# Blakestah's rotating fins

I took a look at Blakestah’s site and the picture of his “rotating fins” and I’m wondering what the design principle is. If a fin reacts to a force applied to it by the water, wouldn’t the reaction be to move back into equilibrium with the force? I haven’t really given much thought to fin design, but is it like handle bars on a bike? If I could somehow cause my fins to rotate while turning, wouldn’t I want them to rotate into the turn? If I turn my bike handle bars into a turn, I turn harder. If I turn the handle bars in the opposite direction of my current turn, without redistributing my weight, it leads to disaster. I envision a system where a fin would rotate towards the rail onto which a turn is being directed. In other words, I’m riding a right, I do a bottom turn, and the inside fin rotates towards the wave, effectively cutting the turning radius of the turn. This would produce more force on the fin, requiring some sort of active system for producing the turn…right?

good question…I am not sure but I think it works more like a firemans ladder truck with the dude in the back turning the back end with a steering wheel in the opposit direction to get the back end of the long truck to make a sharper and more controlled turn. maybe you should try riding your bike backwards. seems like it could work on a surfboard… or is it just another gimmick like the turbo tunnel fin? The rotating fin system claims: to “make surfing easy. That’s all there is to it. It is easy to paddle. It planes early and easily, so it is a wave hog machine.” Wow! now that is amazing…

The principle is that the fin rotates to prevent cavitation/stalling behavior. It effectively lets you use the entire range of turning angles that you would ever use on a shortboard, with one fin, without ever stalling. As you lean into a turn, the single fin approximates the angle of the inside rail fin.

There are a few side effects of this.

1. Turns are smoothly graded. The fin grabs hold more and more as you turn more and more. Unlike boards with toed-in rail fins, you do not need to move it far enough to engage it. Instead of jerking it rail to rail, you can lean smoothly. This also make the system attractive to beginners, because learning to turn a thruster is challenging for a lot of people. Learning to turn a RFS system is comparatively very very easy.

2. Minimal water resistance. It has noticeably less paddling and planing resistance than a three fin board. The retro-fit experiment has been done several times, and the effects are large enough to be noticeable by anyone, easily.

Compared to a classic single fin, you get rid of stalling behavior, and allow turning radii close to a thruster of the same length. Because the fin can turn, you do not use a fin with a pronounced horizontal ‘flex’ tip. Instead, a fin that is highly optimized in its drag/lift ratio is used.

The ‘trim’ behavior also gets worse as the turning radii decrease, so I like to choose a middle ground where I trim much better than a thruster and turn much better than a single. The same is true of hulls, we run slightly flatter rear rocker than thrusters of the same length, and slightly more rockered than a single. Thrusters are made to go rail to rail, to always turn, and they do it well. The RFS allows a board to go straight well, and also to turn well. A fin system should not punish its rider for wanting to go straight.

Honestly, I doubt it is going to supplant the thruster in competition surfing because of inherent limitations in single fins and wide tails. When board lengths get out to about 6’6", its about even with a thruster for turning (for riders about my weight, 150 lbs). And for longer boards it turns very well. Its just not so easy to take a longer board rail to rail, with the RFS it’s made a lot easier.

I’ve spent a lot of the last six months putting people on demo boards and watching their reactions. People getting into surfing, in their first year on a thruster, prefer the RFS 10 to 1 over a thruster, maybe more. Accomplished surfers usually, but not always, like it, but rarely go to great efforts to get one in the short term. However, they are also buying them after test riding, although at a slower rate than the less experienced surfers.

Round and round and round we go, where we stop…

Peter,

I have never tried the rotating fin system but I got to check it out last spring. Shipman has it right on the fire truck idea and I think that fins might be able to operate like handle bars on a bike.

You wrote “If I turn my bike handle bars into a turn, I turn harder. If I turn the handle bars in the opposite direction of my current turn, without redistributing my weight, it leads to disaster.”

This is not true. The harder you turn your bars into a turn the more your bike will stand up and want to go straight. It is hard to feel on a bike but it is painfully obvious when you are on a motorcycle. They call it counter steering… for instance if I was approaching a left hand curve I would actually pull with my right hand and push with my left and the bike will lean and turn natuarlly to the left. On your bike ride with no hands, lean into a turn and check out which way your bars turn.

On a bike, the steering wheel, in front, turns to point in the direction you want to go. The axis of rotation (sighting down the head tube) is in front of the point of contact of the wheel on the ground. This distance is called trail, and it is a stability criteria. With trail, you can ride a bike no hands, and just lean side to side, and the movement of the front wheel complements your leaning. With no trail (if you bent the front fork forwards about 2 inches), you could not ride a bike “no hands”.

However, the front wheel on a bike is in front of the rider’s center of mass.

In surfing, the steering is opposite. Steering elements for stand-up surfing are behind the center of mass. For the rotating fin the center of resistance of the fin has to be behind the axis of rotation so the fin points away from the direction you want to turn.

It is very loosely analogous to the steering of a forklift. The wheels point right, and the front end moves left.

I’ve had some very experienced surfers tell me that the fin ought to turn the other way. However, there are no special instructions required to ride it. You just lean the way you want to turn, and the board turns.

The bike analogy is not a good one to use because of the gyroscopic forces at work. Anyone who’s taken a motocycle riding course will tell you that to initiate a turn you actually steer in the oposite direction you want to go. This is known as counter-steering. Push left, go left, push right, go right. It seems counter intuitive but due to gyroscopic forces it works. Next time you’re riding you bike down to the beach try it. Get going fairly quick, say 15 or so, and push lightly with your left hand on the handlebar. Which way does your bike want to turn. The slower you go the less effect this has, so it’s not as noticable on bicycles as it is on motorcycles but it’s still valid. The point being sometimes what seems counter-intuitive is actually based on physics and Newton’s first law of motion.

I tried it in a longboard and could feel the effect. Similar to a flex fin in that there was some “give” to it. When pushed to rotation, it felt like the fin steered me through turns and cutbacks. I liked it enough to harvest the system from my broken board and will try it on another.

Re: the system itself - it is more complicated than a standard long box. Adjusting the fin fore and aft is not something you can do in the water. Switching out fins is more difficult as well. It is a well thought out and well executed design that works as planned.

Quote:

The bike analogy is not a good one to use because of the gyroscopic forces at work. Anyone who’s taken a motocycle riding course will tell you that to initiate a turn you actually steer in the oposite direction you want to go. This is known as counter-steering. Push left, go left, push right, go right. It seems counter intuitive but due to gyroscopic forces it works. Next time you’re riding you bike down to the beach try it. Get going fairly quick, say 15 or so, and push lightly with your left hand on the handlebar. Which way does your bike want to turn. The slower you go the less effect this has, so it’s not as noticable on bicycles as it is on motorcycles but it’s still valid. The point being sometimes what seems counter-intuitive is actually based on physics and Newton’s first law of motion.

This same “initial turn left to ultimately turn right” (and vice-versa–and keeping upright on the bike) is required independent of whether there is any gyroscopic action (although the latter also enters the mix, if present). It’s just a matter of creating (to initiate, or terminate, the turn) and eliminating (to maintain a constant turn, or no turn at all) the torque generated by a lateral displacement between the combined force of gravity and the centrifugal force acting on the center of mass of the bike and rider, and the combination of the vertical and lateral forces of the ground on the front (and rear) wheel(s). The magnitude of this displacement in the centers of effort is related to the (sine of the) bank angle of the bike, and the distance from the bottom of the wheels to the center of mass of the rider (and bike). Note also that once in a steady turn to the right, the turn is terminated by initially turning farther to the right, and then backing off on the rotation of the handlebars to less than during the turn until the bike is back upright.

The HYPO (HYdrofoil PaipO) board turns same fashion (i.e. analogous to a bicycle)…and there are no rotating parts like a wheel. (Laird’s Air Board, is like riding a unicycle–it must be balanced fore-and-aft, as well as from side-to-side).

MT

Sorry to necrobump an old thread but it seems best to have all the info in one place and this is what a google search comes up with. Here’s a file with an unfinished nasca profile with the blakestah fin base:

I measured it as a 37mm wide, 6mm thick, 12mm deep tab on the base.

Now here’s a .svg you might use as a start to make a lasercutting:

And a base only for 3D printing to check if it fits first, before printing a whole fin:

And now a fully parametric .step file so you can fully edit and thus regenerate the files above:

Reason for all this is that Rainbow don’t seem to have any old stock, the guy who used to manage them isn’t there anymore and they couldn’t find the template. Though Blakestah is a super nice guy so all that wouldn’t be a problem if you need some and they’re an easy base to make, nothing like horrible FCS2.

bought a set many year ago
seems to just smooth the turns out
smooth transitions is what I felt

Firewire made a weird pivoting center fin that I have
that one is a little more clunky
4wsf did something similar as well
allot of guys used those fins

larry, blakestah and spindler were fin geniuses here
robin’s hanalei fins are cutting edge as well

A/J/K-
I tried 3 different softwares (Fusion360, Inventor, and FreeCAD) and I could not get the STEP data to read in parametrically. What did you use for design and output?
What is your target foil? Imported from .dat information or drawn by hand?
It is very clean data and easy enough to rebuild by creating new sketches and projecting the geometry and redoing the extrudes or lofts.
If you do the foil as a loft you can start to taper or curve it closer to a fin shape.
The STL is nice too. The SVG suggests the tab but is not as pristine as the others.

confounded doppelganger…

Don’t know if it would help - I have a pivoting single fin and a thruster set. If you think you could use to scan or whatever, let me know. I could mail them to you but I’d like them back at some point.

Quit moaning. Different login for mobile and desktop

A-
My second reply was meant an alternative to ‘double post’ and not intended to be a commentary on anything else. I now see how it could be read differently after the salutation in the first post, sorry.

My first reply was expressing interest in the files presented and the process getting there. The asymmetric foil is quite interesting.

John-
That is a gracious offer. I am not looking as I do not have the matching boxes or frankly enough skill in the water to know the difference that a pivoting fin would make. Maybe Andy is interested.

What a guy…
You’re the man John.

To the topic… Go quad. Ha!