Split Fin Physics

[Blackwood]

TN Traveler, I'd like to see this thread go back to the physics of split fines. As there seems to be little to no data to base such a discussion on, that seems unlikely. If however its fine for the thread to evolve into subjective assessments of fin performance, I think its just as valid for it to evolve in any other direction any other member would like to discus. I really don't see why someones opinion on a fins performance per dollar, is any less valid than their opinion on the performance alone.

I think there is value in the bolded portion as it pertains to the topic of the thread. Comparing various fins may not answer the OP directly, but I'm having fun constructing hypotheses (right or wrong) based on observations of how a variety of fins feel and perform, however subjective they may be.




If one could show decisively that on fin 'performs better' than all others, then it's likely that many people would buy them, which would allow the mfr to lower the per-fin price as the non-recurring development and production costs would be better amortized. That aside, I personally consider price to be irrelevant in this discussion, though I won't complain about people discussing it.

 

[Kiltboy]

For some reason I like to have both. I blew by people with my Xpert zooms in Cortez and i loved em , although there was always little current. Must swim on back to the boat if I surface away from boat...snorkel check.... I always use full fins when I go home to dive the rivers in Nor Cal.

 

[Kern]

I think there is value in the bolded portion as it pertains to the topic of the thread. Comparing various fins may not answer the OP directly, but I'm having fun constructing hypotheses (right or wrong) based on observations of how a variety of fins feel and perform, however subjective they may be.

If one could show decisively that on fin 'performs better' than all others, then it's likely that many people would buy them, which would allow the mfr to lower the per-fin price as the non-recurring development and production costs would be better amortized. That aside, I personally consider price to be irrelevant in this discussion, though I won't complain about people discussing it.

I'm afraid I lost interest in the thread a while ago. I find it interesting that the post I was responding too was thanked twice. Both times by someone with a vested interest. But then, I'm obviously just another weirdo with hang ups who should take their nasty ideas to another thread.

 

[John C. Ratliff]

SeaRat, I have been curious about the higher performance of full foots. One common difference between full foots and open heels is that many open heels have an empty space around the toe pocket on the underside of the fin. The full foots tend to have a smooth upper and lower profile around the foot pocket. The empty space would be a major source of drag and consequent inefficiency. Of course, the open heels also generally have more protruding pieces of stuff associated with the heel strap. These will also cause greater drag. I'd like to know 1) which open heels do not have that empty space and whether they tend to perform better; and 2) which full foots have an equivalent empty space and whether they tend to perform worse.

Imorin,

You've brought up things I had not thought about for a while. It would be worth looking at though. I know that before the monofins were used in competition, full-foot long bi-fins were used, and were pretty efficient. Because of the split, split fins can provide more thrust with less blade, as the water as channeled to the rear by the action of the blade. And the ones I used in the pool were smaller full foot ones which worked as well as the bigger open heel fins. Drag is a part of the equation, and so has to be taken into account.

SeaRat

 

[John C. Ratliff]

SeaRat

I do not disagree that it is possible to achieve precision positioning with splits - I did it with my Bio-fins for quite awhile - I have just found it is easier with blades. To be honest, the best fins I have found to date that almost do it all are the Tan Delta Excellerating Force Fins - they feel as efficient as a split, but can provide precision positioning like a blade. Of course to achieve the best of both worlds you need to adjust the wings on them to match what you are trying to do.

I just got a pair of Force Fin Splits (Foil Force) and the new "Hockey Fin - both of which seem to have elements that you have identified as being important to fin performance. I have not had the chance to test either, but I will be trying them both out on a trip in May and will post my impressions (probably on the FF Forum). This is not the place to discuss the characteristics of these fins - but why don't you take Bob Evans up on his offer to let you test these - I would be interested in your experience/evaluation.

I am a engineer/scientist and like your methodology - probably as close to an unbiased test as possible. By the way - try the Force Fins with Comfort Inserts instead of socks - it will eliminate the drag and give you a better comparison.

This has been a great thread - hope we can keep it going without individual prejudices influencing the rational discussions.

TN Traveler,

I will probably be contacting Bob fairly soon. But I have been very busy with work, and now my main computer is ready to be picked up but I will need to re-install some fo the software, so those areas have some priority.

SeaRat

 

[Pullmyfinger]

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

That's funny.

-Mitch

 

[tome]

id have to say cressi frogs are a fin ive seen in more locations than anythign else, highly adjustable and size's available seems to make them the choice for most dive operators ive seen, and they have not changed in however many years other than the occasional color change...

great thread, raises some fantastic questions about why people buy particular fins and again comes back to personal preference and training...

 

[Kondado]

Hello everybody,

first things first, thank you all for this very interesting and educative thread. I've read every post you all have written (i.e. 217 posts!) which took me some hours (english isn't my mother language, but even if...).

To the few saying "hey it's just fins" i'd like to say "just go away and leave this thread alone".

It's like if you were buying a car or some boat and say "hey, it's just the engine"...

Now having said that, let's go to the interesting stuff... there were a lot of opinions I found interesting or worth commenting but right now I don't remember all of them, so I'll stick with what I do remember.

Actually the first thing I found intersting and funny at the same time was that Blackwood had exactly the same idea with the drawing he made on the split fin. Indeed it was the same day (that is, yesterday) that I thought about the split fin and said to myself "actually, if it worked like a wing, the 'lift' would go perpendicular to the blade of the fin"?!

After reading the whole thread, I have come to the following conclusion but I don't know if I'm right...

(i am not allowed to post this link, copy and replace the XX with tt)

hxxp://i472.photobucket.com/albums/rr84/marknekk/splitfin.gif

The idea is the following, my guess to why the split fins are easier to kick with, is because of two things

1) Lower resistance because of bernouilli-venturi effect caused by the split fins' blades. That are the two white arrows which together form the "red vector" arrow. So the split fins would actually help or "assist" your kicking.

2) The propulsion itself comes from the water which is pressed at higher speed through the split by the divers kick. Here's where Newton's third law would come into account and which would explain the actual forward movement of the diver.

3) The lower thrust compared to a paddle fin is explained because of the same water going trough the split.

So maybe making a wider split fin would move more water trough the split resulting in more thrust. Although then you would have the problem of fins colliding with each other. It would be interesting to see a wide mono-splitfin.

These are only subjective or pseudo-scientifical thoughts!! But I just wanted to share them with all scubaboard members.

In the end my conclusion is the same as everyone elses: the best fin doesn't exist and will never exist, since it depend's on a lot of factors like your kicking style and much more. It's the same reason there is no perfect car, ship or airplane.

But nonetheless it would be really useful to know the basic principles of a fin. It surely isn't an easy thing, as we can see from the formula 1 aerodynamics, where each team is always doing thousands of tests not knowing exactly why some things work while others don't.

Probably I have forgotten to comment a lot of ideas I had, but well, it's enough for the moment.

Thanks for reading. ;-)

 

[Kondado]

Oh yes, now I remembered one important question I'd like to ask.

Blackwood and "SeaRat" talked about the non-compressibility of water. But then blackwood said that it's not about compression but about density.

Could you explain this a bit more?

Does this mean that there can't be a bernouilli-venturi effect underwater???

I am referring to post #103

John -

Can you post your photos to scubaboard directly? Where you have them is blocked on my webserver (I can't see them).

Quote:

Originally Posted by John C. Ratliff View Post
The part that is wrong has to do with the "flying" and "wing" concepts, as those apply to air and not to water. The reason they don't apply to water is that water is incompressible, and therefore there is no possibility of a "low pressure" area above the wing.

Low speed flight is considered incompressible as well. Incompressible doesn't mean that the pressure is constant. It means the density is constant.

 

[John C. Ratliff]

I'll see what I can do to explain this in another post, perhaps over the weekend (I'm pretty busy right now). In the meantime, if anyone wants to see the drawing that Kondado put together, here it is corrected (added the "t"s instead of "x"s).

http://i472.photobucket.com/albums/rr84/marknekk/splitfin.gif

SeaRat