The never-ending Jetfin debate...

[CincyBengalsFan]

jplacson once bubbled...
Ok, doing my own personal testing (very informal... don't take it seriously) with some friends... the classic Jets do perform better over-all than most, if not all, other fins.

Here's my question... it's pretty obvious why they work well... they're stiff, and have channel thrusts in the middle... although they seem to be optimized for the downstroke like most fins... I was wondering why they don't TRY optimizing for the upstroke...since the human leg is stronger on the downstroke, and weaker on the reverse, optimizing it for the upstroke would actually give you more consistent speed and acceleration...since power would be almost even on both up and downstroke.

And I know the 'frog' ends don't make the fins perform any better... so why don't they try re-styling the things???

Just wondering cuz these fins don't seem to want to go away... and that's fine...if it works, it works... but come on!!! They haven't even bothered changing the style AT ALL! Not even color accents, or something...ANYTHING!!! I mean...don't tell me that by now, they haven't found even a better way to angle the channel thrusts??? Something to make it look a bit modern?

Anyway.. another question, this time to Mares Quattro owners... how does the Quattro perform using alternate kicks? I did some research on the net and so far, the Quattros seem to be lagging just a little behind the Jets when doing the frogkicks.

I don't know what to say, I had jets for 12 yrs. In 1993 I switched to a SeaQuest composite fin (Accelerators) and fell in love with them. I have never gone back to the jets. In fact I use splits now. I can frog kick great with my splits too. Maybe it's just different types of kick. If you angle your foot to the right or left when you kick then you lose all kinds of propulsion to the sides of the fins. That kind of kick requires a good stiff fin. Cressi makes a goodsize super stiff fin that is the fastest thing I've ever used. But I don't really have a use for it. It's like mounting some plywood on your feet.

 

[jeremyrfoster]

jplacson,

I totally agree. And you know, there are those like you that want some decoration and those like me that like it black and plain... they could please both crowds and make both... it's not like they have to make a new mold.

I had never considered the ability to adjust trim with the Jets that SeaJay brought up. I think that's cool.

 

[jplacson]

Ok... now another question... Jetfins, or TwinJets?

What advantages/disadvantages do the twinjets have over the regular jets?

One thing puzzles me... some fins seem to be faster than Jets, except in current... why is this??? Wouldn't it follow that if a fin could propel you fast through standing water, it should be able to do the same in current? (The Bio-fin supposedly is faster than the Jets, but not in a strong current)

 

[jeremyrfoster]

I'm speculating, but I wonder if it's because split fins have a performance curve where they provide more propulsion in the range of speeds that a diver normally swims, but not higher. Swimming against current is the equivalent of swimming very fast (in both scenarios, water is passing the diver at a higher rate). So splits wouldn't give you as much propulsion because they are operating in this high water speed... the vortices (which I believe are the whole key to their performance) must not form correctly in the high speed water.

Jet Fins, on the other hand, may have a more linear performance since they do not rely on these vortices. They would then be able to propel the diver as long as he is able to push back on this fin faster than the water is moving.

So there you have it. In speculation anyway.

 

[jplacson]

That does make sense though.

I'm guessing that the same rings true for the new Volo fins which probably assume a certain "top speed" to maintain it's 'optimum pivot angle' ... if the current is higher than the speed that the diver is finning, then the pivoting blade can't reach it's optimum angle...

 

[SeaJay]

jplacson once bubbled...
Wouldn't it follow that if a fin could propel you fast through standing water, it should be able to do the same in current? (The Bio-fin supposedly is faster than the Jets, but not in a strong current)

My gosh... Sanity!

I completely agree.

If you were to use the example of a diver diving blue water (that is, say, in 1500' of water, where effectively "there is no bottom") that descends to... Say... 100 feet... And let's say that there's 50 feet of vertical vis...

That is, let's say you've got a diver who has no visual references at all. He's just hanging in space somewhere. No bottom, no surface. Nothing. The only way he knows how deep he is is by his gauges...

Can he tell if he's in zero current or 10 knots of current? Of course not. He's just "hanging." Pilots call this "ground speed," which can be different from "air speed." That is, speed relative to the ground ("bottom" in our diver example) vs. speed relative to the air around them ("water column" in our diver example).

So... Does will it affect performance of the fin if there's a current or not? Of course not.

...So if splts are "no good in current," then why would someone claim them to be better when there's no current?

For this reason, I personally am suspect of anyone that claims that their split fins are faster, but not in a current.

 

[oversea]

I currently have the twin jets. I was recently diving a local area in which the current can get pretty nasty. Later that week, I was at my lds and asked about jets for more power in currents. He told me he did'nt care if I used plywood, I would not get any more power than my twin jets have. Is this so? It does not make sense to me but I've never used any other.

 

[SeaJay]

jeremyrfoster once bubbled...
I'm speculating, but I wonder if it's because split fins have a performance curve where they provide more propulsion in the range of speeds that a diver normally swims, but not higher. Swimming against current is the equivalent of swimming very fast (in both scenarios, water is passing the diver at a higher rate). So splits wouldn't give you as much propulsion because they are operating in this high water speed... the vortices (which I believe are the whole key to their performance) must not form correctly in the high speed water.

Jet Fins, on the other hand, may have a more linear performance since they do not rely on these vortices. They would then be able to propel the diver as long as he is able to push back on this fin faster than the water is moving.

So there you have it. In speculation anyway.

I was trying to figure out a way to say this... But you've done a better job than I could have.

I agree with you completely on this, and I think you've drawn an excellent conclusion.

...Which, if you have "suspected" correctly, would show that contrary to the popular opinion of split fin supporters, splits are not "faster." They are "easier" to fin... And at medium effort levels, may provide more propulsion.

The problem is, when a diver REALLY need the speed (which is unusual), it's not there... And when they just want to cruise, it's available to them... Right when they don't need it.

The only advantage I can see from this is perhaps a less tiring dive. Sorta like riding a bicycle in a lower gear.

What makes more sense is to streamline yourself, learn the horizontal attitude, and keep yourself a bit more fit - and get fins that can haul when you need them to (again, which isn't common).

 

[jplacson]

Just another thought on Jet Fins... I noticed that freedivers and spearfishers use those really really long fins... why? Why don't they use Jets?

What are the known advantages/disadvantages of long fins over scuba fins/Jets?

 

[SeaJay]

I've used freediving fins before... They're looooong... And the good ones are stiff, too. They simply put a lot of power to the water for "sprinting" underwater.

The problem is, if you're underwater diving a wreck, the ends are really difficutl to mangage. If you're diving over a reef, the reef can take a real beating for the same reason. And trying to move into the water from a boat deck with freediving fins on is less than fun.

...So, freediving fins don't work for scuba... At least, not IMHO. But freedivers do tend to like the stiff Jets... They just want "more" fin for quick bursts of speed.

To a diver, control and management is more important than "quick bursts of speed."