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

 

Others have pointed out the power versus efficiency reasons for fin selection. There is also the issue of fundamental fluid dynamic principles of wing lift versus wave form thrust as demonstrated by eels, snakes and skaters.

There is a high aspect ratio dive fin that has good performance, and efficiency but is no longer being manufactured. Google Dol-fin and look for the DeeperBlue thread.

Dol-Fin Monofins Review - Introduction - DeeperBlue.com https://www.deeperblue.com/dol-fin-lift-based-mono-fin-review-introduction/

 

Longer fins do not deliver more power per stroke. It is like saying that a larger propeller will deliver more power. The fin should be able to deliver power as efficiently as possible. A swimmer with more available power will benefit from a fin that can deliver more power. However, a weaker swimmer will go slower and tire faster with the previously mentioned fins. Conversely, a dinghy with a ship propeller will not go faster.

 

I am not sure what your point is here. Much of what you say is true. No problem with the statement "A swimmer with more available power will benefit from a fin that can deliver more power. However, a weaker swimmer will go slower and tire faster with the previously mentioned fins."

As for the first part of the post "Longer fins do not deliver more power per stroke. It is like saying that a larger propeller will deliver more power." This is true if the power is limiting as you more or less imply with the last statement "a dinghy with a ship propeller will not go faster."

The dingy with a 1/2 horsepower motor and a tiny propeller will go faster if equipped with a 5 horsepower motor and a suitable propeller. Put the new 5 hp propeller on the 1/2 hp motor and it will likely go slower just as implied with the first statement if the power is limited to the 1/2 hp motor.

I look at this as common sense and obvious but for some it may not be that clear in the dive fin application. In that case it may be useful to compare this with other more familiar examples. A piper cub will not go faster with a P-51 propeller and a P-51 will not go as fast with a piper cub propeller. Each propeller may have 95% efficiency in the designed application but in the wrong application the same propeller is extremely inefficient.

The example of the Dol-Fin is to show that the, low aspect ratio, longer free diving bifins can be improved upon by creating a high aspect ratio monofin.

I do not know if that explanation is necessary but if so I hope it helps.

 

I think we are on the same page. The thread started with the statement that divers with longer fins use less oxygen because they are more efficient, with the trade-off being reduced maneuverability. As a professional diver, I find that inaccurate. I use four different fins depending on the type of dive. For example, in confined spaces a pair of Hollies F2 fins let me turn head over feet easier. In those really long pipes it is possible to use the sides and bottom to push along. Hogs let me have more power per stroke and are very durable, and are my standard for bridge inspections. For wreck diving (recreational fun) I use stiff blade Mares that give me precise control and prevent me from lifting silt and ruining visibility. Hogs are shorter than the Mares, but definitely put more power out, which is noticeable in high current.

 

There are reasons why the longer low aspect ratio free diving fins can be more efficient than the typical shorter fins of SCUBA and Snorkel divers. The answer can be complicated and beyond the scope of this page so I have started a new thread for any that may be interested in a more detailed explanations and discussion.

Bio Inspired Propulsion https://www.boatdesign.net/threads/bio-inspired-propulsion.65990/

A quick answer is that a properly designed long fin does not conform to analysis of high aspect ratio airfoil theory. The high aspect ratio airfoil theory minimizes losses from the so called tip vortex while the long undulating free diving fin recovers and redirects the energy otherwise lost in the vortex trail.

The original question is similar to the study that proved honey bees can not fly. Ignore bad assumptions made in the postulating theory and the answers derived are not representative of the object that is studied.

 

Gonzo,

You may want to look into the Dol-Fin for work in high current environments. It is sturdy and built for speed and efficiency.

 

Those fins are definitely not as good as any of the types we use for commercial work. To start with, they don't have a metal spring in the back to hold them in place. They seem like the cheap ones they sell for occasional snorkelers. For example, they wouldn't fit over hard boots.