Tusa X-pert HBK's v Atomics

[RebreatherBoy]

Gang,

I just got some Tusa X-pert split fins and noticed they were quite a bit more flexible compared to my Atomic split fins.

An instructor told me none of the tech guys wear them as they fluttery their legs and dont accomplish much.

I just ordered the stiffer Tusa X-pert fins, the HBK model which Tusa told me today is much stiffer then their regular colored X-perts.

Anyone have some opinions on how stiff the X-pert HBK models are compared to Atomics?

Oh, by the way, I read some of these esoteric claims that such and such fins use hydrodynamic principles and actually produce thrust and lift......it is all marketing hype! Water is more or less not compressible, so there cannot be much of a pressure differential between the top of a fin being pushed down and the bottom underside of it. All flat surfaces undulated underwater will propel itself forward, and as such, all flat surfaces in some way mimic fish fins. With all the gear plastered all over us these days, for the same output of energy, any properly designed fin, whether jet fin, slotted, angled, split or whatever of the right size and stiffness will for the most part propel you more or less at about the same speed.

 

[CALI68]

Gang,

I just got some Tusa X-pert split fins and noticed they were quite a bit more flexible compared to my Atomic split fins.

An instructor told me none of the tech guys wear them as they fluttery their legs and dont accomplish much.

I just ordered the stiffer Tusa X-pert fins, the HBK model which Tusa told me today is much stiffer then their regular colored X-perts.

Anyone have some opinions on how stiff the X-pert HBK models are compared to Atomics?

Oh, by the way, I read some of these esoteric claims that such and such fins use hydrodynamic principles and actually produce thrust and lift......it is all marketing hype! Water is more or less not compressible, so there cannot be much of a pressure differential between the top of a fin being pushed down and the bottom underside of it. All flat surfaces undulated underwater will propel itself forward, and as such, all flat surfaces in some way mimic fish fins. With all the gear plastered all over us these days, for the same output of energy, any properly designed fin, whether jet fin, slotted, angled, split or whatever of the right size and stiffness will for the most part propel you more or less at about the same speed.

You might be interested in reading this fin report done using scientific method by a University Professor.

http://www.ndsu.edu/instruct/grier/fins.html

 

[O2Addict]

I have not done any research in to fins, but I do have a degree in Naval Architecture and lift and drag are just as important in water as they are in air.

Take a look at the deepflight submersibles, they develop negative lift when propelled forward which overcomes the bouyancy of the sub and "pushes" it downwards.

Also look at sailboat keels, another example where lift underwater plays an important part. It is to do with pressure differentials, and has very little to do with the compressibility of the liquid.

 

[smay28]

lift and drag are just as important in water as they are in air.

It is to do with pressure differentials, and has very little to do with the compressibility of the liquid.

Bingo!!! The water might not be compressed but it is displaced.

 

[Jeddah Aquanaut]

Gang,

I just got some Tusa X-pert split fins and noticed they were quite a bit more flexible compared to my Atomic split fins.

An instructor told me none of the tech guys wear them as they fluttery their legs and dont accomplish much.

I just ordered the stiffer Tusa X-pert fins, the HBK model which Tusa told me today is much stiffer then their regular colored X-perts.

Anyone have some opinions on how stiff the X-pert HBK models are compared to Atomics?

Oh, by the way, I read some of these esoteric claims that such and such fins use hydrodynamic principles and actually produce thrust and lift......it is all marketing hype! Water is more or less not compressible, so there cannot be much of a pressure differential between the top of a fin being pushed down and the bottom underside of it. All flat surfaces undulated underwater will propel itself forward, and as such, all flat surfaces in some way mimic fish fins. With all the gear plastered all over us these days, for the same output of energy, any properly designed fin, whether jet fin, slotted, angled, split or whatever of the right size and stiffness will for the most part propel you more or less at about the same speed.

I can't speak to the hydrodynamics, but I bought a pair of Tusa X-pert Zoom splits recently, and love 'em. I had looked at Scuba Pro's, and thought them too flimsy. Tusa's did have very good reviews, and after borrowing my friend's pair to try, I was hooked. I like the difference in effort. They're got just the stiffness level I want. I haven't encountered a current yet here that I couldn't deal with. If the current is soooooooooooo strong, Tarzan say, "Dive over." I get very good propulsion, and acceleration with them. I wouldn't say they're the best in the world, but as with many other pieces of equipment, it's what one feels comfortable with, right? Anyway, I'm a Trekkie, not a Tekkie.

 

[AtomicWalrus]

Gang,

I just got some Tusa X-pert split fins and noticed they were quite a bit more flexible compared to my Atomic split fins.

An instructor told me none of the tech guys wear them as they fluttery their legs and dont accomplish much.

I just ordered the stiffer Tusa X-pert fins, the HBK model which Tusa told me today is much stiffer then their regular colored X-perts.

Anyone have some opinions on how stiff the X-pert HBK models are compared to Atomics?

Oh, by the way, I read some of these esoteric claims that such and such fins use hydrodynamic principles and actually produce thrust and lift......it is all marketing hype! Water is more or less not compressible, so there cannot be much of a pressure differential between the top of a fin being pushed down and the bottom underside of it. All flat surfaces undulated underwater will propel itself forward, and as such, all flat surfaces in some way mimic fish fins. With all the gear plastered all over us these days, for the same output of energy, any properly designed fin, whether jet fin, slotted, angled, split or whatever of the right size and stiffness will for the most part propel you more or less at about the same speed.

Those fins DO use hydrodynamic principles, and they really aren't esoteric. The split fin design basically acts as a wing in the water, and a wing produces lift according to the following equation:

L=Cl*0.5*rho*V^2*S

where
L=lift
Cl=coefficient of lift for cross-section
rho=fluid density
V=speed of fluid past wing
S=area of wing

Any cross-section has a coefficient of lift when positioned at an angle relative to the flow direction, even a flat board:

Cl=2*pi*alpha

where
alpha=angle between the chord of the section and the flow

Any surface producing lift also produces drag, and the amount of drag is proportional to the lift:

Cd=Cl^2/(pi*AR)

where
Cd=coefficient of drag
Cl= coefficient of lift
AR=aspect ratio of wing (length/width)

Notice that a wing tends to produce less drag than lift. The split fin produces thrust by splitting the end of the fin into 2 wings that twist with the direction of your kick to produce lift. It also produces some drag in the process, and the drag is what you're feeling when you're kicking. In contrast, a conventional fin generates basically all of its thrust through drag - it's basically a paddle that you use to push against the water.

I think what throws a lot of people off with the split fins is that you feel way less resistance when you kick. Anyone used to regular fins interprets that as generating less thrust, but they're actually just feeling less drag. The other trick is that the fins have to be used differently - you can't just kick harder to generate more thrust, you have to kick faster (notice how lift is proportional to the SQUARE of the speed of the flow in the first equation) I think some people like the stiffer Atomics or the stiffer version of the Tusas when they're transitioning from conventional fins because their characteristics are more like regular fins. With a stiffer blade, the wing generates more thrust through drag and a bit less through lift because it doesn't twist as far in the flow. That gives a bit more resistance when you kick and is a little more familiar.

Anyway, sorry for the physics lecture - my background is in fluid mechanics and I get pretty excited about this stuff. I've got a pair of the Tusas myself, and my girlfriend has the Atomics. They both seem to work well, the biggest difference being some of the details of construction. The Atomic has got a nicer fin strap connector, and the Tusa has got a more comfortable angle on it. I also know a lot of tec divers around here who use Atomic split fins and seem pretty happy with them.

 

[O2Addict]

L=Cl*0.5*rho*V^2*S

where
L=lift
Cl=coefficient of lift for cross-section
rho=fluid density
V=speed of fluid past wing
S=area of wing

Any cross-section has a coefficient of lift when positioned at an angle relative to the flow direction, even a flat board:

Cl=2*pi*alpha

where
alpha=angle between the chord of the section and the flow

Any surface producing lift also produces drag, and the amount of drag is proportional to the lift:

Cd=Cl^2/(pi*AR)

where
Cd=coefficient of drag
Cl= coefficient of lift
AR=aspect ratio of wing (length/width)

Takes me back to my Naval Architecture days, although I did not do that well in Aero-Hydrodynamics!!

I bought a pair just to test them, as the concept is so cool. I have to say from first hand experience that it is left "propelling" you. The faster you kick the more thrust develops. But it is weird, especially with the more flexible ones, as it feels like there is no resistance and you are hardly wearing them!!

 

[CALI68]

Those fins DO use hydrodynamic principles, and they really aren't esoteric. The split fin design basically acts as a wing in the water, and a wing produces lift according to the following equation:

L=Cl*0.5*rho*V^2*S

where
L=lift
Cl=coefficient of lift for cross-section
rho=fluid density
V=speed of fluid past wing
S=area of wing

Any cross-section has a coefficient of lift when positioned at an angle relative to the flow direction, even a flat board:

Cl=2*pi*alpha

where
alpha=angle between the chord of the section and the flow

Any surface producing lift also produces drag, and the amount of drag is proportional to the lift:

Cd=Cl^2/(pi*AR)

where
Cd=coefficient of drag
Cl= coefficient of lift
AR=aspect ratio of wing (length/width)

Notice that a wing tends to produce less drag than lift. The split fin produces thrust by splitting the end of the fin into 2 wings that twist with the direction of your kick to produce lift. It also produces some drag in the process, and the drag is what you're feeling when you're kicking. In contrast, a conventional fin generates basically all of its thrust through drag - it's basically a paddle that you use to push against the water.

I think what throws a lot of people off with the split fins is that you feel way less resistance when you kick. Anyone used to regular fins interprets that as generating less thrust, but they're actually just feeling less drag. The other trick is that the fins have to be used differently - you can't just kick harder to generate more thrust, you have to kick faster (notice how lift is proportional to the SQUARE of the speed of the flow in the first equation) I think some people like the stiffer Atomics or the stiffer version of the Tusas when they're transitioning from conventional fins because their characteristics are more like regular fins. With a stiffer blade, the wing generates more thrust through drag and a bit less through lift because it doesn't twist as far in the flow. That gives a bit more resistance when you kick and is a little more familiar.

Anyway, sorry for the physics lecture - my background is in fluid mechanics and I get pretty excited about this stuff. I've got a pair of the Tusas myself, and my girlfriend has the Atomics. They both seem to work well, the biggest difference being some of the details of construction. The Atomic has got a nicer fin strap connector, and the Tusa has got a more comfortable angle on it. I also know a lot of tec divers around here who use Atomic split fins and seem pretty happy with them.

Wow! Now that's what I call a post! That is a keeper! Very Interesting and intelligent!