**Split Fins – Let the Bashing Begin!**

[richerso]

If one looks closely at the top paddle fins of today, the Mares Avanti Superchannel, the Aeris Mako, etc. they all have flexible center panels, much like a taped split fin.
Pesky

Ahh yes. They all have flexible center panels; BUT, they are not split.
The geometry of the stiff outer rails coupled with the flexible center panels will "trap" the water in that cup and expel it aft -- this means you get pushed forward.

FWIW, there is still some spill over around each of the two rails. I once saw a marketing brochure for split fins that showed this really cute "twin swirl" of water swirling off the aft end of the split fins. I think the marketing claim was that two swirls of water coming off the split fin was somehow better than what paddle fins can produce. The problem is, if you look critically, that a paddle fin has spillover off each rail. You know what that spillover looks like (if I were a marketing person, I would create this poster) ?? It looks like two swirls of water coming off the back of the paddle fin ! ! !

By the way, for anyone who doubts spillover effects; just go talk to Boeing. The tips of aircraft wings suffer from tip spillover effects as well. That's the reason we are starting to see those wierd upturned tips at the end of Boeing wings. It's an attempt at reducing spillover. The outer rails on a nice paddle fin perform the same function; although fin rails are noticeably less pronounced (obvious for some folks) than the wing tips of a Boeing aircraft.

 

[pescador775]

Rich, old news and I'm not sure why this info is directed at me. However, compared to the numbskulls on the previous page it is refreshing to read. Thanks.
Pesky

PS, winglets supposedly prevent "spill up" from the high pressure zone under the wing. If the air is allowed to migrate over the edge vortices are created which increase drag or harm stability, not sure which, maybe both.

 

[TheAlmightyHubbard]

//Hugs ScubaPro Twin Jets

 

[Clammy]

I have a solution to everyones fin problems!

Trained pet dolphin with a harness.

 

[Bob Evans]

Ahh yes. They all have flexible center panels; BUT, they are not split.
The geometry of the stiff outer rails coupled with the flexible center panels will "trap" the water in that cup and expel it aft -- this means you get pushed forward.

FWIW, there is still some spill over around each of the two rails. I once saw a marketing brochure for split fins that showed this really cute "twin swirl" of water swirling off the aft end of the split fins. I think the marketing claim was that two swirls of water coming off the split fin was somehow better than what paddle fins can produce. The problem is, if you look critically, that a paddle fin has spillover off each rail. You know what that spillover looks like (if I were a marketing person, I would create this poster) ?? It looks like two swirls of water coming off the back of the paddle fin ! ! !

By the way, for anyone who doubts spillover effects; just go talk to Boeing. The tips of aircraft wings suffer from tip spillover effects as well. That's the reason we are starting to see those wierd upturned tips at the end of Boeing wings. It's an attempt at reducing spillover. The outer rails on a nice paddle fin perform the same function; although fin rails are noticeably less pronounced (obvious for some folks) than the wing tips of a Boeing aircraft.

All paddle fins have water spilling off at 180 degrees when it is pushed through the water. That is why you can see some of the fins on the market wobble when going through the kick cycle. Split fins do not have spill over as much because there is a split more of the water escapes through the split. Some of the marketing that is used in demonstrating water reactions - “twin swirl” was taken from marketing that was used in the 90’s from an old the Foil Force Fin ad.

We used these “twisted Swirls” to drive the point home that the fin has 2 foils turned into a curve shape and made of a material that retains its shape, cupping the water and does not move away from each other. It just bends and snaps back to recover – creating energy. Our Foils don’t move away from each other losing energy. http://www.forcefin.com/FF_PAGES/FF_Products/foil_force_product.htm

Winglets- I first started playing with winglets in the 80’s then patented them on my Flying Force http://www.forcefin.com/Merchant2/m...uct_Code=FY&Category_Code=51&Product_Count=11
in 1995, but before that I had numerous companies seek out the proto types to test out in various transportation industries.
The group from San Diego involved with the Stars and Stripes (America 3) racing in the world cup used the winglet designs from Force Fins to perfect their high tech keel to obtain maximum speed without drag. They actually took the fin and studied its characteristics in the Boeing wind tunnel. Winglets are proven and that is why I decided to put them on the Flying Force Fin. Just a bit of history and more to come about winglets, VBC-variable blade control. A fin can be smart!

http://www.youtube.com/watch?v=ACOl9ud6lTE
Flying Force Fin in action!

 

[richerso]

It's all good!
Pescador, it wasn't directed at you. I was agreeing with your point. My comments weren't supposed to annoy you in any way. I apologize if it came across that way. Winglets do work. That's the whole point of using them. All paddle fins do have water spilling off at 180 degrees. That's the whole point again.

It's all good. It's all a good argument for not using split fins.
Then again, I might still cut six splits in my fins

 

[Teamcasa]

Here's a question:
I contend that a significant amount of your speed is determined by how strong a swimmer/kicker your are, the fin type or style is only a minor part of the equation.
What do you think that percent is?

 

[richerso]

Here's a question:
I contend that a significant amount of your speed is determined by how strong a swimmer/kicker your are, the fin type or style is only a minor part of the equation.

That's probably a true statement.

There is a simple law of thermodynamics at play here. You (the diver) must put WORK into propelling yourself forward. Period. Now there is a question of fin efficiency. What quantity of work does your fin impart to the water? We can have a discussion about fin efficiency. Off hand, I don't really have a good idea about how to go about measuring fin efficiency in the field; at least not yet. I haven't put a whole lot of thought into it. But back to my point on work:

I read somewhere in here that someone was lamenting that their "split fins were so easy to move through the water -- my legs don't cramp, etc, etc, etc" This is all fine if you happen to like this particular feature about your fins. I am not going to tell you (any diver) what type of fins you should like. You are allowed to like what you want. There's something in here about free choice; but I digress again. But you must recognize that the fins are easy to move through the water because they are not doing any work on the water.

I have also read that someone talked about their tremendous speed through the water and all they had to do was use a high frequency kick (presumably low amplitude ??). This is nice too; especially if you like this particular feature of split fins. Now this works because you (the diver) are compensating for the split fins poor transfer of work energy to the water by increasing your work input to the fins. Your increase in work input is in the form of a high frequency kick (presumably easy kick as noted above). Hypothetically, if you or I could kick our fins (any fin) at 3600 kick cycles per minute, then we would all be flying through the water regardless of fin design or fin efficiency.

It is possible to generate work W by:
1) high frequency, low amplitude, small effort kicks and
2) low frequency, high amplitude, large effort kicks

and have the work output W of both types of kicking be equivalent. It's just a question of which style do you like? At some point, you can get into some discussion about resonance effects (as they relate to mechanical systems, fluid systems and human anatomical systems); but we'll save that for a later discussion.

Today's lesson is about WORK and the First Law of Thermodynamics.

 

[ZzzKing]

It's all good!
Pescador, it wasn't directed at you. I was agreeing with your point. My comments weren't supposed to annoy you in any way. I apologize if it came across that way. Winglets do work. That's the whole point of using them. All paddle fins do have water spilling off at 180 degrees. That's the whole point again.

It's all good. It's all a good argument for not using split fins.
Then again, I might still cut six splits in my fins

You never apologize for annoying us...

 

[meesier42]

Here's a question:
I contend that a significant amount of your speed is determined by how strong a swimmer/kicker your are, the fin type or style is only a minor part of the equation.
What do you think that percent is?

absoultely true, as a good swimmer without fins will likely keep up most people in the best fins. nothing will compete with proper training, technique, and fitness

but the question is not truly about speed, I contend that the root of the speed question is actually about efficiency. As with any diver, the amount of work that the individual can generate with their legs is independant of the fin. So the basic hypothesis is that if a diver goes faster in fin A than fin B; then fin A must be the more efficient fin. since the work done by the diver is constant, the difference is how the fin tranlates that work into forward thrust.

So what percentage of the equation is the fin... well that depends, what is my goal?
1) if I want to go for 30 minutes at 30feet and never go more than 200yards from the boat, then the fin basically doesn't matter
2) if I want to go for a 60min@60' with a AL80 and have to cover 2miles during the swim from the beach and back (common for many of my shore dives). Then the efficiency of the fin matters a lot.

the real question is how do you measure efficiency of a human biomechanical interface and not fall victim to the problems of exercise theory and individual preference. Well, its not easy, as if you let each diver fully acclimate to any one fin prior to testing, then you create a problem of having enough time between test that the individuals fitness, hydration and the hundreds of other factors that can modify your output come into play and skew your test. I only know of 1 test that has taken significant efforts to eliminate these problems and you have to do a FIOA request (that will likely get denied) to get the information, but if they do give it to you please share. This test was conducted at the University of Buffalo in 1992-1993 and the US Navy paid the bill, the test included 200 divers and 15 pairs of fins. All were monitored in a highly controled flume tank with O2 consumption (VO2) among many other variables being measured. But of course nobody wants to believe the limited results that have been released.


edit- richerso thanks for such an intelligent post