Split Fin Question

[Pinch me]

Curious... How do split fins work? I've googled this question and got this video (The differences between split fins and paddle fins - YouTube) that mentions vortices pushing you along instead of pure power or thrust from the fin/leg. Seems like you lose thrust with your fin cut in half, and vortices will not give you as much thrust (or at lest it doesn't seem like it).

This makes no sense to me, can someone smarter than me please explain?

 

[scubadiver888]

I'm not that smart but empirical evidence tells me split fins give you more thrust to a point. Back when I was getting my AOW (a while ago) I was partnered with a 20-something with ADHD (literally). It took everything I had to stay with this guy. Research told me I should switch to split fins. I had to drop out of the AOW course because of work. I re-scheduled my AOW training for a few weeks later. For the next dive I bought a pair of Apollo bio-fins. When I went diving I paired with the instructor for my AOW training. I exerted ZERO effort on our dive and at one point we were doing the underwater navigation. I was following the compass and she was counting the fin kicks. When we got back on the boat she gave me heck and wanted to know why I was swimming so fast. It took everything she had to keep up with me. She meant to grab my arm after 30 fin kicks but instead had to settle for grabbing my fin after 40 fin kicks.

At first I was thinking she had gone insane when I realized, "new fins!" The split fins made a HUGE difference.

Years later, I realized that if I got caught in a strong current, I had a hard time getting out of the current with the split fins. No matter how hard and fast I kicked I kind of max'd out and barely beat the current. I now swim with paddle and blade fins. I just decided I need to get my legs in good enough shape that I can power myself through the water. My split fins have actually been unused for 4 or 5 years now.

 

[cb5150]

Oh dear....this is gonna get ugly.

 

[Phil_C]

*Ding* *Ding* - seconds out! - round 2!

P

 

[BRT]

How do split fins work? I'll take the side that says they work way better than anything else and you will die if you don't use them.

 

[Adventure-Ocean]

I used split fins for years as I got old. I found them easier on my legs. I did not need speed or power, I just needed to manuever. Adventure-Ocean

 

[NAM001]

Lets not be to engeenering like here.

Givens. Nothing, nothing gives more thrust than a non split fin. all things being equal except the split.
The split acts like a relief valve such as an opv on a compressor.
If you have a stiff blade split fin it will act like a non split untill you exceed the force required to split the fin. from that point no mattter how much you try the split portion will not generate
anynmore thrust, at least not proprotional to the effort used in kicking.
The softer the paddle of the fin the lower the amount of force needed to open the split in the fin.
So you can call the split a thrust bypass device. The less thrust the less resistance on your legs, less resistance less strain and pain for those not used to using fins.

I have used them and thought they are great in lakes and still water. However i could not go against a current that others were able to do with their non splits. That was 1/2 of a Belise drift dive down the creek. Since then I use non splits. There are those that sware by then but I dont. I have found that if you have minimum resistance in the water you get much more out of them. That means good buoyancy and trim. Coupled with a good frog kicker, one could probably make them work ok because of the lessor resistance from being in trim, and the kicking style. If yuo want an example, put your self in the water virtically and see how much of your body you can lift out of the water by kicking.

 

[Just_Me]

I have and use both. SP Jets and the split SP Twin Jets. I like them both for different reasons.

The Jets give better thrust, which is a confined space or a heavy current really is nice to have. This is a drag car.

The twin jets give easier cruising speed. For a calm water open swim is very nice to have. This is the NASCAR.

Both will do the same things. Neither are best in all situations. Need to get off the line fast or turn on a dime, use the blade fin, there is a leg penalty as you have to work harder when pushing the fin blade in both directions. Need to cruise and sight see and your doing 4 or 5 Caribbean dives a day and you don't want to work very hard, use splits.

 

[KeithG]

Need to cruise and sight see and your doing 4 or 5 Caribbean dives a day and you don't want to work very hard, use splits.

Or a smaller blade fin....

From an engineering perspective the "relief valve" analogy above is good.

 

[Lorenzoid]

Curious... How do split fins work? I've googled this question and got this video (The differences between split fins and paddle fins - YouTube) that mentions vortices pushing you along instead of pure power or thrust from the fin/leg. Seems like you lose thrust with your fin cut in half, and vortices will not give you as much thrust (or at lest it doesn't seem like it).

This makes no sense to me, can someone smarter than me please explain?

So far, none of the replies actually attempts to answer the question of "How do split fins work." This is probably because there does not appear to be any principle of physics behind split fins that magically gives them what marketing people would like you to believe: that you can get the same amount of thrust as non-split fins with less effort. I suspect the "vortices" explanation in that video is bunk, at least to the extent that it is interpreted as implying the split gives the fin some sort of hydrodynamic boost. The explanation sounds a bit like an analogy to an airplane wing or other airfoil (or hydrofoil, I suppose), where you've got some sort of lift-thrust relationship induced by the shape of the structure. But I have yet to hear anyone explain it using physics in a persuasive way. The "vortices" explanation in the video smells like marketing hype to me. Physics says you don't get something for nothing.

I believe the "relief valve analogy" that KWS and giffenk mention is apt. An analogy to a gearbox might be, too. If you want to exert less effort than you would have to exert to get X amount of thrust out of a paddle fin, split fins let you do that, but at the cost of the resulting thrust being less than X. (Hey, how about designing fins with an adjustable split width, so we can "shift gears"?)