From: Christian Gerzner <christiang@cc*.co*.au*>

> I'm sure that someone's going to clobber me for this but:

You'll get over it.  A couple of filters in your email program will wipe out
the worst of it.


> "For every action there is an equal and positive reaction"

True enough.  Keep that in mind.  We'll be back to that later.

> is what I think, or something very like it, someone very famous once said.

Newton.  I don't recall if it was Huey or Fig.

> If that is the case, and I think it is, then an "easier but more
> efficient" fin is an impossibility.

This is where you make a wrong turn.  If that is true, then a "harder but
less efficient" fin would also be an impossibility.  I have a big drawer of
exhibits that prove such things do indeed exist.

> A more slippery car, or diver, is a possibility, has nothing to do
> with force/energy/action/reaction.

True enough.

 > Folks,
>
> Consider yourselves as engines, which you actually are.
>
> - If you use Force fins you will kick x times to achieve a given
> distance (all else being equal) and expend y energy.
> - If you use split fins you will kick x times to achieve a given
> distance (all else being equal) and expend y energy.
> - If you use absolutely the best fins you will kick x times to achieve a
> given distance (all else being equal) and expend y energy.
>
> The ENERGY requirement remains the same for a given distance, never
> mind the fin (all else being equal). Other things can, and will, vary.

I have no idea what you mean by "all else being equal."  If the water flow
is improved, say by installing vents and calling it a "Jetfin," work
performed for energy input will improve.

> The mere fact that some novice raves about farce (sorry, Force) fins
> and their (ease of use) efficiency has everything to do with their
> (lack of) energy efficiency. The less energy demanded, the easier to use.

I'm having a little trouble staying with you here since many people report
very good results with the fins.  I refuse to believe they are all deluding
themselves.


> THE POINT?
>
> The energy you expend will, never mind the fin, achieve the same
> distance (all else being equal).

There's that phrase again and I still don't know what this "else" is.

> I think I'll now take up tiddlywinks and unsub real quick. ;)

That's one way.

The reason the split fins get rave reviews is that the flex and the center
venting reduce the energy wasted in pushing water up and down (cross axis)
and direct more water straight back (along axis).

Just watch someone flutter kick close to a silty bottom and you'll see that
considerable force hits the bottom from an altitude of about two feet or so.

Directing that force to the rear with blade flex or not wasting energy on it
in the first place with center venting will improve efficiency at one thrust
level.

The stronger your legs are, the more thrust you can develop and the stiffer
the fins need to be.


David's comments about swimming a couple of 95s in the Pacific Northwest are
really telling.

The flex is optimized for a fairly narrow range of thrust and when David
wants to speed up, he kicks harder and drives the fins out of the thrust
sweet spot.  It takes him a couple of kick cycles to overcome inertia and
get back within the range of thrust where the fins work well.

My guess is his fins are an RCH too flexible for his leg strength.


If you put a spaghetti legged tourist in the water, the fin would hit him in
the back of the head before it was too flexible for his leg strength.  Thus
the raves from the reefs.


The split fins really fall on their ass when it is time to frog kick,
something I hardly every do, but many tech divers do all the time.  That
cross axis water is how a frog kick works and diverting the cross axis water
to along axis leaves the diver banging his heels together and barely moving.

There is some promise to the fins, but they aren't ready to sell me a pair.

Don Burke


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