Freedive fin efficiency test results

[ishuyi]

Pure speed is a misleading way to rate fins. Drag puts an upper end on speed, so the top speed difference between types of fins is very compressed by the drag limit, ie, it takes a lot more thrust to make much difference in speed and is hard to measure. Further, fins act very differently under different power inputs. Blade thrust comes from the shape of the blade under stress, too much and you overpower the fin, poor thrust. Too little and the fin does not bend enough to produce good thrust. Soft fins bend more easily and are more efficient at low speeds (also easier to push). With correct finning technique they can be as fast or faster than stiff fins. With poor technique, high power input can overbend the blade and you get poor thrust. Stiff fins will produce great thrust if you really put the power to them, but are inefficient at lower power levels(and lower speeds), because they don't bend enough at low power inputs.

Real world examples: I had a set of stiff Specialfins Hybrids that were definitely the fastest blades I ever used and fastest over as long a distance as I could maintain( several hundred yarda), but they were so stiff that they did not work well at low speed and way over powered my legs during a long days diving. I used a set of c4 vtr25 blades for a week of 20-70 ft reef crawling. They were wonderful, super efficient, super easy on my legs, plenty fast enough. Then I took them deep, 100+ ft and had no thrust at all when I make my turn. I was overpowering a very soft fin. Scared me silly till I figured out what was going on.

Your concept of “overpowering” a fin is helpful to me.

I have been using snorkeling fins for some time and am used to strong powerful strokes to propel myself in the water. When I try on freediving carbon bi fins with soft stiffness, I noticed I tend to overpower them. So I don’t feel the thrust they are supposed to give me.

I am in the midst of buying my first customised fin and am trying to decide whether to get one with soft or medium stiffness. From what you have written, there seems to be different purposes for different blade stiffness. But matching our power to the stiffness of the fin blade seems to be important for optimal performance of the fins.

My concern with choosing a soft blade is that I will overpower it, as my past experiences have shown me. However you mentioned that learning the right technique can actually prevent this and even enable a softer fin to achieve a speed that is more associated with a stiffer fin. Is my understanding right?

Given my circumstance, do you have any insights that can help me decide between a soft and medium stiffness blade? Learning to fin with correct technique is definitely one of my goals with my new fin.

 

[cdavis]

What kind of diving do you do?

Soft fins need a more straight knee kick and a little patience to kick them softly while speed builds up. its easy to learn.

I use a medium blade because I do a lot of different types of diving and often need the acceleration that comes with stiffness. My kind of spearfishing really needs quick acceleration. I've used very soft carbons and they are fabulous for me for reef cruising in less than 28m, but acceleration is limited. In shallow water or not spearfishing, I did not notice the lack of acceleration. Those blades are probably fine below 28, but I tend to overkick them when I get excited or turn in deep water. if my use fit the softer fins, thats what I would have.

 

[ishuyi]

What kind of diving do you do?

Soft fins need a more straight knee kick and a little patience to kick them softly while speed builds up. its easy to learn.

I use a medium blade because I do a lot of different types of diving and often need the acceleration that comes with stiffness. My kind of spearfishing really needs quick acceleration. I've used very soft carbons and they are fabulous for me for reef cruising in less than 28m, but acceleration is limited. In shallow water or not spearfishing, I did not notice the lack of acceleration. Those blades are probably fine below 28, but I tend to overkick them when I get excited or turn in deep water. if my use fit the softer fins, thats what I would have.

Thanks for your kind reply! Your kind of spearfishing sounds very thrilling. and reassurance that learning to kick with soft fins isn’t that hard to do.

I do pure freediving at the moment. In the open water and swimming pool. Though I think I am easily excited by nature, probably causing me to overkick very often.

I think situations that require an acceleration in my case would be more for openwater rescues, which do you think would be harder to do with softer fins?

 

[cdavis]

never thought about that one, not sure how rescues would be. Never heard of anyone having a problem. I've had the dubious honor of playing safety diver for several open water BOs. Never a problem with my mediums.

For pure freediving, I'd go soft. If you used them all the time, I suspect that you would learn to avoid overkicking. If you overkick very soft fins, its pretty obvious and you can adjust. I have a set of soft blades that are not nearly as soft as those carbons. I did not think I was over kicking them until I saw a video of a 30+ m dive. Clearly overkicking, but I had not noticed.

 

[ishuyi]

Thank you for your advice! I have also decided to go for the soft and learn to avoid overkicking it! Perhaps also taking videos of myself to compare what overkicking and normal kicking looks like to increase my sensitivity and awareness.