The hydrodynamics of Jetfins

[DPOP]

I recently got into a heated debate with a non-diver friend who claims that while the jetfin may be great and popular, its "jets" are simply for show, and cannot possible improve the characteristics of the fin. Can anyone refute this claim?
why are Jetfins so great? does it have to do with the "jets"?
Thanks
David

[Walter]

Blindfold him and have him swim across the pool with the vents blocked and with them open. Don't tell him which is first.

[DPOP]

Good idea, but I still want to learn the hydrodynamics, in order to make a logical argument.
David

[Walter]

If I can find an old Scubapro catalog, I'll tell you how they explain it.

[jonnythan]

Well, it seems to me that the idea is like this:

Imagine if the jets weren't there. The fins have to push away water in order for you to move them and create linear force. They have to push away *all* that water on the underside of the fin for a 2 foot or so stroke. If you put some nice big vents near the foot pocket, the water can slip right *through* the fin instead of having to get out of the way. And since the vents are right where the fin flexes the least, pushing against the water there would just add vertical resistance, which doesn't contribute to propulsion. This way, you're really only pushing against the water on the part of the fin that flexes upward, so the reaction force moves you forward.

[rcohn]

There were test done that showed there is no difference in efficiency between vented and unvented fins, an article on this can be found at Undercurrent online. Of course if you take a pair of fins designed with vents and then block them you will change the fin's performance.

Ralph

[Walter]

I've yet to find another pair of fins that work as well.