The physics of fin movement

[NAM001]

using fins with flexable blades you can get the bending of the tips like shown. touching the fns at an angle adds support during that portion onf the kick. with a stiff fin , the touching part has less benifit. Most people do nt know where thier fins are at any way. Not being a smart but , but it is true. watch most divers and thier fins are all over the place. on a forward kik where you have 90% thrust and 10% drag , you have no place to go but forward and no one cares, you have 80% of dynamics still pushing you. on a back kick where you have at best 55 thrust and 45 drag the margin for negative effort has reduced to the 10 percent margin. Every bit of technique is needed to maintqin the 10% ballance of desired motion over opposition.

 

[bluemed]

What andrew is really saying is tha the biggest problem usually is, is that the diver lifts his feet, and the blades pointing towards the surface, than there is No way you can move backwards since the blades will push the water bacwards and your body forwards.
It are the sides of your fins and not the blades.
I have made a video kicking backwards without fins and you can see what I mean.
BACKKICK WITHOUT FINS (Here you can clearly see how to) - YouTube

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[boulderjohn]

What I found most interesting in this video is not apparent in the video.

When I was training in UTD, we had online training resources to various skills, including the backwards kick. I remember we had a number of discussions about this skill and how it was taught. I don't have access to those slides any more, but my memory is very clear because of the discussion points. What we were taught back then is very different from what is in this current video. Back then, the emphasis was to point the fin tips downward on the outset of the power stroke. That is clearly not part of the skill as described and performed now.

 

[bluemed]

Hi Boulder, the idea is not to point the tip downward on the outside of the powerstroke (2) but on the loading phase (1)..
you see...
The tip should point "outside" on the powerstroke and downward, in the sense..of a 90 degree angle with your legs or parallel to your body..not
pointing to the surface.

---------- Post added April 14th, 2013 at 10:49 AM ----------

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

He who speaks without modesty will find it difficult to make his words good.
Confucius

 

[boulderjohn]

Hi Boulder, the idea is not to point the tip downward on the outside of the powerstroke (2) but on the loading phase (1)..
you see...
The tip should point "outside" on the powerstroke and downward, in the sense..of a 90 degree angle with your legs or parallel to your body..not
pointing to the surface.

I understand what is being said. What I am pointing out that what he is teaching now in that video is NOT what he was teaching a couple of years ago.

 

[boulderjohn]

OK, as luck would have it, I am currently teaching fin kicks, including the back kick, to someone who is an expert on this topic. There are two topics in this thread. The first is the one Lynne raised, and the second one was implied (but not stated precisely) in my observation that the technique being described here is not the same as the one I originally learned from the same teacher. I assumed there must have been a reason for the change. I thought a good reason was that the old method (fin tips pointing down and out to make the front of the fin the primary source of resistance) tended to push water downward toward the silt, but I questioned its power since the sidewalls of the fin had less surface area to provide the resistance for propulsion. When I tried it, I was surprised at how well it work. I showed my student both methods, and he was able to talk about both issues from an expert point of view.

1. Lynne's Original question: He sees no reason why pushing the fin tips back at an angle would be any different from going straight back. He is with Lynne on this--it shouldn't matter.

2. Relative power behind the two techniques: He wrote out the equation for calculating drag force, but I lack the ability to make the symbols here. The key concept in the equation is that the most important factor is the velocity of the movement. All other factors in the equation are multipliers, but velocity is a multiplier after it is squared. In examining the shape of the kick, he observed that, yes, the older (to me) kick style provided more surface area to create drag, but, perhaps, it is more awkward and less efficient because of the anatomy of our legs. The method being shown here allows more velocity, and because velocity is squared, it makes up for the lesser surface area. If you are able to perform the motion twice as fast, you get four times the power.

We also talked about my opinion that it is important that the recovery stroke be done slowly to keep from cancelling out the effects of the power stroke, and he said the same principle applies. If your motion is half as fast, the power is 1/4 of what it would have been.

He edited this post before I sent it. I hope it is helpful.

 

[TSandM]

Thanks, John!

 

[Oldbear]

OK, as luck would have it, I am currently teaching fin kicks, including the back kick, to someone who is an expert on this topic. There are two topics in this thread. The first is the one Lynne raised, and the second one was implied (but not stated precisely) in my observation that the technique being described here is not the same as the one I originally learned from the same teacher. I assumed there must have been a reason for the change. I thought a good reason was that the old method (fin tips pointing down and out to make the front of the fin the primary source of resistance) tended to push water downward toward the silt, but I questioned its power since the sidewalls of the fin had less surface area to provide the resistance for propulsion. When I tried it, I was surprised at how well it work. I showed my student both methods, and he was able to talk about both issues from an expert point of view.

1. Lynne's Original question: He sees no reason why pushing the fin tips back at an angle would be any different from going straight back. He is with Lynne on this--it shouldn't matter.

2. Relative power behind the two techniques: He wrote out the equation for calculating drag force, but I lack the ability to make the symbols here. The key concept in the equation is that the most important factor is the velocity of the movement. All other factors in the equation are multipliers, but velocity is a multiplier after it is squared. In examining the shape of the kick, he observed that, yes, the older (to me) kick style provided more surface area to create drag, but, perhaps, it is more awkward and less efficient because of the anatomy of our legs. The method being shown here allows more velocity, and because velocity is squared, it makes up for the lesser surface area. If you are able to perform the motion twice as fast, you get four times the power.

We also talked about my opinion that it is important that the recovery stroke be done slowly to keep from cancelling out the effects of the power stroke, and he said the same principle applies. If your motion is half as fast, the power is 1/4 of what it would have been.

He edited this post before I sent it. I hope it is helpful.

John,

Here is the NASA equation: The Drag Equation