Split Fin Physics

[mdb]

@mdb: To clarify, your company did not give "free" fins, discounts, money, or any special "consideration" to military personnel or any U.S. government entity in exchange for being able to use the testimonials that you reference and that are posted on the following website, right? I realize that the website is run by Nature's Wing who holds the patent license to your fin design and you probably have limited input (if any at all) into its website content.

I'm not criticizing you for obtaining direct or indirect military endorsements at all. I'm just pointing out that it's effective marketing. Force Fin does it. APS Mantaray has a Navy Seal endorsement on its website, too.

Excerpted from the Apollo Biofin (Europe) website: : : Apollo Europe : :

Bubbletrubble: You are right. We did not compensate in any way for the comments/ endorsements shown on the Nature's Wing site.

The fact that military divers choose our fin is not because these divers are stronger or better swimmers etc., it is simply because the fin is, in their opinion, the most efficient for the task at hand. We are not exclusive. The military does use other fin models.

 

[Bubbletrubble]

Bubbletrubble: You are right. We did not compensate in any way for the comments/ endorsements shown on the Nature's Wing site.

The fact that military divers choose our fin is not because these divers are stronger or better swimmers etc., it is simply because the fin is, in their opinion, the most efficient for the task at hand. We are not exclusive. The military does use other fin models.

Thanks for your response, mdb.

 

[Teamcasa]

---

---

A ScubaBoard Staff Message...

This is a thread on the Physics of Split Fins, not the marketing, personal choice of paddles vs splits. Some off topic post have been removed.

---

 

[mdb]

Dave:

Thanks for cleaning up the thread a bit. Blackwood started a good discussion that, in most cases,
continues without much rancor. It is simply trying to get to the answer of why the split fin works.
I look forward to more input and am enjoying the overall civil tone of this discourse.

 

[fnfalman]

Actually, The fins are at the high end of the scale. They can purchase older style fins for much less. They spec bio-fins based on performance, not price.

Now and then the military got it right. Most of the time GIs get crap for gears.

 

[fnfalman]

@lmorin: I agree in principle with what you're saying. However, from a fin manufacturer's perspective, why even bother with doing any real R&D studies when one can effectively market the product by using testimonials from Navy Seals or other military special operation forces? By and large, the consumer public is stupid. People think: "If it's used by elite military forces, then it must be the best gear in its class and it's probably worth the additional cost." Navy seals have different goals when selecting a dive fin than the average recreational diver. They also have a different level of fitness and may have different kicking form. It's also unclear whether the military folks were somehow compensated (money or free fins) for their endorsement.

Military testimonials. Gotta love 'em (from a marketing perspective).

That can apply to solid fin manufacturers that the US military uses and the regulators that they use too.

Just because the military uses something, it doesn't mean they're always the best.

 

[mdb]

Now and then the military got it right. Most of the time GIs get crap for gears.

fnfalman:

I must agree with you. As a former Marine we always felt we did more with less and got the leftover gear. Our 1st Sgt. in Vietnam used to trade our Jungle Utilities to the Air Force in DaNang for cases of beer. The USAF got to look "gungy" and we got our beer.

In the case of our bio-fins most of our DOD orders state "Your products have been specifically requested, and your help in the matter is greatly appreciated."

We are always honored to oblige.

 

[Blackwood]

You're right. Hmmm...what about because of the split, the water being channeled back as an expanding vortex - a sort of controlled eddies? That would explain why big kicks don't do anything but wasting energy but small kicks with fast cycles really drives the split fins?

My hypothesis is that it simply decreases the resistance of kicking. It also decreases the amount of water moved, hence the necessity to kick with a higher frequency. The extreme case is: take your fins off. You can kick much more rapidly, but each kick reacts much less force.

I don't suspect that there is any sort of controlled pattern behind the fin, split or otherwise. It's all chaotic. The diagram certainly looks cool, but I have a hard time believing it. I've seen actual wing sections in wind tunnels (which are designed to keep the flow attached as far back towards the training edge as possible) that don't behave as cleanly as well.

I may be convinced that curving the blade into an almost half-funnel shape may force more water backwards rather than over the sides. To that end, a wide flimsy center with strong edges with no split may be more efficient.

 

[Eric Sedletzky]

My hypothesis is that it simply decreases the resistance of kicking. It also decreases the amount of water moved, hence the necessity to kick with a higher frequency. The extreme case is: take your fins off. You can kick much more rapidly, but each kick reacts much less force.

I don't suspect that there is any sort of controlled pattern behind the fin, split or otherwise. It's all chaotic. The diagram certainly looks cool, but I have a hard time believing it. I've seen actual wing sections in wind tunnels (which are designed to keep the flow attached as far back towards the training edge as possible) that don't behave as cleanly as well.

I may be convinced that curving the blade into an almost half-funnel shape may force more water backwards rather than over the sides. To that end, a wide flimsy center with strong edges with no split may be more efficient.

Yeah, like Voit Duck Feet or the Voit Vikings.
Those would be the exact type of design you're talking about. Except the Duck Feet had three ribs - one down the middle.
Only thing is they're foot foot fins.

 

[Blackwood]

Duck Feet are stiffer down the center than I was meaning.

Think if you held it out of the water by one edge, it would fold along the length of the blade under its own weight.

When you kick up, the blade of the fin when viewed on the back edge would be almost a cup-up semicircle (depending on blade shape, stiffness of the various components, and how hard you are kicking), somewhat like the permanent shape of the Foil Force (less the split). When kicking down, it would assume the opposite curvature. On both strokes, the curvature in theory helping direct more water backwards, decreasing energy loss.