This picture was sent to me by a surfer curious about fins, it was scanned from a magazine. He made the astute point that you can see the wash from all three fins in the picture. I roughly circled them on the left image, the right is an identical copy without my marks.
The inside rail fin is for all intensive purposes operating in nearly complete laminar flow. At this resolution you cannot even see much evidence of the eddies that flow around the tip, or the low pressure vortex that raked low aspect ratio fins generate.
The rear fin is somewhat past its stall angle. The flow is noticeably turbulent, it is probably not far from stall. This was one of my theories (although I am SURE someone else had it first), that the rear fin entered an angle of attack that was in the lower end of the stall range to keep the nose straight enough. It is a stabilized to prevent the board from spinning out, and to keep the rail fin in a powerful “lifting” range of angles of attack. The stall causes drag in the direction of flow. So, basically, if the rail fin generates too much lift, the nose will turn, and the angle of attack on the rear fin will increase. There will be more drag at the rear fin, which will prevent the nose from turning. It is a feedback system designed to keep the rail fin operating in a stable, powerful, zone. It is the essence of a thruster.
The outside rail fin is useless (until, of course, you turn the other way). It is way way past its stall angle. The high pressure side of the flow is ON THE OUTSIDE OF THE OUTSIDE RAIL FIN (note the cavitation is on the inside), so it is operating at a negative angle of attack relative to its foiling. And even though the rider is tubed at one of the steepest waves on the planet, the outside rail fin is still in the water (barely).
I hadn’t really thought about how the rear stabilizer would turn side-on and engender a low pocket on the non-flow side such that there would be such powerful drag it would tend to keep the inside fin AOA operative.
Is that called a “stall” when a stabilizing fin, dual-foiled, is turned side-on to flow? I thought that term was about a phenomenon of proper foils, asymmetrical ones. I dunno, just seems like a different thing. The stabilizing fin is still, as you say, functioning, albeit radically, whereas when it’s a rail fin or an airplane wing, a “stall” is a loss of function due to the same separation. I guess you could argue that the wing is still operational in a purely Newtonian sense though.
Good brain candy, DB
Not you, but a lot of people know a little Bernoulli and forget the rail. Worth noting that the main drive/hold element is the hull. Newtonian hold/lift/drive.
I just realized that’s what “drive” is. A family of Newtonian phenomena affecting the hull and fins, lift/control surfaces all.
It is a stabilized to prevent the board from spinning out, and to keep the rail fin in a powerful “lifting” range of angles of attack. The stall causes drag in the direction of flow. So, basically, if the rail fin generates too much lift, the nose will turn, and the angle of attack on the rear fin will increase. There will be more drag at the rear fin, which will prevent the nose from turning. It is a feedback system designed to keep the rail fin operating in a stable, powerful, zone. It is the essence of a thruster.
On the inside rail just forward of the fin you can see the flow is most pronounced. Without looking at the rail contour, it appears to be the point where the rail transitions from a hard down edge to a softer rail forward. But, I also believe this area shows evidence of the low pressure area outside of the fin. It’s this combination of contours that makes it work.
The trailing fin is acting as a stabilizer. But, you can get stabilization through longer fin profiles as well.
And I agree at this point the outside fin is just going along for the ride.
What if that outside fin angle was changed specifically for left hand barrels… or if the outside fin was simply omitted… or if it was removed and reglassed just inside the visible line that shows the board penetrating the water? Any ideas? Someone try it, film it, and let us know how she goes!
Carl Ekstrom has made a bunch of asymetrical boards like that. In all kinds of variations.
Personally, I like to think I don’t favor my front or backside. I want my board to work the same in both directions. I love going frontside. I love going backside. I love waves that change direction for a jacking reform. Give me symetry and I’ll adjust my body english to suit the direction I want to go.
Maybe blakestah’s trucks are the answer… if they can adjust on the fly, and find the right angle, they will create less turbulance when you’re not looking for them to bite.
How do you think SurfTrux would handle the Chopes power? Okay, we know that it’s your original product, so answer as if we were your children risking our lives on your handiwork, not potential clients.
This is slightly off topic, but I just wanted to know if I’ve got this right. In the images below, I describe how a single-fin board turns. Here’s the first image:
The red dot is the center of gravity of the surfer as he ‘pushes down’ on the board, the blue line is the ‘waterline’ of the oncoming water on the bottom of the board. The images are all from a ‘bird’s eye’ view.
In the first image, board and rider are going straight into the water flow. I do realize that I am simplifying the situation by not taking into account the slope of the wave, but bear with me…
In the second image, the rider has leaned his/her weight left, casuing a CG change. The third image shows the CG change (downward in the image) affecting the board by sinking the left rail. As the left rail sinks, more left rail/outline is in the water than right, as can be seen by the change in the blue line. This means that the nose will yaw (point) right before the board turns left, is that correct? I am assuming so, because look at the next image:
as the nose yaws to the right, the fin assumes an angle of attack, causing the resulting lift and drag forces. The lift force makes the fin act as a rudder, yawing the nose left, in the direction of the intended turn. The board will keep turning as long as it has forward propulsion (provided by the wave) and the CG is is still not centered over the stringer.
Is this a decent analysis of what is happening? If so, I’ll try my hand at a few similar diagrams will try to describe what is happening with a thruster arrangement. If not, back to the drawing board…
Although if this is correct or close, I can see why a curvier outline turns much easier than a more parallel one… It makes the fin engage at a higher AOA (Angle of Attack) for the given amount of CG shift, everything else being equal…
How do you think SurfTrux would handle the Chopes power? Okay, we know that it’s your original product, so answer as if we were your children risking our lives on your handiwork, not potential clients.
There does not seem to be any difference in hold between SurfTrux and a thruster on the same board. I had it in two boards, a Flyer (small wave board) and a Patagonia epoxy rocket-something-or-other, a board with a narrower tail. The Patagonia board held a lot better, but it had thinner rails and a narrower tail and more rocker ie: those seem to impact hold more than the difference between SurfTrux and a thruster.
Bud’s SurfTrux board was ridden at Backdoor by his friend, and he said hold on steep faces was no problem.
In fact, no one has ever had an issue with hold being particularly different - either better or worse.
What if that outside fin angle was changed specifically for left hand barrels... or if the outside fin was simply omitted... or if it was removed and reglassed just inside the visible line that shows the board penetrating the water? Any ideas? Someone try it, film it, and let us know how she goes!
Very roughly speaking, the water flows are closer to being perpendicular to your blue lines. Way back I posted some test data from a test I conducted using a clear sheet board model and attached streamers. $5 test.
Cant say much about your other thoughts cuz the basics threw me off.
I commented cuz you took the time to make diagrams, and thats worthy of a response. HTH.
The railsaver is visible. But so is the turbulence, here is the full resolution closeup…
The other cool thing, if you look REAL close, is that the inside rail fin re-directs the water towards the tail ONLY until it crosses the rail (compare the angle from the rear of the fin base to the wash at the rail, and the angle of the wash off the rail). This is why if you mount the inside rail fin too far out, you lose drive.
Now, my question is, as is in the drawing, if the fin lift force is working to ‘straighten’ the board out (making the fin go parallel to water flow again), where doe the force come from that overpowers it and keeps moving the nose left? I’m sure this is right in front of my face, I just don’t see it…
Does the bottom of the surfboard provide this yaw force, with the angle it makes with the oncoming water flow? If so, is the fin just there to stabilize, and not help in the actual turning of the nose, but to keep the bottom/hull from ‘sliding/slipping’?
Thanks for your help, I’ve thought about how this works on and off for a while, but never quite got my head around it…
After thinking about what you said, I think this drawing sums it up better:
Since more rail line is in the water on the Left side, there is more drag there (to say nothing of the rail being more submerged, making it much more so). This force is the dominant one as far as rotation is concerned, and therefore the fin’s contribution to the overall rotation of the board is not the dominant factor (unless the fin is HUGE, it now makes sense how you can ‘overfin’ a board; it won’t want to turn).
