Spread 'em! [Fin-gers & fast swimming]

[wet]

ScienceDirect.com - Journal of Theoretical Biology - The constructal-law physics of why swimmers must spread their fingers and toes

The constructal-law physics of why swimmers must spread their fingers and toes

Abstract
Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The larger force means larger body mass lifted and greater speed, in accord with the constructal theory of all animal locomotion. The spacing between fingers must be twice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D–0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. For example, the total force exerted by two optimally spaced cylinders exceeds by 53% the total force of two cylinders with no spacing when Re=20. These design features hold for both time-dependent and steady-state flows.


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Highlights
► The total force is 53% greater when the fingers are spaced optimally. ► The optimal spacing is twice the boundary layer thickness of one finger. ► The speed advantage comes from the greater force, which lifts more mass above water. ► The theoretical predictions are confirmed by computational fluid dynamics simulations.

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So a slight spreading of the 10 fingers & 10 toes gives better results.

 

[azapa]

This is very interesting. I like no fins and have tried everything from tight, almost cupped hands, to loosely open hands, and the later feels like it grabs more water. What to other DNF/CNF divers think?

 

[Fondueset]

I'm with you Azapa. Drawback is it's hard to maintain the exact spacing. I've also been practicing with Darkfin Gloves - they are fun and an easy way to build strength, but they don't seem to reduce the stroke count much in horizontal swims.

 

[pingshui]

If you watch olympic swimmers you will see that none of them have the fingers tight anymore. Making "cups" won't work and looks funny as well The only moment when tight fi.ngers make sense is the underwater recovery phase during breaststroke.

 

[growingupninja]

If you watch olympic swimmers you will see that none of them have the fingers tight anymore. Making "cups" won't work and looks funny as well The only moment when tight fi.ngers make sense is the underwater recovery phase during breaststroke.

Many years ago when I was a serious young swimmer I remember being taught this at a training camp but didn't adopt it (I obviously wasn't serious enough!)... Was a lot of mental overhead to add to training when you are working at the kind of physical intensity that was expected.

But as far as fluid dynamics from comp swimming to freediving, I am always fascinated, although I think the reduction in speed when freediving changes things a great deal. But in regards to comp swimming and freediving, I think that since many freedivers learn their technique in a full wetsuit, they don't have the benefit of feeling the flow of water on their skin. When I used to swim for real we called it 'feel for the water' and it took a long while to develop and was the first thing to go if you took a week off. If you are feeling the water correctly you are making miniscule adjustments to every stroke on a mostly subconscious level to find clean (nonturbulent) water. Anyone that can swim does it on some level but not--myself included these days--like we felt it. Or thought we did. That is actually part of why we shaved; body hair is drag but naked skin also transmits better feel for the water. Or so we believed. This was also back when new suit fabrics with less drag than skin were only just beginning to appear on a few of the female Olympians.

Uh, my point is that watching even high level no fin swimmers I see a lot of variance in arm pulls--sometimes it looks very much like the way I was taught to do it and other times it looks relatively inefficient, but not say that what anyone else is doing is wrong. I also suspect that some freedivers who are struggling to learn dolphin kick could learn faster without a wetsuit. A theory, impossible to test but food for thought...

 

[wet]

Sort of an interesting article on water boatmen beetles with superhydrophobic wings, the concept seems to link finger spread force and penguin feather-bubbles.

Nature inspires new submarine design
Research on superhydrophobicity has seldom focused on insects that live in water. In this report, the authors investigated the water-dwelling water boatman, which belongs to the Corixidae family of the order Hemiptera, suborder Heteroptera. The water boatman can swim freely and breathe in water. The study showed that the superhydrophobicity of the water boatman's hind wings plays a crucial role in its swimming, breathing, and balance, as well as its ability to escape from the water surface under unfavorable conditions.

The water boatman's metapodia are quant-like and keep swinging while it swims; counterforces between the metapodia and water push the water boatman forward or downward. When the metapodia stop swinging, its body begins to float upward because of the buoyancy induced by the perfect superhydrophobicity of the water boatman's hind wings.

Superhydrophobic surfaces are usually induced because of the synergistic effects of hierarchical micro/nanoscale binary structures and low surface free energy. Scanning-electron microscopy studies of the micro/nanoscale structure revealed that the surface of the water boatman's hind wings is composed of mastoids and nanorods with diameters of about 80nm and 50-100nm, with typical mastoid-to-mastoid and rod-to-rod distances of about 50-200nm and 300-1000nm, respectively. Moreover, the insect's wing surface contains low surface energy protein, lipid, and chitin materials, which are hydrophobic. The superhydrophobicity of the water boatman's hind wings is presumed to originate from the combination of such a hierarchical surface structure together with hydrophobic materials contained on the wing. Water contact angles on the wing surface were measured to be 159° and the glide angle was about 8°.

According to Cassie's theory, contact between water droplets and the water boatman's hind wings is a composite contact of solid-liquid-gas. On the rough hierarchical surface, air can be trapped within the micro/nanoscale structures on the wing surface, which makes it difficult for water to enter into the structures to fully wet the surface. The overall contact area between water droplets and air trapped within the micro/nanoscale structures can be roughly estimated to be about 9% (the corresponding contact area between water drops and solid wing surface is about 91%).

The superhydrophobicity of the water boatman's hind wings enables it to swim freely, breathe in water with the assistance of air trapped on its hind wings' surfaces, and escape easily from water area under unfavorable conditions without being affected by moisture.

Penguin feather bubles article:
http://fossilpenguins.wordpress.com/2012/05/29/bbc-one-the-blue-planet-frozen-seas-leaping-for-land/
Air has lower viscosity than water, so adding a layer of air around an object can help it cut through the sea more efficiently. Engineers have even applied this concept to make speedier torpedoes. Scientists studying film of diving penguins found that Emperor Penguins store air in their plumage, which gets compressed as they dive. Moving from deep water to shallow water lowers the pressure on the air, just like when one takes the cap off a bottle of soda. As the penguins near the surface, they are able to shift their feathers so as to release the air, which escapes in bubble form. This creates a smooth layer over much of the penguins plumage, which cuts down on friction and drag, allowing the penguin to build up a serious speed. An Emperor Penguin can reach velocities of up to 3 meters per second as it jumps out of the sea onto land. This may seem like fun and games, but when a bird needs to emerge from a hole in a thick sheet of ice or make it up a steep cliff, high speeds
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so air layer at skin surface reduces viscosity...

 

[Simos]

This is an interesting topic - as a kid, I took a few swimming lessons and (unfortunately) I was taught to keep my fingers together and 'cup' my hands.

I've been absolutely rubbish in swimming and when I took up freediving I tried to learn how to front crawl, as I needed it to be able to pool train and keep up with the others.

One of the things I changed was to start spreading my fingers and I felt it made quite a bit of difference - funny how such a seemingly small thing can make a difference.

Oddly, I haven't been able to unlearn it for breast stroke and DNF and still cup my hands for some reason. The recovery does feel better with cupped hands in DNF but doesn't make sure to pull like that, no idea why I keep doing it!!

 

[pingshui]

Cupped hands cannot be really relaxing, can they? This phenomenon is really interesting and I have seen it many times when I taught swimming to adults.

What you could try is swimming breaststroke arms only and use a slight dolphin kick for your legs (maybe even with short fins, if that is more comfortable for you). This way you can fully concentrate on your arms and hands. This is also a good practice to get more fluidity in your overall movement.

But to comfort everyone who is struggling with breaststroke technique: it IS the hardest of them all

 

[growingupninja]

Cupped hands cannot be really relaxing, can they? This phenomenon is really interesting and I have seen it many times when I taught swimming to adults.

What you could try is swimming breaststroke arms only and use a slight dolphin kick for your legs (maybe even with short fins, if that is more comfortable for you). This way you can fully concentrate on your arms and hands. This is also a good practice to get more fluidity in your overall movement.

But to comfort everyone who is struggling with breaststroke technique: it IS the hardest of them all

I actually do a breaststroke pull with a dolphin kick all the time; the frog kick doesn't agree with everyone's knees, not that I am logging anywhere near the kind of mileage these days where it would be a factor, but it is a habit.. Without fins not terribly efficient unless used as part of a push-off.

One thing I have noticed watching NF is some top people do the finish under the body, making a keyhole same as I was taught to finish a butterfly stroke for hydrodynamic reasons, and how I swim NF/underwater breaststroke, while others use a very wide stroke, sweeping their hands all the way to their hips. The first way engages the tricep more and naturally flows into a dolphin kick. I wonder if there is reasoning behind the preferance for these choices?

 

[pingshui]

Breaststroke really isn't too great for the knees, so you made a good choice using dolphin kick in training. For many competitive swimmers it is either problems with the knees (breaststroke) or the shoulders (the other disciplines) that will make them quit eventually.

The keyhole style stroke is way more effective as well as efficient. You will see this stroke also in olympic breaststroke after the push-off at the wall.
Well, actually for freestyle you don't do that much of an s-stroke or keyhole stroke anymore. In freestyle the hand follows a rather straight line now:


If you do the wide stroke, like a half circle with your arms, you can only use your shoulder muscles for propulsion and especially in the end phase of the stroke that won't bring you very far. In the keyhole stroke, as you said, the triceps will do the most work in the end phase. In untrained people this technique (actually the same stroke pattern in principal applies to freestyle, only that you move your arms separately) is very tiring for the triceps and they will automatically skip the last stroke phase or get very inefficient. Anyway, this should not be relevant in freediving, where we don't swim so far and fast.

 

[watts]

I found the most propulsion came from a continued keyhole stroke where you use your forearms a lot and the armstroke flows not 2 movements like some do.
The loose fingers technique I found best. Didn't feel right with super wide fingers.
I might try it again and compare. The width of the stroke is probably more important.