Vents on Jet Fins?

[Eric Sedletzky]

What exactly are the vents supposed to do on Scubapro Jet fins?
I was looking at them and wonder if they are actually counter productive?
It looks to me like on the downward stroke of a standard from the hip flutter kick that water will gush through those vents but what would be the benefit?
It seems like that’s wasted thrust.
I know those Scubapro version fins are from a 1965 design but what sort of science were those fins based on? Or was there any?
Was it a marketing gimmick?
People use them now doing all sorts of frog kicks, helicopter turns, backing up, etc. and love them, but back then nobody was doing that stuff (that I’ve heard about anyway), it was all standard flutter kicking.
I can confirm that they do suck for regular flutter fin kicking, but they do work well for everything else.

 

[tridacna]

I bet @Bob Evans could explain…

 

[theduckguru]

Reduce drag.

 

[theduckguru]

Reduce drag. The kick style is supposed to be a short flutter kick from the hip or knees.

 

[Robert H. Diver]

There’s a reason those type of fins have those, like duck said ^, reduces drag combined with a stiffer fin enables a more powerful kick.

 

[Robert H. Diver]

.

 

[rx7diver]

What exactly are the vents supposed to do on Scubapro Jet fins?

I wonder if you can think of a single fin as consisting of two blades, one slightly overlapping the other. So what actually contacts the water on the downward stroke is effectively a longer fin (a longer blade).

Also, will water being pushed through the space between the two blades be accelerated, Bernoulli-like, like what happens between a jib and a mainsail on a sloop, effectively giving a greater push?

Thinking out loud.

Any hydraulic engineers here?

I was told a very long time ago that when the design was still under patent, manufacturers who wanted to copy the design were not allowed to overlap the two blades.

rx7diver

 

[Eric Sedletzky]

What I do know is that with my freediving fins they are just a straight flat blade but long.
They are about the same amount of resistance to move but I get at least double the speed.
It’s a slow long flutter kick from the hip.
With Jets if you try that you will wear yourself out and not really get anywhere, at least not for me.
Like I said, they work great for all kinds of other finning techniques, but for standard flutter kicking they suck.
I’m still in the market for something else. I just don’t what yet.

 

[Trace Malinowski]

Sometimes when engineering meets human performance there is a disconnect. One of the best tutorials on flutter kick problems can be found in Umberto Pelizzarris Manual of Freediving. My ex-girlfriend, a pro figure skater, had the leg and ankle strength to make the engineers smile as she churned water making power fins seem as easy to bend as children's "flippers." You can spend half your life in freediving fins as I did and still have some weakness in the stroke. I have to remember to not let the fins drive my toes inward on a down thrust. I have to point my toes and think about a flutter when I demo a flutter. Figure skating since age 7 made her a demonstration kick model in the first few minutes of training.

 

[Still Kicking]

I am not sure what the vents on Jetfins are supposed to do for thrust or what they do in the real world. It would be interesting to put them in a wind tunnel and look at laminar flow (has that ever been done for Jets?)

My layman's observations are that the vents, sharp edges and stiff rails provide a lot of surface area that translates into torque for frog kicking, helicopter turns, backward kicks etc. This is why they are beloved by many tech divers and others who like the precise control they provide in the water. No doubt here's a performance penalty for that, but so be it.

Freedive fins rock for straight line kicking (down, forwards, up) but provide much less ability to turn in tight quarters, as scuba tends to require.