Confused about fin efficiency.

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fuzzybabybunny

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So I'm really confused about fin efficiency. What exactly does it mean? Power outputted by your muscles per unit of forward momentum?

A freediver needs big long fins because he needs to be able to go far with a slow, oxygen-conserving kick. This to me means a very efficient fin. But I hear that those split fins are very efficient, but aren't recommended for freedivers? Huh?

And what about stiffer scuba fins? Wouldn't added stiffness mean more power required for the kick, but also more forward momentum? Couldn't a freediver use long slow kicks with a scuba fin and still be fine?
 
No, when it comes down to it, different designs do different things, very few designs do anything well and often work in spite of whatever design changes are in them.

Freediving blades are efficient in a wide slow cadence kick and develop very good power/speed, but their maneuverability is lacking, and their shear size makes them clumsy when trying to be close to reefs etc. I can't say I have ever even hear of anyone attempting helicopter turns or back kicking with them.

Stiffer Scuba fins- well not all are stiffer, in fact many are very soft so..
Stiff Paddles- ie Jets, Hollis Fin, Turtles, Rockets, etc...- good frog kicking (very low speed) at very low output, very good fine control. However they are difficult ot maintain a powerful flutter kick and speed with them, sure people will tell you they can do it, but actually maintaining it for a mile long swim trying to overcome a 1-2 knot current, well, I haven't seen that yet either.

Softer Paddles- Volo's and Superchannels for example- much better fins for a flutter kick, but they begin to sacrifice the fine positioning control that the stiff paddles offer.

Splits- the concept for splits was conceived in Itally back in the 60's or 70's but as far as I know, it was never brought to market, just a dead patent. The concept sat dead for years, until Bob Evans playing with his Original Force Fin started expirementing with how far he could bring the splits between the blade tips back. His testing brought the first Split fin to market (now called the Foil Force Fin) showed it at DEMA where some guys literally stole it off his table and changed all of the critical aspects because it was too hard and too expensive to mass produce then relabeled it "natures wing". These splits work in spite of the split, hence the reason lots of testing has shown them to significantly improve in performance by taping the splits shut with duck tape. Their performance is based on their flexibility, similar to the softer paddles. But "most" splits work best with a very high cadence kick that is at very low power. They can be efficient at short distances, but I have rarely found anyone that can maintain the 3+ knots these fins are capable of for more than a few yards, much less the endurance to actually get somewhere quickly, the cadence is simply too high.

Then there is the 4th style of fins- Force Fins, sure lots of people are going to say they are junk and they are too expensive, eh whatever, this company is basically alive today because of military special forces around the world continuing to buy them. They are short, flexible and responsive. They have the fine positioning control that stiff paddles have, they have the maintained speed that the flexible paddles do, but none of the cramping. Its a weird match made in heaven (or California in this case). Efficiency of these is impressive, but don't fit a lot of people because they don't "feel" like they are going anyway, but I want to actually go there, not feel like I worked to get there.

But that isn't the whole story on efficiency. there is of course another dimension, length, width, stiffness and foot pocket design. If the footpocket doesn't hold the foot in the right place, contact/leverage the heel correctly, flex in the right place (pivot under the foot not in beyond of it). The story goes on forever, which is exactly why so many fins (ok basically all of them) on the market absolutely suck.
 
Efficiency is how much muscle power is transmitted to the water, without being lost in the fin.

Efficiency isn't everything to a freediver, though. A short stubby stiff fin can have very little loss, and thus be termed "efficient"; however, it will take a lot of muscle power to move it through the water.

Activating lots of bundles of muscle fiber burns oxygen.

Thus, most freediving fins are what seem to be "floppy" when compared to a stiff scuba fin. This softness, though, allows very low oxygen consumption. The long length increases surface area to regain thrust.

C4.jpg


Most freediving fins retain their efficiency by having very "snappy" materials in the blade, which returns the energy needed to bend them back to the water, at the end of each stroke.

Freedivers face a balancing act: soft for oxygen conservation; or, stiffer, for thrust from the bottom, when the wetsuit is compressed. Generally, freedivers start out with a soft blade, and graduate to harder bifins as their depth increases. The switch points usually are 20m and 40m. For example, C4 fins are sold in stiffness ratings that correspond to the target depths.

When compared to freediving bifins, split fins are very inefficient - significant energy is lost in the split fin blade flapping back and forth.



Hope this helps!


All the best, James
 
Thanks for all the tips! I'm definitely beginning to understand the whole efficiency thing. Looks like as a freediver, a split fin really isn't for me, despite its popularity. Are stiff fins generally really easy to kick? You can buy cheapo US Divers type fins and whatnot from any sporting goods store and those all have pretty soft plastic that bends easily. When I swam with them they were really easy to kick and could go at a pretty good clip if I kicked frequently, but slow kicks resulted in practically going nowhere.
 
Thought about the same question years ago; an analogy in my profession is called 'impedance matching'

Throwing a basketball, you may reach perhaps 50 feet away, a tennis ball perhaps 100 feet away, a baseball perhaps 200 feet away, a cotton ball perhaps 10 feet away, a bowling ball 10 feet away.

The one that reaches farther and uses the most power from your arm muscles is not the lightest nor the heaviest; is the one that 'matches' your arm muscles.
For fins, it has to be similar. Certain stiffness/size should fit better for you, based in your legs strenght.

A 5 year old kid will probably have better results with the tennis ball. Its weight matches his arm better.
 
meesier42:
actually maintaining it for a mile long swim trying to overcome a 1-2 knot current, well, I haven't seen that yet either.

That's a job for a scooter, not a fin.
 
Thanks for all the tips! I'm definitely beginning to understand the whole efficiency thing. Looks like as a freediver, a split fin really isn't for me, despite its popularity. Are stiff fins generally really easy to kick? You can buy cheapo US Divers type fins and whatnot from any sporting goods store and those all have pretty soft plastic that bends easily. When I swam with them they were really easy to kick and could go at a pretty good clip if I kicked frequently, but slow kicks resulted in practically going nowhere.

Just to add to what fdog has said, it is also very easy to learn to do back kicks, frog kicks, and pretty much any kind of kick a scuba diver would want to perform, with freedive fins. I use C4 freedive fins myself, the 40 meter version... I use them for scuba, tech, or freedving. I frequently frog kick with them, and find this a very effective kick for certain situations---dolphin kick when I want lots of speed, normal flutter kick most of the time, but generally mixed in with some frog kick from time to time. I could drag most split fin divers through the water faster than most could swim, and still have better air consumption than the split wearer at cruising speed--efficiency of the c4's, and the fact that the C4 allows cycling fitness to translate perfectly to finning efficiency.
DanV
 
https://www.shearwater.com/products/perdix-ai/

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