Why are longer diving fins more efficient?

Freedivers use longer fins than scuba divers, who use longer fins than snorkelers. They get more power per kick and burn less oxygen, with the trade-off being reduced maneuverability. But the longer fins have a reduced aspect ratio, which seems to me the driving factor for efficiency, since there is no difference in foil sections (although some fins do have fences on the sides). How can these two opposites be reconciled?

 

power

you can drive a shorter fin with less power

the legs are a variable here

Now, it is possible a longer fin on any leg is harder to maneuver, but so is a longer vessel.

 

That is not correct that scuba divers use longer find than snorkelers. Freedivers are like sprinters, they need a short burst of speed. Scuba and snorkel divers usually use fins that match their physical condition. It also depends on the object of the dive. I am a commercial and also a recreational diver. For commercial diving I wear heavy rubber fins (HOGs), which take a beating and don't break. For recreational wreck diving, I use fins with plastic blades that are more efficient but wouldn't last more than a couple of days in commercial work.

 

That's still dodging the hydrodynamic aspect of the question. Obviously there is some limitation on how wide bifins can be if you want to flutter kick (since you have to move your feet past each other), but why do monofins have such low aspect ratios of 1 or 2 to one instead of something wider?

 

The longer fins deliver more power per stroke. They require more strength and conditioning of normally little used muscles. They use more oxygen per stroke than typical snorkeling fins. But also require fewer strokes to cover the same distance. Trained divers utilize a kick, kick, glide pattern similar to ice or roller skating. Vacation snorkelers who constantly kick lack the training and conditioning required to benefit from the longer fins.

The size of any fin duel or mono is limited to the strength and anatomy of the human user.

 

A quick Google search lead to this article
Best Freediving Fins of 2021 https://www.theadventurejunkies.com/best-freediving-fins/

It's not just the shape and length, but the stiffness and curve.

I started diving as a kid in our swimming pool before BCs and single house regulators were common. By the time I got certified at age 14, there were no double hose regulators any more and the regulator had moved from the tank to the diver's mouth. One of the things that first struck me about the difference between SCUBA fins and snorkeling fins was the stiffness. Over the years, they grew bigger and longer too.

I also would note that the fastest fish in the ocean do not have long trailing fins but tall thin rigid crescent fins

I would note also that sculling oars, like propeller blades, trade powerful wide blades for faster long skinny blades.

There is speed associated with high aspect ratios and power associated with large area low aspect ratios. It works that way with a sail also.

-Will

 

I don't know if I fully agree with this. Power is a function of total area. Speed a function of efficiency. With higher aspect ratio comes better efficiency. So, why don't monofins resemble something like the high aspect swordfish tail instead of a squarish trapezoid? It doesn't have to be bigger, and therefore harder to use in order to be more efficient.

 

There's more involved than just geometry. High aspect ratio sails have their limitations for other reasons than sail area. Certainly a sail with a high aspect ratio can have the equal sail area of a low aspect ratio, but to do this moves the center of effort higher. That means that under certain low wind conditions or high hull friction conditions, more heeling would be the result of that efficient H.A.R. sail vs the L.A.R. sail. As with sails, diving fins have other considerations besides straight forward geometry. Long stiff fins would require less work to flutter the legs, but may be difficult to power a quick burst of speed to execute multiple jack knife surface dives that a shallow reef snorkeler would do.

Loose flexible fins lack the thrust necessary to move an equipment laden SCUBA diver and who wants four to six feet of fin in the way when kneeling on the bottom to get a good underwater picture?

-Will