Well here's the deal... Every test that has been done that I've read has shown the split fins rate as the fastest... but then again - who races underwater... but also in tests when divers swam a course at a constant speed over a given distance, the air savings for divers using split fins was huge... about 30% less air used.
I did not believe any of that stuff... so one of my vendors - Apollo - sent me an underwater speedometer and told me to test for myself... so I did. I have not found a fin that will take me any distance faster than the Apollo bio fins - and with less effort. And I tested with and against current in Cozumel, calm water in cayman, and dry suit diving in texas lakes.
Also I've been able to do some good tests on divers as I'm a Naui Course Director, and our DM and Instructor candidates have to do a timed snorkel swim... They all take a few cracks at it to get the time down... so we have them try different fins. The Bios are consistently faster, and the divers suffer far less fatigue and leg cramps.
I've got some Bio's and Quattro's and a bunch of others sitting by our pool - and invite anyone to jump in and compare - or take a pair out to the lake.
If the technology didn't work, why would the fin that was started by Apollo now be copied by Tusa, Aeris, ScubaPro, Genesis, Sherwood, Atomic.. and on and on.
The Volo's go for more money than the splits - so if selling the most expensive fins was my goal... I'd push those... but that would be a bit of hypocrisy on my part as if you ever run into me out diving - I'll guarantee you see me in splits - and I can dive anything I want.
You can listen to opinions on a board all day long - but the only real test is hop in the water with them. If you do... let me know the results. (although I know already!
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I only "push split fins" because they push me