Who dares to suggest a reason for this?

[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]

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]

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]

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:
[ame="http://www.youtube.com/watch?v=4Ddjgj6h-qs"]YouTube - free style kicking[/ame]

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.

​

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.

​

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.

 

[Bill]

Trux
Great diagrams. I learned two things about the Aqueon design. One has bothered me for decades and the other I missed. More respect for the designer/engineer all the time. Can't wait to see what an 80 meter monofin diver does with his, when it arrives in Kona.

 

[ADR]

... One has bothered me for decades and the other I missed.....

Hi Bill,

You can't leave us guessing, please spill the beans on both of those.

 

[Bill]

Hi Andy
Nothing new for you, just has to do with the way the foil works and why the bungee was so effective. In my mind I always wanted to move the pivot of the foil to the front and make it more like a fin. The bungee seemed out of place compared to the quality of the rest of the machine. Trux' diagrams let me get the visualization right. It's fun to put something new in this old RAM. Sort of like the new word I learned a few weeks ago, whinge.
Soon we'll get to see if the memory from 1971 matches the reality of '08.

 

[ADR]

Hi Bill,

We need to get you or make you another Aqueon, as you speak of it as you would a long lost brother. It doesn't seem right that you no longer have one. If I make one at some point I think I'll make one for you too.

If the angle of attack on the Aqueon was maintained by the bungee then I'm stumped a little by the Lunocet. From the pic and movement in the video I can't work out if it is a simple forward pivot with a stop point or if there is some internal spring system on the shaft of the pivot.

 

[trux]

If the angle of attack on the Aqueon was maintained by the bungee then I'm stumped a little by the Lunocet. From the pic and movement in the video I can't work out if it is a simple forward pivot with a stop point of if there is some internal spring system on the shaft of the pivot.

I am curious about it too, but assuming from the photos available, I do not think there is any such mechanism. To me it looks more like you control the blades by the sideway inclination of your feet. I may be wrong though.

​

 

[Bill]

The way it works IMHO. It uses the flat plate in the centre (with lots of leverage) to change the angle of the foils. Like a boost tab on an an aileron or elevator.

 

[Bill]

By the way Andy, I think your holes are a good idea to eliminate the part of the fin that produces thrust in the opposite direction. Not needed on the 'up' stroke because the ankles can easily adjust, so just have a one-way panel that opens on the down stroke. Even better have it partially open so that it acts like a slot on a wing or a jib on a sailboat.
Very limited improvement though, that part operates in 'dirty' water.

 

[HJ]

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.

Sorry, trux. I thought you said right angle (90degrees).

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.

A flat or non-flexible blade or foot will deliver thrust through the arc of movement as it approaches the 0 angle, just like your first diagram shows. FYI: I just came back from the pool. I timed a barefoot 25m with dolphin kick. I then put on a pair of very short, very stiff fins (the kind used for swimming training). I reduced my time by 15%. Then I put my foot against the fin and I observed that the fin increased my surface area by about 15%! (just guessing)

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.

One has thrust force throughout the arc moving toward 0 angle, but it diminishes as your approach 0 angle, as you show. I'm not sure about the resting part. Maybe mono kickers can do this because the kick tempo is slower than for swimmers. When I dolphin kick with fins I believe I am doing constant motion, although I am not positive about that.

Unlike that, at flexible fins, or the Aqueon, you transform the kicking energy into the forward motion for much longer parts of the cycle.

Yes, I think you're right about this.

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.

I like it.

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. 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.

Yes. This makes sense. The only thing I would add is that the upward kick is much weaker (and forces generated) because of the anatomy of the human body.

Good job!

 

[ADR]

The way it works IMHO. It uses the flat plate in the centre (with lots of leverage) to change the angle of the foils. Like a boost tab on an an aileron or elevator.

I think you are right as the fin also does the impossible and flexes in a concave fashion when the load comes on. This would support what you are saying.

What did you think of my theory for v-bend in the other monofin thread?

 

[trux]

What did you think of my theory for v-bend in the other monofin thread?

Not sure if you ask me or Bill, and not sure if you mean this thread: http://forums.deeperblue.net/monofins/70457-waterway-glide-ld-long-distance-monofin-review.html, but if so, then unlike at Lunocet, that monofin bends into the V in the wrong way, oppositely (unless I interpret the Glide photo incorrectly).

Lunocet creates a concave V, the Glide monofin a convex V. While the active concave profile of Lunocet prevents the water from escaping sideways, and pushing it backward instead (using so more energy for the propulsion), the passive convex profile of Glide does the exact opposite and pushes even more water sideways than a standard monofin (which is already quite bad from this point of view too) and wastes a lot of energy unnecessarily.

There is another passive way creating a concave V profile at a blade that I would love to see at a monofin. It is used at many modern scuba bi-fins (for example Mares Volo) - they have reinforced sides and a soft central part (or even a slit at split fins). It then creates the concave V passively while you are kicking and helping the propulsion considerably. It just needs to be put into the right dimensions and stiffness for a monofin, but I am persuaded it would work at the monofin as well as at bi-fins. I already created a thread about it - the thread http://forums.deeperblue.net/monofi...tfin-monofin.html?highlight=split+fin+monofin from 2005 was actually my very first post at DB