Split Fin Physics

[Blackwood]

Preface:

I have no reservations that split fins are easier to kick than paddle fins. They "give" against the water. That's a no brainer. For some, that suits their leg strength, endurance, and style. For others, a blade works better. This thread (I hope) won't become "blades are better" or "splits are better" and I'll request that any such replies be removed.

Rather, I want to discuss the physics of split fins. In particular, I often read here and in magazines that split fins use hydrodynamic lift to thrust the diver forward just like an airplane or ship's propeller does. From Atomic Aquatics:

The Atomic SplitFin slices through the water with two wing-shaped surfaces, creating lift and forward propulsion more like a propeller.

I have yet to read any explanation for how that happens other than "it's the Bernoulli principle" (which for the unindoctrinated is a theory of lift that in short states that the faster a fluid moves, the lower its dynamic pressure, and thus a shape which causes the fluid to move faster over one surface than the opposite will generate a net pressure differential across the shape resulting in a force).

In the above diagram, the integral of the pressure distribution gives the net lifting force.


Unfortunately, even if the deflected shape of a split fin forms a hydrofoil, I can not for the life of me see how any lift it may generate would contribute to a net force in the forward motion of the diver (i.e. thrust).

Let's assume it does generate some lift. I'll consider the up stroke. Since split fins have a more-rigid outside edge, when kicking down, the split fin's blades deform up towards the middle like in this picture from Atomic's website. (The opposite is also true).

In their diagram, they show some stuff and then say "resulting lift" and put an arrow forward.

Compare the Atomic diagram with the airfoil above. See anything interesting? Like the direction of lift? The inwards deformation of the split fin forms a bit of a foil shape, but it looks to be in the wrong orientation.

If I can see any lift, it's the pink arrows perpendicular to whatever component of flow is in the direction of the orange arrows (foil cross section in red).

One could say that once the kick breaks the neutral plane, those pink arrows will be going on average "more forward," but even if that's true it's counteracted by the half of the kick which "lifts" backwards.

Atomic's annotations say "high pressure side" and "low pressure side," but that's not hydrodynamic lift. It's just moving a blunt object through the fluid which creates a wake (sometimes thought of as "suction"). The same thing happens with a blade fin.

And even if you can call it lift, how does it translate to DIVER motion? Seems to me that since the blade is so flimsy, it would just act locally (i.e. on the fin blades themselves) to straighten them out some.

My impression is that the physics between blades and splits are the same, not like "a paddlewheel versus a propeller". You push against the water, it pushes back against you (thanks Isaac). Since the blades turn off-perpendicular to the water against which the diver is kicking, that wake is reduced, easing the work of kicking. However, the more it turns the less it pushes against the water, which is why a long wide flutter is less effective than a short narrow one.

Can any split fin experts/designers/builders show me where my cursory analysis is incorrect? Any other thoughts?

 

[diversteve]

I'm guessing you've seen this: Nature’s Wing - SPLIT-FIN TECHNOLOGY

 

[deco_martini]

The split fin design that Atomic and others use is licensed from Nature's Wing. I doubt the marketing guys that made the pictures with the arrows had any idea how the fins actually work.

 

[Blackwood]

I'm guessing you've seen this: Nature’s Wing - SPLIT-FIN TECHNOLOGY

I have, it begs the same questions I posed.

Lift or "suction" dove propels the diver forward through the water like lift pulls an airplane upward.

How? What's creating it (the lift or "suction"), how is it oriented in the proper direction, and how does it translate to diver motion?

The split fin design that Atomic and others use is licensed from Nature's Wing. I doubt the marketing guys that made the pictures with the arrows had any idea how the fins actually work.

I'm sure you're right. To be clear, I'm not asking this to denounce or impugn split fins. I just wonder if the marketing line is accurate and where my thinking is wrong.

 

[mdb]

Marc:

You are asking good questions in a non confrontational manner-a nice change for the never ending split/paddle debate.

When Apollo was first approached with the Nature's Wing fin it had already been declined by most major SCUBA manufacturers. They were afraid. They thought it was too radical.

The Founder and President of Apollo Japan, Seiji Hattori, a very experienced and talented engineer, when he first tested the prototype fins directed that we sell our paddle fin molds-which we did-Dive-Rite purchased the molds. Dive-Rite now has an excellent paddle style fin.

Mr. Hattori then went on a quest to find the right material and the right design parameter to produce the optimum split fin. It took more than a year and a half and many millions of dollars. We use all natural rubber and the very time consuming and expensive compression molding process to produce the Apollo bio-fins.

The others soon copied us, naturally, many using injection molding, inexpensive materials, and pricing up against us, just because they could. That's business. We do not dwell on that. What we want to do is produce the finest fin we can.

The split fins, as you have noted, work on the basis of different pressure flow. Since water is much denser than air the fluid dynamics work to create the forward thrust.

When first used many divers do not think the fins are working properly-little resistance. I was one of those divers. I prefer the XT-Extra Torque-version since I can "feel" the fin and open my kick to a wide flutter or scissor kick if I prefer.

The most efficient version of our split fin model-not all fins are created equal-is the original Apollo
bio-fin pro.

A look at our very pronounced leading edge and precisely placed trailing edge does show the concept. There is less effort and the diver moves forward with both the up and down stroke.

As mentioned before the video coverage @ Apollo Sports: Dive Gear shows the fin in use, even comparing it to our former Paddle fin-The Apollo Prestige. The Apollo Prestige fin was state of the art, reversed foot pocket to reduce drag and the very highest grade polyurethane for snap and recovery.

In any case; you are asking good questions in a good manner. Thank you for the discussion.

 

[Crush]

The split fins, as you have noted, work on the basis of different pressure flow. Since water is much denser than air the fluid dynamics work to create the forward thrust.

For what it is worth, I agree with Blackwood's vector analysis. Again, FWIW, I did well in fluid dynamics courses at university. In Blackwood's analysis, the net effect of the airplane-like fin shape will have the effect of resisting the deflection of the blade as it pushes against the water. Three things should be noted: (1) the effect is likely to be negligible owing to the relatively slow rate of flow parallel to the camber surface; (2) the same effect can be achieved by simply using a stiffer material; and (3) the effect of turbulence, which has been neglected, is likely to be substantial.

mdb's "explanation" when you strip out all the praise of the inventor, amounts to "fluid dynamics work to create the forward thrust" which is akin to saying "mathematics works to put money in my bank account" - the argument, as presented by mdb, is rubbish.

Now, split fins may work for you, or they may not. If they do - great. To date I have not heard any compelling scientific reason for their use. What I have heard, to date, is pseudoscience.

Note: I find that split fins simplify my choices when pairing up with insta-buddies on dive boats.

 

[Bubbletrubble]

The split fins, as you have noted, work on the basis of different pressure flow. Since water is much denser than air the fluid dynamics work to create the forward thrust.

@mdb: Could you please elaborate on this statement? It is very incomplete. Diagrams illustrating vector forces throughout the various phases of the kick cycle would help. If possible, please assume that your audience has a fairly good understanding of physics.

 

[mdb]

SPLIT-FINS - SPLIT-FIN TECHNOLOGY has several illustrations that illustrate the method that produce thrust and force with split fins. These illustrations have been posted a number of times.

There will always be the types who say-this can't be true-I love my "whatever" fins. So be it.

Paddles work for ducks and frogs. A properly designed split fin, using the correct materials, provides power, thrust, without excessive strain. Just a look at a Dolphins fin or a whales tail should be enough to know that Mother Nature did not design paddle style fins for these mammals, their fins deflect and twist like a properly designed split fin.

Another good design is the Force Fin, based on Bob Evan's study of the Marine world.

This argument will go on and on. Decide for yourself. All the best for lots of good diving, no matter what fins fits your fancy.

 

[Blackwood]

This argument will go on and on. Decide for yourself. All the best for lots of good diving, no matter what fins fits your fancy.

Again, I'm not trying to tell people what's best for them. I'm not even expressing an opinion about what's better for me. Just trying to satisfy my own curiosity.

I've had some fun discourse with Bob, one result of which is action on his part to change some of the language on his website.

It's posts like this one that sparked my interest in the subject (name removed):

If displacing water is the way to go for propulsion then how come we done away with paddle ships and went with propeller ships?

That post was with reference to a 'paddle vs split' conversation. I don't see how it can be an apt analogy, but the idea is supported by at least some marketing language.

Here's a good diagram from Boeing which shows a wing, flow, and lift.

I modified it to make it like a prop: the 'wing' rotates about the center axis, and the resultant lift (aerodynamic force perpendicular to the freestream, which in this case isn't air flow relative to the plane but rather relative to the prop blades) is in parallel to the direction of motion.

A paddle wheel and a propeller are functionally different and work on different principles. A split fin, as far as I understand, is a modification of a paddle which decreases the work of kicking, but it's not a different paradigm.

 

[Crush]

SPLIT-FINS - SPLIT-FIN TECHNOLOGY has several illustrations that illustrate the method that produce thrust and force with split fins. These illustrations have been posted a number of times.

There will always be the types who say-this can't be true-I love my "whatever" fins. So be it.

Hey, if you like them, use them.

I looked at the links on the page you referenced. I could find no credible arguments in favour of split fins in any of the links. If anyone can catch what I missed, please let me know.

As for inspiration from nature, I believe (but could be mistaken) that the fastest marine mammal is Commerson's dolphin (Cephalorhynchus commersonii) while the fastest marine animal appears to be the Sailfish (two species, both are in the genus Istiophorus). To my eye they do not have split fins, but decidedly non-split crescent-shaped fins (especially the Sailfish, which is faster than any dolphin or whale). The crescent shape has sharp trailing edges which help to reduce drag.