Please try this test on your fins!

[irishsquid]

the hockey fin is a very different fin indeed. the demo was actually done using a Hockey Fin in some really special material. At many request I was able to convince Bob to make the Hockey Fin for me, its purpose designed to excel at underwater hockey. The first version, the one that you see earlier in this thread leaping off of Bob's knee is in XXL, much too big a boot to work really well for barefoot use, but using a pair of comfort insteps in each fin I was able to play hockey quite well in them, I have since started using them for diving as they just fit my drysuit boot. I have found this first version, while requiring some tuning for hockey, turned out to be one hell of a diving fin. It is very similar to the Excellerator, but has the "whiskers" built into some very low profile tabs at the end of the blade that are very effective at stabilizing the fin despite their small size.

For frog kicking they are noticeably different than the Excellerator. While similar in its ability to perform very fine movements with absolute precision. The far end of the Hockey fin is beefed up which shifts the flex profile enough to allow an extremely powerful frog kick, my initial dive with them showed that I was kicking once for every 2 times the guy with me was (in Jets/Turtles)

Are these the ones you are talking about?

 

[TN Traveler]

Yes - that is the "Hockey" fin

 

[meesier42]

I think the Force Fins look really neat -- I'd love to try a pair sometime Among other things, I love the idea that you are powering them with leg and front of ankle and not with foot and toe (I know, you do power regular fins with your legs, but there is that pointed toe thing with the full foot vs. the open toe...)

I have a question though, and what it's probably going to do is show my complete ignorance (but that's okay - I'll learn something new. )

I was thinking about physics law of Conservation of Energy: Wouldn't that mean that if a fin is more resilient, like a Force Fin, that you have to put more energy into it in the first place? I mean, so that it can then "store" it and give it back to you in the way that a Force Fin does?

Or can the fin "give back" more than you put in? (Maybe it's not a closed system? Or... something else physic-sy?)

I imagine my first thought is not right for some reason, but I don't understand physics enough to know why it's not right.

B.

Later thought: Maybe it's something about the way or timing of how you get the energy back; so that it doesn't even matter if it's not *more* energy than you put in. Like how a snappy wrist makes a Frisbee go.

Even Later thought: But hmmm, the propeller analogy. I guess the propeller doesn't give you back more energy than your engine puts in, but rather wastes less energy than something else that you might put on the end of a shaft. So maybe that's the key: The more resilient fin wastes less energy?

Last and latest thought: I think I'm confusing two characteristics here (maybe). One being the resiliency (snappiness), and the other being the shape (impeller vs. propeller in drawing). With a propeller you can control how hard an engine works and then get either more low end torque or more high end speed, I think (but I think you have to give up some of one to get more of the other, and then there is the idea of matching the propeller to the recommended RPMs of the engine....). With an impeller... I'm not sure how that fits in.

Okay, straighten me out here! Before I think more

ok well you hit on a bunch of topics, so I will try to answer them in a way to clarify vice confuse.

Conservation of Energy (1st law of thermodynamics).
a) Force Fins have roughly equal to "most" other fins in terms of actually stiffness, for example doing a cantilever beam and hang a fixed weight from the unsupported end of the fin, both will flex roughly the same amount, so there is little additional energy imparted for this effort
b) "most" fins have no noticeable rebound due to the materials they are made from (TPU), so when you kick down on them they take the shape of least resistance and at the end of the kick simply float back to their natural shape. There is no "spring" in them that can store potential (spring) energy. With the material that Force Fins are made from as you kick the fin will still take the shape of least resistance, but at the end of the power kick it will recoil using the stored "spring" energy at a rate that will maintain pressure on the face of the blade thus continuing the power stroke for a very short period
The propeller analogy is different all together concept. With most fins, they act only to move the diver by creating a positive angle of attack and face pressure (similar to a canoe paddle). in this water will simply move from high to low pressure in the fastest way possible, meaning that you will have significant side slip of water (look at the vortices that come off your canoe paddle), which is my most fins have lateral ribbing, attempting to reduce the amount of side slip and resultant shutter the fin exhibits. For the Original Force Fin (no ribbing whatsoever) the shape of the fin (negative taper, or gets wider down the length) much of the side slip is reduced as the water tends to come off at an angle anyway and the lateral edges will tend to keep more of the water on the surface of the blade. But there is a second effect, as the blade flexes significantly (upwards of 90degrees) the path of least resistance will be more directed down the blade anyway channeling more water off the end of the blade and producing thrust.

 

[Blue Sparkle]

Thanks, meesier

I think I understand what you are saying about the rebound. So it's like there is less *lost* energy, or IOW, you get back more of what you put in. Nice.

I probably read too much into the propeller diagram, but because it looked like it was comparing propellers to impellers (such as a raw-water impeller on a cooling system), I couldn't stop thinking about it, and what it meant I vote for changing the first graphic to a canoe paddle I'm from northern Minnesota, so yep, I've watched a million of those swirls flow past the boat

Thanks again,
B.

 

[Blair Mott]

Rebound and Snap- is a Force Fin priority!


Blue Sparkle- thanks for the post! We love the canoe paddle analogy

 

[KCIREVAM]

As far as Force Fins go no other fins will stand up against them. I've used other fins and I don't get the same performance, with less stress on my legs while underwater.

 

[uncleavi]

The real test is on your body.
Sometimes I have to do 4 dives a day on a day boat and this is after prepping the boat and other little jobs. I am no spring chicken anymore so, the main advantage I have with Force Fin is that I get the power I need with out it hurting my high mileage body(a lemon).

 

[Bob Evans]

Uncleavi, Another famous diver in the U.K. was reviewed in DIVER Magazine this month. His favorite fin's....guess. HOPEFULLY BLAIR CAN POST details. Meanwhile enjoy the water for me. Brother Bob