Split Fin Bashing?

[dmoore19]

Not overly concerned about it. I deal with fluids and hydraulics daily, I don't claim to be an expert when it comes to fins. There IS a pressure differential from the one side of the wing/fin to the other. How that differential is generated may be different but none the less it is there. There are a lot of other resources to show this is true but Wiki explained it in a fairly simple way. Actually I believe it has more to do with velocity than density. But that is another can of worms killed needlessly.

I will bow out now because it really makes no difference anyway.

Dmoore19

I wouldnt take wiki's definition too seriously. First of all lift does not counter drag. Lift counters gravity. thrust counters drag. when lift adn thrust exceeds gravity adn drag the plane will fly. Next a wing and a foil like on a plane although yeilding the same result does it is opposite ways. the "lift from a wing is from the surface areas being different causing a change in densit of the air above as compared to below the wing. In reference tothe top surface teh wing is said to lift up. from the lower side of the wing the wing is said to be pushed up. This motion is caused by force created from differences in fluid(air) density.

Now from a water stadpoint you can not achieve the same effect by the same means. Ther is no change n fluid (water) density. The force is derived by the teh wing or foil physically interfacing with the water and only moves cause the wing mass is less than the water mass. Both the wing and water are non compressables and it is a attle of which one will give in first. You will not find a submarine with airplane like control surfaces. both sides are alike. the top surface is the same area as the bottom.

Next you could get a plane to fly with flat wings angled up instead of wings with upperand lower surface areas differing. It just takes a lot more speed to create the force to get the LIFT. The example of pushing your self off a wall is a very accurate discription of hydrodynamics. Which by the way if you were bigger than the wall the wall would move and not you.


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[Garth]

None of these posts tackle the majority of issues I see with people using split fins.
1: used in the absence of good buoyancy control
2: used by instructors that sell split fins
3: create a "split fin soup" of flying debris and sediment in the water column when used with problem number 1.

When there is no sediment to worry about I see no reason why others cannot use split fins.


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[REVAN]

The definition of "lift" can only be achieved in a compressible gas.
Hydrolic means hydro or water and water is not compressible.
From the diagram you attached you can see the area of low pressure and the area of high pressure, this applies to AIR.

Hope this helps

Wow! Peoples understanding of fluid mechanics is eye opening. No wonder there are so many useless gimmicks in fin design. Few of the customers know of what they are buying.

Lift has nothing to do with the compressibility of a fluid. Air can be considered incompressible at speeds below Mach 0.4 . The same goes for water, but the speed of sound in water is so high that Mach 0.4 in water is up somewhere around 3 thousand miles per hour.

Lift requires circulation and creating circulation in a fluid by mechanical means requires viscosity. Air and water both have viscosity and both can generate lift. All liquids and gases have viscosity (except for cryogenic superfluids).

Lift is a definition. It is simply the component of fluid dynamic force that is orthogonal to the direction of the freestream flow. There is no up or down associated with it. Drag is the component of the force that is inline with the direction of the freestream flow.

Paddles create force that is primarily drag oriented. Foils and wings create force that is primarily lift oriented. Scuba fins tend to be somewhere between but usually leaning more toward the paddle functionality. A few systems like this X-15 scuba fin (in the youtube video link below) are true foils and demonstrate what is possible when applying propulsion technology that is leveraged from utilizing lift forces to create thrust rather than using drag forces to create thrust.

James Bond Would Love this X-15 SCUBA Ready Monofin - YouTube

---------- Post added May 27th, 2014 at 09:39 AM ----------

Paddle fins vs. Split fins:

I think some people bash split fins because they are power limited. In fairness, paddle fins are power limited also, but usually have a higher power limit than split fins. Paddle fins can be used efficiently at low power settings by frog kicking. When high power is needed they can be flutter kicked to activate the fin flexure and generate thrust from the recoil of the blade. Split fins are usually quite soft and use the flutter kick at low power settings. If you add too much power input, they will over flex and lose efficiency. This, of course, all depends on the fin design and the blade stiffness. There are both stiff and soft flexing paddle fins out there as well. These are just some general observations for many of the common fins of each type I have seen in use.

 

[BDSC]

None of these posts tackle the majority of issues I see with people using split fins.
1: used in the absence of good buoyancy control
3: create a "split fin soup" of flying debris and sediment in the water column when used with problem number 1.

When there is no sediment to worry about I see no reason why others cannot use split fins.

My experience has been that divers with poor buoyancy can silt or stir things up no matter what type of fin they use and diver with good buoyancy control won't no matter what type of fin they use. "Split fin soup" has little to do with split fins except being part of that title.

 

[GrandpaScuba]

Some people like them, some do not. It is a time-honored tradition that divers dream up rationalizations as to why equipment they don't like is "bad."

 

[victorzamora]

Wow! Peoples understanding of fluid mechanics is eye opening. No wonder there are so many useless gimmicks in fin design. Few of the customers know of what they are buying.

THIS is my biggest issue with all gimmicky fins. Divers walking out of a shop having paid $200+ for some stupid fins that have "extra thermo viscous turbulo-lifting encabulators with force-enhancing high-tension spring bearings." I'm not saying the fins suck, but they're gimmicks that people get suckered into.

Lift requires circulation and creating circulation in a fluid by mechanical means requires viscosity. Air and water both have viscosity and both can generate lift.

Not being picky.....but there are inviscid models of lift. I spent FAR too much time on inviscid lift model calculations to think anything else.

Lift is a definition. It is simply the component of fluid dynamic force that is orthogonal to the direction of the freestream flow. There is no up or down associated with it. Drag is the component of the force that is inline with the direction of the freestream flow.

(sarcasm) Nuh-uh!! You can't lift things down, can you? /argument! (/sarcasm)

I think some people bash split fins because they are power limited. In fairness, paddle fins are power limited also, but usually have a higher power limit than split fins. Paddle fins can be used efficiently at low power settings by frog kicking. When high power is needed they can be flutter kicked to activate the fin flexure and generate thrust from the recoil of the blade. Split fins are usually quite soft and use the flutter kick at low power settings. If you add too much power input, they will over flex and lose efficiency. This, of course, all depends on the fin design and the blade stiffness. There are both stiff and soft flexing paddle fins out there as well. These are just some general observations for many of the common fins of each type I have seen in use.

There are two other reasons (other than gimmicky sales crap) that I can't stand split fins for: power limited is one. The other has also been said before.....there's nothing you can't do with paddles you can with splits, but the opposite can't be true.
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I know I was being overly picky, but it was mostly as a joke. That was a good post.

 

[Garth]

My experience has been that divers with poor buoyancy can silt or stir things up no matter what type of fin they use and diver with good buoyancy control won't no matter what type of fin they use. "Split fin soup" has little to do with split fins except being part of that title.

Alrighty then. So you have some sort of experience with split fin divers that don't silt out an area that has difficult bottom?

Can you provide video proof that a split fin diver can fin near the bottom without disturbing the silt?

I don't care what fin you use as long as you don't kill the viz... Is all I'm saying... I don't give a crap what color, shape, split, paddle, spring, dimension or any other component you carry...

I mean that in the most fair, reasonable way possible. My apologies to proficient split fin divers who don't stir up the silt.

Garth


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---------- Post added May 28th, 2014 at 08:08 PM ----------

Yes non split fin divers can kill the viz as well. I make sure I provide feedback to the people I dive with regarding their ability to maintain a high viz environment. If someone is stirring up the silt I tell them about it so they are aware of it as some people don't ever look behind themselves...


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[boulderjohn]

Alrighty then. So you have some sort of experience with split fin divers that don't silt out an area that has difficult bottom?

Can you provide video proof that a split fin diver can fin near the bottom without disturbing the silt?
....

Yes non split fin divers can kill the viz as well.

Disturbing silt is primarily a result of kicking technique. A flutter kicker near the bottom will stir up silt regardless of the fins being worn. I am quite sure I can go through a silty area with spit fins without stirring up silt. I won't do it as efficiently as I would with good, stiff, paddle fins, but I can do it.

 

[BDSC]

I am quite sure I can go through a silty area with spit fins without stirring up silt. I won't do it as efficiently as I would with good, stiff, paddle fins, but I can do it.

Sorry John, you just saying it isn't good enough. You need video proof!

 

[boulderjohn]

Sorry John, you just saying it isn't good enough. You need video proof!

It's pretty hard to find someone with good frog kicking technique wearing split fins. It would have to be done for the purpose of demonstrating it for your benefit. I think anyone who has had the proper training will agree with what I wrote without hesitation.