Split Fin Bashing?

[mdb]

All seem to have the desired configuration for the dive at hand. All the internet conflict is just BS.

What we really want is to get weightless and enjoy the Underwater World.

 

[DaleC]

The bumble bee analogy doesn't really translate. In that case the comparison was between two different means of propulsion. Swimming attempts to use fins in much the same way fish do, though less efficiently because we don't have bilateral symmetry frontal/dorsally. That's why most swim fins are designed asymmetrically.

I think observing fish fin shape, and the mechanics used to propel them is pretty important and translates to diving fin shape. just as bird wing shape can translate into aircraft wing design. Observing how fish/birds use their propellants can teach a diver how to use theirs.

Your fin shape dictates the how you will kick it and what it will be best for. If you want to know that just watch this whole biosystem of organisms already using those fins shapes. will it be an exact match - probably not; but if one is thrashing about in freediving fins, kicking monofins from the ankle of trying to derive great speed out of splits one can pick up the right way to use them.

ps - gone divin. No replies for a while.

 

[boulderjohn]

The bumble bee analogy doesn't really translate. In that case the comparison was between two different means of propulsion. Swimming attempts to use fins in much the same way fish do, though less efficiently because we don't have bilateral symmetry frontal/dorsally. That's why most swim fins are designed asymmetrically.

Fish are shaped differently than humans, and their bodies move differently than humans. Bumble bees are shaped differently than fixed wing aircraft, and their wings look and move differently than fixed wing air craft. It would be wrong to assume that wings of a fixed wing aircraft should look like a bumblebee's wings. In fact, such a craft would be unable to fly.

Why then should a human's fins look like a fish's?

 

[DaleC]

Human fins do not look like fish fins as I stated, because we do not have symmetry in the frontal/dorsal plane. However, the means of propulsion are similar in that we move the fin back and forth to produce lift (or as we call it, thrust).
This is not my opinion. Just look at long free diving fins and how a similar fish fin (eel) moves. Now watch how a free diver generate the best thrust from his/her fin. Look at a monofin and then observe a dolphin. Watch a proficient monofin diver (like the video I linked earlier). Similar.
Anyone wanting to generate more thrust with paddle fins would benefit from watching pelagic fish generate thrust by using flank muscles (not flutter kicks) etc...

 

[Frosty]

cutting through all the rubbish-My wife uses splits and I use paddle fins as well as Vole's
My wife is physically incapable of "overpowering" splits and finds them very easy on her ankles and hips. She can drive paddles and snorkle type full fut fins but doesn't like them and finds them exceptionally hard on her legs..
In a straight out drag race she gets off the line faster than me but once the paddles are driving i'll outrun her.

 

[Garth]

http://youtu.be/ADfHfssrv_I

Funny there are moments of this old video that have split fin silt outs and regular fin silt outs. This was a pickup dive. Not sure I ever dive with these fellas again...

The thing that I have experienced with divers with split fins is that the majority of them aren't good divers.

There may be good divers that happen to use split fins but most of the divers I see off the coast of North Carolina and in the quarries are frequently vertical and have little in the way of fin awareness. They silt up the bottom, have horrible buoyancy control and are out of shape. I do know two divers that sometimes use split fins that are in my opinion good divers but when they use split fins they also stir things up.

What the heck do I know though. I only have an opinion..

Sorry split fin users but I find them to be a bit of a mess,
Garth McClune


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[reghunnicutt]

Hey Garth.....I'm taking split fins to American Quarry this morning because of the controversy and Jess said they are easier on the legs and ankles. I'm not back finning, and wanted to give them a whirl since I was shamed into paddle fins on my first tec class over a year ago. I'm diving with some guys off the NC forum so I'm sure they'll be talking behind my back.


Reggie in Midland, NC

 

[Garth]

Haha! Reggie, no one cares unless you stir up the bottom!! :-D however if you are going to American Quarry you can stir the bottom all you want as I will be as LKN Quarry.


As far as split fins being easier on your legs and ankles, this may be true. What I've found is that the people who say that are often found kicking the entire dive regardless if they are going somewhere or not. Proper buoyancy prevents the need to kick when staying in one location.

Have a great dive Reggie with Jess and enjoy American! I need to make it out there. Is Jess diving his Rebreather?

Garth


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---------- Post added May 25th, 2014 at 06:34 AM ----------

One other thing to consider if your ankles and knees are hurting would be the trim of your body. If you are in a semi vertical position yet you are trying to get somewhere horizontally you will have more drag against your body. This could be some of the appeal for splits as well. If you wear ankle weights and prefer a vertical position in the water split fins may be for you.

Garth


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[Eric Sedletzky]

I don't know if it's true or not:

...but I was told the whole "Lift" idea in fin propultion is a myth because water doesn't compress like air. So with that in mind, the whole theory of lift with an uncompressable medium is false. With air the theory is a fact because an object will always be moved toward an area of low pressure because the medium (air) is trying to equalize, and this is only possible in a meduim that can compress. The wing just happens to be in the way of this attempt of air to equalize so it gets moved in the direction of the low pressure, which in the case of the design of an airplane wing is up. The top plane measurement of the wing is longer than the bottom measurement causing air to take longer to move over the surface of the top plane thus causing an area of low pressure.
If you were to put the same wing underwater and move it like it was moved in air it would just dive and go in circles because of the shape. Water fins, like on a sub or keel fins on sail boats or rudders are symetrical and to change directions they must be turned to cut through the water in a different direction. This has nothing to do with the concept of "lift".

With a prop on a boat, the screw action of the prop turning is essentially cutting a path through the water like it would cut a path through clay and the prop is secured in a semi dense meduim (water) similar to how screwing a screw into wood would be. Different pitch props are numbered 15 pitch, 17 pitch. That means that the prop will move 15" or 17" inches forward in one revolutuion - revolving a dense non compressable medium.
Fins do basically the same thing to a degree, your just pushing yourself off a semi dense non compressible medium. This whole idea that split fins create a spinning vortex and that's what propels you is not true. What the water does behind the fin after it has been agitated and sent into motion doesn't have any affect on any forward movement. The actual movement of the fin is what moves you created by it cutting through the water and the immediate pushing affect is where the propultion comes from.
Splits have two surfaces that cut through the water to the two inside planes and when the fin is moved up or down the cutting action kind of happens at a slight angle to the direction of travel off both egdes of the blades instead of straight off the back of one surface like conventional paddle fins.

Aircraft props on the other hand use both the concept of cutting like a screw and lift. The blades of an aircraft prop are angled like a boat propeller and the blade itself has a shape like an aircraft wing with one side flat and the other side bulged for lift, so it gets an added bonus.

This is just what I heard though.

 

[dmoore19]

There is still a pressure differential.