling like a shopping cart wheel?Or should it be loose and pivoty? Must loosen the board up while maintaining drive. Very cool idea.How's it surf?
The force is easily changeable. It has a pre-load on the resistive forces in the center position which stops oscillations. Wobble is a significant problem without this attribute.
People generally like it quite stiff. So stiff that the fin rarely if ever touches the sides of the pie cutout - so that all the turning force is stored in the bumpers.
Different physics describe the interactions between the hull and fin when you use one fin, compared to when you use three. So, the hull is more like a singlefin hull than a tri-fin. Less rocker, wide point a little further forward. You can generate drive, turn easily, and it paddles and planes easily. Floats through whitewater sections like butter. But I encourage people to try it and make up their own minds. With a little fiddling to dial the fin in (bumper forces, mainly), people have been quite happy.
Building a fin of the configuration of the one diagramed in the initial post would be tricky for sure. You would have to carry plenty of structural integrity in the leading edge of the fin. The hollowed out part of the fin with the thin sides would be very delicate and in a bottom turn the inside surface would flex into the outside surface. It doesn’t look to me like a viable design but I’m just another guy in the line up with an opinion.
Doc, that method of anchoring the fin is about the same as what I could come up with on my own, but I don’t see a way around problems with material fatigue.
Blakestah, you also bring up good points about having controlled rigidity. I thought I was going down a different path, starting some discussion, but I see you’ve beaten down this path before and I am just headed towards your rotating fin! I emailed you some time ago with questions about the fin and will probably email you again in the next month or so to order one, whenever I get around to building my single fin. Keep up the great work!
