Pervasive Fallacy about Split Fins

[ONESPEED]

Post edited to be less likely to get me reprimanded.

 

[Matt_NJ]

splits are for people who cant use proper fins, eg. small women who dont have the leg muscles to use real fins. being a man i use cressi reaction's. there great fins.

That's interesting because i'm a 280lb man, and was a competetive swimmer up until about 8 years ago yet I love my split fins. Maybe I should ditch them and use a paddle I did not realize that fins were the high heels of scuba diving......:mooner: . I find them easier to use than a paddle fin.

 

[HowardE]

splits are for people who cant use proper fins, eg. small women who dont have the leg muscles to use real fins. being a man i use cressi reaction's. there great fins.

How do we know you're really a man?

 

[ClayJar]

Now this is where I think some of the problem is.
The above logic is flawed.

Thrust can produce acceleration but
acceleration is NOT equal to thrust.

Acceleration is defined as:
The rate at which an object's velocity changes with time.
Thrust is defined as:
A force that pushes an object forward.

Force = Mass * Acceleration.

(Thrust is a component of the net force, of course.)

It is possible to have lots of thrust and have ZERO acceleration.
In fact you could have thrust and be slowing down or even
be moving backwards.

With non-zero net force, there is non-zero acceleration, but it is possible to have significant thrust and zero acceleration as long as you have other forces which cancel the thrust to give zero net force.

For example, I could strap a rocket onto a huge
boulder and have millions of pounds of thrust and no
acceleration if the rock doesn't move.

The force in that case is the net thrust of the rocket. The mass is not, however, the mass of the boulder. The mass, in the case of a boulder rigidly fixed to the earth is the mass of the earth (including the boulder). The acceleration, then, is equal to the thrust of the rocket divided by the mass of the earth.

While the acceleration is so miniscule as to be indistinguishable from zero given the precision of the experiment, it is indisputably *not* zero.

(Strap two rockets against the boulder with equal thrust in opposite directions along the same line, and you'll have no acceleration... but I haven't seen anyone skilled in head-finning. )

Likewise, if I am swimming, at some point, I will reach a
terminal velocity and will stop accelerating and level off into a
constant speed.
This is the point where my fluid resistance or drag
has come into equilibrium with the thrust my fins are producing.
I still have thrust, just no more acceleration.

There is no likewise here. In this case, the net force is equal to the thrust minus the drag. Once you've reached steady-state and have zero net force, you will indeed have zero acceleration.

It is also a false assumption to assume that the thrust
would be the same if the speed is zero or 4 kts (4 kts is FAST BTW).
There could (and will) be all kinds of efficiencies or inefficiencies that only
show up once dynamic fluid resistance enters into the picture.

I would not be surprised to find that the thrust of a split fin is greater at forward velocities greater than zero.



It seems, from my experience using both split and paddle fins, that the "effectiveness" of a paddle fin peaks right around zero velocity and decreases with increasing velocity, while the "effectiveness" of a split fin at zero velocity is less than the paddle fin but increases with velocity, at least to a point.

• If I'm doing a dive where maneuvering is of primary importance (a cavern, perhaps), the better fin style for me is a paddle fin.
• If I'm doing a dive where forward velocity is of primary importance (against a current, perhaps), the better fin style for me is a split fin.
• If neither maneuvering nor forward speed are primary concerns (a bridge span in the Gulf, perhaps), either style would work adequately, so the better fin style for me is determined by the dive plan. (Photography? Paddle. Tour the area? Split. et cetera)

(Strangely enough, I don't see many people arguing about whether tennis shoes or hiking boots are the best footwear. Tennis shoes are terrible for hiking and hiking boots are pathetic for tennis. I happen to have more than one pair of footwear, and I don't consider myself a sellout to either the tennis or hiking camps. Oh, and some of those cross-training shoes are terrible for both hiking and tennis. )

 

[in_cavediver]

Oh, and some of those cross-training shoes are terrible for both hiking and tennis. )

Would those be 'semi-split fins!

 

[djanni]

I'm living part time in the Philippines right now and dive with a lot of "small women" who are instructors or just very experienced divers. None of them use splits and can dive circles around most of the men that dive with them.

I don't use splits nor have I tried them so I can't make a comment about their effectiveness.

You should try a pair. Then you'll know what all the hype is about.

 

[djanni]

Here you go Bill, I find the videos interesting ... http://www.scubadiving.com/2005fintest_protocols

The videos are nice.

One thing that cought my eye in the speed test was the lack of any noticable disturbance of the botton even though the diver was about 2 feet off the bottom and kicking a hard as possible.

Sorta dispels the Myth that Split Fins stir up silt more than paddles.

 

[ClayJar]

One thing that cought my eye in the speed test was the lack of any noticable disturbance of the botton even though the diver was about 2 feet off the bottom and kicking a hard as possible.

Sorta dispels the Myth that Split Fins stir up silt more than paddles.

No. It doesn't.

• Split fins provide more cruising thrust.
• For every action, there is an equal and opposite reaction.
• Greater cruising thrust, then, directly correlates to greater net force propelling water rearward.

I will gladly concede that split fins may provide greater cruising thrust. It naturally follows, however, that there is increased potential for silting.

It's certainly possible in many circumstances to fin along without silting regardless of the fins you're wearing, but if split fins are more efficient at moving water, they are necessarily more efficient at silting. Q.E.D.


Okay, here's an interesting idea for everyone to ponder and discuss:

• For a given amount of effort, split fins can provide more cruising thrust than paddle fins, or split fins require less effort to produce a given amount of thrust.
• When diving in inclined trim (i.e. upright or "leaning forward", as opposed to horizontal), to maintain depth while kicking, a diver must be negatively buoyant equal to the vertical component of the thrust vector. (Thrust_v = Thrust * Cos(Theta), where Theta is the inclination from horizontal)
• If the diver in inclined trim is kicking at all while maintaining constant depth, the diver must maintain constant thrust in order maintain constant depth.
• In split fins, the effort required from the inclined diver to maintain constant depth (by kicking to provide constant thrust) is significantly less than that required if paddle fins are used.
• Therefore, divers in inclined trim who are diving split fins may well have significantly less motivation to correct their inclined trim.
• Therefore, any perceived difference in the rate of silting divers between those wearing splits and those wearing paddles may be a secondary effect of the efficiency of the split fins.

In other words, could split fins simply be reducing the penalty divers would be paying for inclined trim, which results in less incentive for the divers to learn horizontal (non-silting) trim? If that is the case, it could be said that the "problem" is that the fins are too good for the inclined divers' benefit.

(In some sports, athletes train with equipment which magnifies any errors in their form. By augmenting the penalty for poor form, they force themselves into proper form and become much more skilled. Would divers learn better form by diving cumbersome fins? )

 

[thomjinx]

Thanks, "Temple" for taking this on. Like many here, I use both types of fins and have proven to myself at least, that what 'feels like' is happening is often not the case. If a fin feels like it is offering a lot of resistance when you kick it, it is easy to assume that it is giving you more thrust. But where a paddle fin works more like a paddle, a split fin works more like a propeller. I have timed myself repeatedly over a closed course and found that I couldn't always tell what was actually happening by how things felt. Even comparing paddles to paddles (of different types) and splits to splits, many times what I thought was the fastest fin was not. I also proved to myself that learning precisely what types of kicks work best with each fin can have a significant effect on the results.

I am one of those weirdos who can indeed move backwards in splits. But, surprise, it takes a different technique than with my Jets. And for me, at least, a helicopter turn is easier and quicker in the splits, but again, it requires a different technique. Who'd a thunk it? And believe it or not, on tight swim throughs in Cozumel, I was stirring up almost no silt compared to divers with paddle fins, because I was able to use a much smaller flutter and less of the thrust coming off the fins was off to the side.

I am not saying split fins are the right tool for every job, just that they are too often dissed on this board for the wrong reasons.

As always, your mileage may vary.

Thanks again for addressing this trend of misinformation.

 

[Temple of Doom]

It seems, from my experience using both split and paddle fins, that the "effectiveness" of a paddle fin peaks right around zero velocity and decreases with increasing velocity, while the "effectiveness" of a split fin at zero velocity is less than the paddle fin but increases with velocity, at least to a point.

I don't at all want to suggest that I know more about your experiences than you, but do you think the following perhaps better explains what you've noticed?

The thrust for any fin will be pretty much equal for all speeds of the diver in the water (even 0 speed). Also, tests have demonstrated that when tied to a stationary object splits output more thrust than paddles (it's pretty much proportionate to the speed differences between the two fins when swimming fast).

I think the reason that paddles feel faster (and actually, over *very* short distances are faster) from a standstill is not because they have more thrust at 0 speed, but that they have more thrust per single down kick (though much less in the up recovery). I explain why in post #35 of this thread: http://www.scubaboard.com/showpost.php?p=2915080&postcount=35

A paddle will likely beat a split-fin after 2 kicks and 1 recovery (but not after 1 & 1 or 2 & 2), but the additional kicks of the split-fin will catch up the net thrust very quickly, not because they have more kicks, but because they are more efficient.

Fins of equal efficiency will have the same net thrust, but paddle fins will have surges of speed then slow-downs, versus split fins that have a relatively more constant velocity. If you measure two fins with equal efficiency at a given time either one may appear to have generated more thrust, but their average thrust will be pretty close equal after a very short distance.

Craig