The physics of fin movement

[TSandM]

In his recent VIDEO, Andrew Georgitsis advises people who are working on the back kick to tilt the fins at about a 30 to 45 degree angle from the horizontal, to form a V, before extending the legs in the loading phase of the kick. According to him, extending the fin when it's flat will cause the flexible fin tip to catch the water, bend, and increase resistance. Whether this happens or not, I'm not sure, and that's not my question.

It seems to me, though, that if it DOES happen, what angle the fin is at to the bottom (or surface) is immaterial. If the fin is pushed backwards along its longitudinal axis, the same phenomenon should occur, regardless of the orientation of the fin.

Physics was not my strong suit, so I'm asking . . . Am I missing something here?

 

[ScubaInChicago]

In his recent VIDEO, Andrew Georgitsis advises people who are working on the back kick to tilt the fins at about a 30 to 45 degree angle from the horizontal, to form a V, before extending the legs in the loading phase of the kick. According to him, extending the fin when it's flat will cause the flexible fin tip to catch the water, bend, and increase resistance. Whether this happens or not, I'm not sure, and that's not my question.

It seems to me, though, that if it DOES happen, what angle the fin is at to the bottom (or surface) is immaterial. If the fin is pushed backwards along its longitudinal axis, the same phenomenon should occur, regardless of the orientation of the fin.

Physics was not my strong suit, so I'm asking . . . Am I missing something here?

I call BS on this one... Outside of another revenue stream buy charging $149.95 UTD Online Store - +1 855 DIVEUTD (348-3883) or +1 760 585-9676 - sales@utdequipment.com for "the" fins that will allow you to back kick, the video does not support what he is saying. The divers fins do not touch on the loading phase and don't touch till the actual back kick. It's the same resistance flat or at 45 degrees, if the tips were together I might see the argument that it reduced possible flex of the tips.

 

[g1138]

I've always tilted my fin tips to point downward in relation to my 0 degree horizontal plane. I would say 2-5 degrees though.
If you look closely at all the back kick videos, you'll see every diver putting downward tension on their fin tips. Not doing this leads to your fin tips pointing skyward, so you need some sort of force to keep those tips level or down.

It's definitely not a huge 30 degree angle. The only important part is downward tension to keep ankles from flopping your tips upward.

Also I still don't believe you need to tap on the loading phase, but that's just me.

 

[Doppler]

As with so much New Age scuba advice, many explanations of the forces at play with a back kick seem to be over-wrought bull****.

I believe the video makes a good point with regards the angle that fins best slice through the water. The explanation is a good one... however, the reason is a more valid ergonomic argument than a physics one, surely?

Water resistance is water resistance and the brief explanation of fluid dynamics seems a little suspect. On the other hand, most divers find it comfortable and easier to maintain position in the water column when the fins are thrust backwards at an angle (45 degrees or otherwise). At an angle I have found it much easier to track the fins backward through the water... keeping my feet flat is... well, uncomfortable.

 

[johnkendall]

In his recent VIDEO, Andrew Georgitsis advises people who are working on the back kick to tilt the fins at about a 30 to 45 degree angle from the horizontal, to form a V, before extending the legs in the loading phase of the kick. According to him, extending the fin when it's flat will cause the flexible fin tip to catch the water, bend, and increase resistance. Whether this happens or not, I'm not sure, and that's not my question.

It seems to me, though, that if it DOES happen, what angle the fin is at to the bottom (or surface) is immaterial. If the fin is pushed backwards along its longitudinal axis, the same phenomenon should occur, regardless of the orientation of the fin.

Physics was not my strong suit, so I'm asking . . . Am I missing something here?

As others have said, there is little difference between flat and 30/45deg if the fins are not touching. And for someone who is good at a backkick their fins generally don't touch, and they are able to move the fins at whatever angle they want. However when *learning* to back kick, it can make it a lot easier to have the fins at an angle and touching together. This stiffens the fins and so helps to reduce the wobble in the fin tips.

Looking at that video though, the diver is going far too fast with their kick. The back kick should be slow and smooth, not frantic.

HTH
John

 

[HenrikBP]

I'm not sure about the physics part of it - resistance is resistance no matter the angle of the dangle

The point about having the fins touch is really just what I was taught when I was first introduced to the back kick: you set the kick up by trying to "place" the bottoms of your feet together. Since most of us aren't flexible enough to fully accomplish that, we end up with the blades together at an angle. Also, if I try to keep that angle on the power stroke of the back kick, I find I get a bit more propulsion.

I learned an important thing about back kicks recently: it is mostly a maneuvering kick, and if I try to turn it into a "power kick" it will usually mess up my form and get me nowhere. So more "form" and less "power" is - for me - the better way to go.

 

[Oldbear]

Could someone recommend a good "Back Kick" video(s)?

 

[drbill]

Physics was always my weak spot too... but I wonder if his assertion jives with the recent confirmation of the Higgs boson. After all, it changes our entire understanding of physics! Tee hee.

 

[Doppler]

Could someone recommend a good "Back Kick" video(s)?

The one attached to the marketing message selling fins (link above) is all you need.

Then forget videos and get into a pool and practice.

And then you need a video... of yourself. Also helps if you can actually see someone doing it... for real... up close.

Good luck.

---------- Post added March 14th, 2013 at 12:22 PM ----------

Physics was always my weak spot too... but I wonder if his assertion jives with the recent confirmation of the Higgs boson. After all, it changes our entire understanding of physics! Tee hee.

Well, your assertion certainly adds weight to the matter.


:cool2:


Sorry.

 

[Oldbear]

The one attached to the marketing message selling fins (link above) is all you need.

Then forget videos and get into a pool and practice.

And then you need a video... of yourself. Also helps if you can actually see someone doing it... for real... up close.

Good luck.

:cool2:


.

Thanks...