What's the purpose of the holes in jetfins?

[diver_doug]

I'm hoping for a fairly technical answer. I see how the holes would reduce resistance/effort, but since water is just flowing through those holes it seems like they would reduce propulsion and control as well. But given that the Jetfins have been that way for many years, and many other fins have a similar design (including the relatively new F1's) I'm assuming that the holes must be beneficial. So what exactly do those holes do...and would there be a benefit to adding holes to paddle-style fins that have none?

 

[meesier42]

according to the most comprehensive study on fins really ever completed at the University of Buffalo... nothing.

to quote them
"The use of vents, either forward or rearward facing does not improve economy as was seen in this previous study, apparently water does not pass through the vents, thus they do not relieve the negative thrust in the recovery phase. Also, the vents would presumably "leak" water, and hence reduce the pressure difference that results in thrust, during the power phase." from Evaluation of fins used in underwater swimming, by DR Pendergast et al, circa 2003.

 

[diver_doug]

according to the most comprehensive study on fins really ever completed at the University of Buffalo... nothing.

to quote them
"The use of vents, either forward or rearward facing does not improve economy as was seen in this previous study, apparently water does not pass through the vents, thus they do not relieve the negative thrust in the recovery phase. Also, the vents would presumably "leak" water, and hence reduce the pressure difference that results in thrust, during the power phase." from Evaluation of fins used in underwater swimming, by DR Pendergast et al, circa 2003.

hmmm...it would be interesting to know if anyone on the board that has a pair has ever tried taping them closed to see what difference (if any) resulted.

 

[Scott L]

I'm hoping for a fairly technical answer. I see how the holes would reduce resistance/effort, but since water is just flowing through those holes it seems like they would reduce propulsion and control as well. But given that the Jetfins have been that way for many years, and many other fins have a similar design (including the relatively new F1's) I'm assuming that the holes must be beneficial. So what exactly do those holes do...and would there be a benefit to adding holes to paddle-style fins that have none?

I thought I was bored tonight.

 

[diver_doug]

Maybe they're like the "speed holes" Homer added to his car.

 

[ptyx]

Just an hypothesis - but the holes might be there to fine-tune the flexibility of the fin: encourage the fin to flex just after the foot pocket rather than on the foot itself or further down.

 

[txapacheguy]

I too have wondered about the slats/holes in the jetfin.

Perhaps if I ever become highly skilled enough at my "propulsion techniques", I will tape a set up and test them out. Maybe someone should tape up a fin on one foot, but leave the other one alone and see if there is a difference between the left and right leg, or if the person tends to list/turn to one side or the other with this configuration....

 

[drdaddy]

I too have wondered about the slats/holes in the jetfin.

Perhaps if I ever become highly skilled enough at my "propulsion techniques", I will tape a set up and test them out. Maybe someone should tape up a fin on one foot, but leave the other one alone and see if there is a difference between the left and right leg, or if the person tends to list/turn to one side or the other with this configuration....

This is hilarious. I am laughing so hard my eyes are tearing up. I don't know which is funnier - whether you are serious or whether you are not.

Thank you for the good laugh.

 

[vladimir]

This is a bit of a reach, but you may find it interesting. It's from an article in the Journal of Applied Biomechanics, which concludes that the optimal finger position for a competitive swimmer is slightly spread apart. Since it's the Journal of Applied Biomechanics, they've actually gathered some data to support the conclusion, but they don't do a lot of theorizing. (I guess you have to go to the Journal of Theoretical Biomechanics for that.

One can note that the drag coefficient presented the highest values for an attack angle of 90° in the three hand models. The lift coefficient resembled a sinusoidal curve across the attack angle. The values for the lift coefficient presented few differences among the three models, for a given attack angle. These results suggested that fingers slightly spread could allow the hand to create more propulsive force during swimming.

Anyway, here's the reach: what if the vents in a jet fin (or the split in a split fin, for that matter) are analagous to the gaps between a swimmer's fingers? As I understand the article (and I don't pretend to have studied it carefully), the flow across the dorsum of the hand prevents flow through the gaps between the fingers (when held at the optimal spread), effectively enlarging the paddle surface. Of course, one crucial difference between a hand and a fin is that you're stuck with the hands you got, whereas you can build a fin as big as you like. So, if my theorizing has any validity (and I wouldn't argue strenuously that it does), the vents serve to make the fin behave as if it were a larger fin--as large as that fin would be if you filled in the vents. The net benefit: a little weight savings above water, a little plastic or rubber saved in production, and a marketing coup, like the tailfins on an old Cadillac.

 

[bowlofpetunias]

Don't forget the other benefits hang them on a peg to dry.... run a pretty ribbon through them to add some personality.. so you can pick your fins out of the pile on the boat..... escape route for small fish that get caught in the vortex you created finning with all the Power Jet fin Owners claim to get...

:doh: I forgot to mention.. a good way to hook a John line so your hands are free....