Who dares to suggest a reason for this?

[trux]

Originally Posted by harald johnson

Hey, Trux... talk to me before you add this photo to your collection; I'd like to be able to give you the correct information. Thanks.

Send me please a PM, or email to ivo at apnea.cz

Originally Posted by harald johnson

Cool. What club is this? And what university? Where?

The club is Cyrnéa Lyon, and the University is Université Claude-Bernard Lyon 1, Laboratoire de Méchanique des Fluids et d'Acoustique

[trux]

BTW, Andy, can you tell me where you found the photos (the ones from the first post)? I'll add them to my collection too, but would like to credit the original source, if possible.

[HJ]

Originally Posted by Bill

The very simple reason is speed (power to drag). Sailfish and Wahoo are usually considered the fastest fish and at 30-40 knots they have a P/D ratio about ten times the best human. If you'd really like to feel puny calculate the speed required to lift the centre of gravity 3-4 meters out of the water in a dolphin show and then consider that no human has been able to get to 1M or even clear of the water.

Hey Bill... I think what you're saying doesn't rule out what I'm saying. Speed is Distance over Time. Power is Work over Time. We can increase P/D by training (for power) and reducing drag (by design).

Imagine the the most extreme High-Aspect-Ratio (HAR) monofin: wide span, little chord (front-to-back). SEE DRAWING. Would it work? I don't know, but take a look at Cal Gongwer's invention which uses similar "wings" but in a very different way:
http://fksa.org/showthread.php?t=4372

Hey Dave: I just realized that I have video on my Olympus 770SW waterproof digital camera. Maybe I'll make a stiff HAR fin and video it some day! Fun discussion, guys. -- HJ

[trux]

That's entirely different case. At Aqueon, the mechanism assures the ideal angle of attack of the hydrofoil during almost the entire kick (both upward and downward) - that's achieved with the variable angle of the hydrofoil. If you fix it on the feet like in your picture, you will just move water up and down, and only a very small part of the motion will be transformed into the forward motion. Well, it may work somehow better if you manage to keep the ideal angle of attack with your feet, but I am afraid it won't be very easy.

[Bill]

I tried extreme aspect ratio in some indoor model airplanes and in air the problems that it caused out weighed any advantage. In water where weight and stiffness are easy to manage it may be worth it. Move the foil about half a meter back and use Congwer's design for support and angle of attack and you'll have the fin that I've wanted to try since I lost the Aqueon.

[HJ]

more thinking on a Sunday morning (U.S., GMT -5)...

if a fin is a foil attached to the foot, basically extending the leg, then why wouldn't the same physical principles apply as if you had a big foot? i.e., lift and drag. The angle of attack relates to the angle of the leg/foot/fin as it moves through the water, no?

See drawings for two extreme aspects and a couple of in-betweens... -- HJ

[trux]

You simply need a variable angle between the foil and the feet, to keep the ideal angle of attack constant (in relation to the forward direction) during the entire kicking phase. You will not manage it with a fixed foil/blade attachment as at the fin in your photo. You need a mechanism like at the Aqueon, adjusting the feet-hydrofoil angle during the kicking phase so, that the relative angle to the forward direction is ideal (probably somewhere close to 45 degrees), all the time, except of the two dead points at the top and at the bottom of the kicking wave.

[Kars]

So the aerofoil needs to rol to a certain degree up and down? Like a tailwing on a fighterplane that can be made. Some springs, or flexible material will provide a gradual adjustment in relation to the power applied.

The counterbalast aerofoil is still looking rather unnatural to my eyes, but maybe a diver can benefit from having an aerofoil in his stretched out arms?
Jan Ploeg English Version

From Jan, I learned that monofinswimmers gain much speed from swimming with a waterwing. I think it's forbidden to use in dynamic competition freediving, not explicitly by the aida rules but indirectly the organisors apparently may ban anything they think is not fair.

Kars

[apneaboy]

The fin is a hybrid that i put together from my Waterway glide footpockets and a standard blade from a waterway model 1 (medium stiffness)

Andy how well did this hybrid fin work by the way. Was it better or worse than the two different fins used to make it?

[HJ]

Originally Posted by trux

You simply need a variable angle between the foil and the feet, to keep the ideal angle of attack constant (in relation to the forward direction) during the entire kicking phase. You will not manage it with a fixed foil/blade attachment as at the fin in your photo. You need a mechanism like at the Aqueon, adjusting the feet-hydrofoil angle during the kicking phase so, that the relative angle to the forward direction is ideal (probably somewhere close to 45 degrees), all the time, except of the two dead points at the top and at the bottom of the kicking wave.

Good explanation. But here's another way of looking at it: the foot *is* the foil or fin. Here's an example from swimming (surface). The swimmers with the best kicks are able to point their toes on the down-kick so that the tops of their feet and toes form a line with the shin bone (I have a great picture of this somewhere). As this leg-foot unit is driven down, the angle of attack changes and lift forces are generated that move the body forward. (not as much as the arms, which are sculling their way through the water like propellers, but that's another story) The greater the ankle flexibility and the bigger the foot, the more powerful the swimmer's kick. Recall the swimmer's short fins called "Zoomers"? I'm looking at one right now. All it does is add surface area to the foot. It creates a bigger foot. Most people with Zoomers (or other very short, stiff, stubby fins) can kick slightly better (with more thrust) than they can without them. Now take it another level and increase the foot's surface area again. Double the size. Then triple. Each time, you will get more thrust, but each time, it will cost more in energy. At some point, it's not worth it. (this is similar to the reason long-distance swimmers don't usually use a 6-beat kick; it draws too much energy out of the system; I use a modified 4-beat kick or a 2-beat if going long)

So I'm pretty sure that some kind of stiff monofin securely attached to the foot will provide propulsion. Is this the best solution for freedivers? Probably not. And I'm not sure about the foot-fin attachment; I think that needs to be solid/secure. You need power applied directly to the foil/fin that's attacking the water in my view.

[trux]

Originally Posted by harald johnson

Here's an example from swimming (surface). The swimmers with the best kicks are able to point their toes on the down-kick so that the tops of their feet and toes form a line with the shin bone (I have a great picture of this somewhere). As this leg-foot unit is driven down, the angle of attack changes and lift forces are generated that move the body forward.

I am afraid you still do not understand what I mean with the angle of attack - it is the angle between the blade or hydrofoil (or the foot in the case of a swimmer without fins) and the line of swimming direction (more or less the prolongation of the body). It is not the angle between the shin and the feet (or blade). And since the shin continually changes the angle to the swimming direction, during the kick cycle, the angle of the blade (or feet) needs to change equally in the opposite direction equally, to compensate for the change and to keep the angle of attack constant. In the case of no-fins kick, or a non-flexible blade fixed to the feet, it is simply anatomically impossible even for the most skilled swimmer to keep always the feet in the right angle to the swimming direction. It is in the right angle for only short part of the cycle - it is when it actually provides some thrusts. For the remaining time, the energy is wasted into moving the water mostly only upward or downward.

So again, having a non-flexible blade, with a fixed angle to the feet (or to the shin), regardless of the blade type, size, or shape, very simply cannot provide sufficient thrust, because it will be in the ideal angle of attack always only during a small part of the kicking cycle.

[HJ]

Originally Posted by trux

I am afraid you still do not understand what I mean with the angle of attack - it is the angle between the blade or hydrofoil (or the foot in the case of a swimmer without fins) and the line of swimming direction (more or less the prolongation of the body). It is not the angle between the shin and the feet (or blade). And since the shin continually changes the angle to the swimming direction, during the kick cycle, the angle of the blade (or feet) needs to change equally in the opposite direction equally, to compensate for the change and to keep the angle of attack constant. In the case of no-fins kick, or a non-flexible blade fixed to the feet, it is simply anatomically impossible even for the most skilled swimmer to keep always the feet in the right angle to the swimming direction. It is in the right angle for only short part of the cycle - it is when it actually provides some thrusts. For the remaining time, the energy is wasted into moving the water mostly only upward or downward.

Yes, we are talking about the same thing. It is the angle between the foil/wing/fin/sail and direction of travel. What we disagree on is this: the object (shin, foot, fin) is moving through a range of motion except when it stops and goes the other direction. Throughtout this range of motion, LIFT and DRAG forces are being created. There is one sweet spot where the lift forces are at maximum, and I guarantee you that it is not at 90-degrees or even close to that. The goal is never to "keep always the feet in the right angle to the swimming direction", or you would go nowhere. I have seen this over and over with poor swimmers who have no ankle flexibility (many triathletes or runners, by the way). All swimmers know (or must learn) that they must kick within a shallow "arc" or range of motion, maybe around 25-degrees from the direction-of-travel line (on each side) and with pointed feet (on the downstroke; the feet relax a bit to recover). Otherwise, there is too much drag created and not enough lift. But throughout a correct range of motion, there is always lift and there is always drag, but the resultant vector lift forces are what count and keep you moving forward. Here is a good swimming kick:

YouTube - free style kicking

So again, having a non-flexible blade, with a fixed angle to the feet (or to the shin), regardless of the blade type, size, or shape, very simply cannot provide sufficient thrust, because it will be in the ideal angle of attack always only during a small part of the kicking cycle.

I have already proven the opposite many times, although I guess it depends on your definition of "sufficient." If the foot (or foot+fin) is bigger, you will generate more thrust, but at some point, it will not be worth the drain in energy. Assuming that you are kicking correctly.

Again, I am not an engineer or physicist, just a layman, but I have these views from direct observation and a bit of reading. Don't forget: Bernoulli's Principle and Klaatu Barada Nikto!

[trux]

Nobody told the angle is 90 degrees. The angle is more like 45 degrees, but may be different - that also depends on the speed and the profile of the hydrofoil (if profiled hydrofoil is used), and some other factors.

However, I have to repeat again, with plain feet, or with a short non-flexible blade fixed to the feet, you will simply not manage to set the blade/hydrofoil/feet into the right angle during the entire cycle (minus the dead points), while you do manage keeping good attack angle for considerable parts of the cycle with a flexible blade, or with a hydrofoil with a variable angle, like at the Aqueon.

So, of course, you can propulse even with the non-flexible fin, but you need to exercise full force in the moment the blade is in the best attack angle, and then relax for the rest of the cycle. Unlike that, at flexible fins, or the Aqueon, you transform the kicking energy into the forward motion for much longer parts of the cycle.

To make it more clear I designed the following (very) simplified diagrams. They both represent only one half of the cycle (the downward kick). The real-life situation is little bit more complex (beside others, to certain limited degree, you improve the angle of attack with your feet and knee bends), but for understanding the principle the diagram is more comprehensible in this way.


The first one if for a fix blade/hydrofoil - the blade is in the right attack angle only in one part of the top half of the kick. Only there it transforms the kick energy into some thrust, but the transformation is the lower, the further down you kick. At the bottom you actually brake. I designed only the more interesting vertical and horizontal resulting forces, but correctly the force has both vertical and horizontal elements at each point.

fix-angle.gif

There could be a second diagram for the upward kick that would look practically identical, just the smaller green part (thrusts) would be at the bottom.


The second diagram shows a system with a variable angle of the blade or hydrofoil (meant variable to the leg). It stays in the ideal angle of attack during almost the entire half cycle. There would be an inefficient part only at the dead end, during reversing the kick from down to up, when the hydrofoil flips to the opposite direction, taking again the ideal angle of attack for the upward kick. But the entire upward kick would be fully efficient (green) again.

variable-angle.gif

At a profiled hydrofoil, the situation is little bit different than at a flat blade. While hydrofoil may work also with a fixed angle (at high speeds and a very low kick amplitude), you need to maintain the attacking blade side under the right angle as much as possible during the cycle, to keep the efficiency high. You can achieve it either by limiting the kick amplitude or prolongating the lever (the angle at the extremity of the kick must be smaller than the angle beween hydrofoil sides), or you need to add a variable angle similarly like at the example above.

[trux]

If you are interested in the physics / mechanics of hydrofoil / fin propulsion, you will find some interesting analysis, math, and diagrams for example at this website: Isentropics.org

Specifically, you can have a look at the analysis of dolphin propulsion: Mouvement d'une nageoire caudale de cétacé et efforts résultants

[Bill]

Harald
Thanks for the video. Now I understand how some people can make a non-angled blade work. A coach told me a long time ago to only kick for balance. You 'showed' me what he was talking about. It's not just ankle flexibility, that swimmer's shin bone has a different angle than mine.