The physics of fin movement

[Peter Guy]

Oldbear -- a tip on learning the backwards kick -- try it barefoot first to get the motion. Get in the pool, no gear (perhaps mask/snorkel) but a kick (float) board for your arms/chest. I found, and have since worked with several students this way, that learning the kick barefoot is ever so much easier. "Swim" laps backwards to get the motion, then add fins.

 

[Oldbear]

Oldbear -- a tip on learning the backwards kick -- try it barefoot first to get the motion. Get in the pool, no gear (perhaps mask/snorkel) but a kick (float) board for your arms/chest. I found, and have since worked with several students this way, that learning the kick barefoot is ever so much easier. "Swim" laps backwards to get the motion, then add fins.

Thanks great tip. My GF and I swim laps a lot so great time to practice. I think before I do that I will lay on the livingroom ottoman and practice while watching the video...I think someone called this "table dancing" a GUE Fundies thread :dance2:

 

[TSandM]

I'm pretty sure I do extend my feet with them at an angle to one another, as AG demonstrates, but I also have the distinct impression that I'm scooping with the TOP of the fin, rather than the side boards, when I get the power of the kick. I dunno . . . maybe I can get somebody to video me so I can see.

Even if you touched the edges of the fins together, as demonstrated, how would that stabilize the floppy tips as you extend the fins? I don't think anybody's ankles are flexible enough to put the entire bottom surfaces of their fins together

 

[g1138]

I'm pretty sure I do extend my feet with them at an angle to one another, as AG demonstrates, but I also have the distinct impression that I'm scooping with the TOP of the fin, rather than the side boards, when I get the power of the kick. I dunno . . . maybe I can get somebody to video me so I can see.

Even if you touched the edges of the fins together, as demonstrated, how would that stabilize the floppy tips as you extend the fins? I don't think anybody's ankles are flexible enough to put the entire bottom surfaces of their fins together

I used to think that, but them I tried really exaggerating. Scooping the tip topside didn't work at all. Scooping the tip downward made it seem normal.
So for this reason I no longer tell divers that their fins direct water in up from the tips.
I say it's a mix of sidewall and bottom-pointing flats of your fins that work together to send you in a straight line backwards.
Pointing my fin tips down just happened to flex the tips up (on video), which made me think originally that I was scooping tips up.

And it makes sense, if you're scooping tips upward, shouldn't the tips be flexed downward? I never see that in proper back kick videos.

 

[ams511]

Well, your assertion certainly adds weight to the matter.

Dark matter or the normal variety?

 

[Belce]

you know one of the things that makes a game like golf hard is that when you do it correctly <swing> its natural, when it doesn't work it is because you pushed, hurried and meddled with the natural order of things. don't overthink this. load by placing your fins where you want to go, move yourself by bringing your fins back to yourself like a heart.

when I was learning this I was told to think that the outer edge of calf actually did the proplusion.

 

[NAM001]

i AM NOT VERY GOOD AT BACKING. sometimes i do ok and mostly i am not. certainly the thing about touching fin tips must aid in the initial muscle memory training process.

 

[denniscloutier]

First, a disclaimer: I can't back kick.

I think there might be something to what he says, but I don't think it has anything to do with fluid mechanics. I think having the two fins pressed together at right angles braces them so they are harder to bend in the upwards direction. If the tip doesn't start to deflect, then it is less likely to catch in the water. This is like taking a steel flat bar and bending it into an angle iron. It is a lot stiffer once you have two sides at right angles.

Anyways, this is pure theory since I can't actually do this myself, so I could be wrong.

 

[befee]

UTD Equipment Z-Side Mount System Fitting & Function - Part 2 of 3 - YouTube AG catches himself.

 

[mala]

to back kick i angle the blades as the fins move away from each other and let them go horizontal as they return together.
as long as the power bit is done faster than the return then the physics of fluid dynamics will give you reverse thrust.

stiff blades do it best and split fins dont do it at all.