Help me understand the effects of current on fins

[jd950]

I really do not want to start another split vs. paddle fin thread. But in those threads the same issue keeps getting brought up. It is the idea that certain fins "don't do well in current" or "don't do well with a large diver or a diver with a lot of gear. Yet those same fins seem to propel people just fine in the absence of currents or heavy loads. I am not trying to start a fight, I really just want to understand the theory behind this concept.

It seems to me that if I try two different brands or styles of fin and one moves me faster, or moves me at the same speed with less effort, then that fin will continue to perform better if I am in current or pick up a camera. Isn't current or a big camera or a big diver really just more resistance? Why doesn't the faster fin retain its edge?

Is the idea that above a certain amount of resistance some fins fail to work as intended? Does the fin collapse or the split portions fail to move correctly, or do more flexible fins fail to flex as they should or over-flex?

I assume that with fins, concepts such as mass and momentum are not at work, right?

[FONT=&quot]Here is a quote that illustrates what I am asking: "[/FONT]We got stuck in strong current once and a couple buddies with split fins were stuck in it near a wall, causing a bottleneck. We both moved out into heavier current and passed them easily."

I don't understand how this could be, unless we are comparing stronger swimmers to weaker ones and the fins had nothing to do with it. I also gather the guys with split fins never made it back?
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[diver 85]

Who really cares about the theory----get a pr. of each & try them under the same conditions--ie back to back short dives......I used non splits(Dacor Turbo's) for the 1st 20+ years of diving-----Great fins.....have used Split fins, Aeries Duo Velocity, for the last 4 or so years(when all my Turbo's gave out or I lost them, had II's & III's)-----Great fins.........Have used the latter in 5+ knot currents........So I have no complaints about either.....

 

[danvolker]

I really do not want to start another split vs. paddle fin thread. But in those threads the same issue keeps getting brought up. It is the idea that certain fins "don't do well in current" or "don't do well with a large diver or a diver with a lot of gear. Yet those same fins seem to propel people just fine in the absence of currents or heavy loads. I am not trying to start a fight, I really just want to understand the theory behind this concept.

It seems to me that if I try two different brands or styles of fin and one moves me faster, or moves me at the same speed with less effort, then that fin will continue to perform better if I am in current or pick up a camera. Isn't current or a big camera or a big diver really just more resistance? Why doesn't the faster fin retain its edge?

Is the idea that above a certain amount of resistance some fins fail to work as intended? Does the fin collapse or the split portions fail to move correctly, or do more flexible fins fail to flex as they should or over-flex?

I assume that with fins, concepts such as mass and momentum are not at work, right?

[FONT=&quot]Here is a quote that illustrates what I am asking: "[/FONT]We got stuck in strong current once and a couple buddies with split fins were stuck in it near a wall, causing a bottleneck. We both moved out into heavier current and passed them easily."

I don't understand how this could be, unless we are comparing stronger swimmers to weaker ones and the fins had nothing to do with it. I also gather the guys with split fins never made it back?

This would actually be a great time for Bob Evans to chime in, but for the moment, here's what I have to say. With a traditional fin design, the blade pushes the diver forward, efficiently, as long as the down kick produces an "optimal" bend in the fin...this is easy to visualize with the very long freedive fins, because everything happens in slow motion--the down kick begins a bend, that is easy to hold along time, because the fin is so long it can move down slowly--meaning the bend is there pushing efficiently for a long time. With shorter, stiffer fins, you need much more kick turnover, and the "sweet spot" for the right power to get an optimal bend is harder to find.
The idea with split fins, was to "rip off" some of the Force Fin ideas about less effort and higher kick turnover, essentially to create a smaller ( an easier gear to push with) that would move the diver with less stress to the muscles, than you would get from something like a standard jet fin or a quattro type fin.
The problem with the splits, is that they are too flexible to allow large thrust when needed, and the fins become somewhat unuseable in certain high current situations...An even worse scenario occurs with many splits in places like Boynton Beach, where a current can run 30 to 45 degrees off of the reef line...so you can't just go with the reefline unless you can counter the side current--and with many splits, they have some kind of major failure in producing thrust of any useful kind, in the side currents. On several occaisions, I have had to litterally drag divers with splits across the reef crown, so that they could stay on the reef--fortunately my freedive fins have no such issue with side currents. And interestingly, Force Fins or jets don't either....still, the corrupt in the dive industry love the money they make from splits, so you will read their praises constantly in the ad driven dive magazines, like rodales.

I will post more later.

Regards,
Dan V

 

[Teamcasa]

[FONT=&quot]Here is a quote that illustrates what I am asking: "[/FONT]We got stuck in strong current once and a couple buddies with split fins were stuck in it near a wall, causing a bottleneck. We both moved out into heavier current and passed them easily."

I don't understand how this could be, unless we are comparing stronger swimmers to weaker ones and the fins had nothing to do with it. I also gather the guys with split fins never made it back?
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The answer is more the diver than the fin. No two divers are the same or have the same physiology.

The facts are that a strong diver with excellent kicking power is not going to have the same issues that a diver with poor skills, regardless of their fin selection.

 

[jd950]

Who really cares about the theory----

Well, actually, I do. I am not even looking for fins; mine seem to be fine. I also tend to feel fins are fins and the differences are not great in most circumstances, but I am curious about the "science" behind the claims, if there is any, and figured I could learn something. I look forward to being educated on this topic.

 

[Puffer Fish]

Dave has it right.

The fact is, that split fins, when used correctly, provide both more speed and more force than any other type of fin...the key is "when used correctly".

The kick required is so different from standard fins, that most people don't know how to do it.

There has been a huge amount of testing regarding this, but there are a lot of people that still try to use them as if they were jets, because that is what they believe fins are supposed to be.

One of the more interesting speed records for fin testing is that a pair of split fins has the fastest speed doing the frog kick (also every other type of kick, but that should be expected)... even after seeing a video of it, I cannot do that kick, but clearly at least one person could.

The scuba mag tests for fins are actually very realisting testing, except everyone there was taught how to use the fins. (as was the case in the Univ. Penn testing, and the testing done in England). The english testing was force, not speed..and splits were still the best.

Unlike what most people think, split fins are not modified force fins... they work by letting someone use the large muscles of the leg, and oddly enough, work better at a faster speed. The basic concept is to pull the fin up, bending your leg...as the fin bends (cups if you will), you suddenly push down.. sort of a cross between a bicycle and walking up a very steep hill. The cupped fin catchest the water and throws it directly backwards.... which is hugely more efficient than jets.

But this is a very unnatural motion for a fin and requires some timing to get correct..

It is why there is such a love/hate issue associated with them.

The current issue is a huge red Herring..a physically fit person, who knows the correct method, can swim around 20 -30% faster that the best one can do with any jet or paddle fin. If you use the same fin method as you would with jets, you will be able to go around 50 to 70% as fast, simply because their design does not work with that method.

I swim, with fins, usually twice a week, doing at least a mile each time, except when I am using jets.. as the slower speed takes so much longer.

Mind you, I love doing the frog kick with jets...as I am not coordinated enought to do it with split fins, and they are easy to a nice relaxed kick with.

I find a nice comfortable speed for me is around 1.4 miles per hour....I can swim that fast with jets, but not for very long...how fast could I swim for say 100 yards? Would guess it was around 1.9 mph. My best with jets is about 1.5... or very similar to what testing has reported, except I am not as strong as some of the testers.

So, if I were in a nasty 1.7 knot current... I would only be able to make headway with splits.

A very simple question for any "expert" in fins, is to simply do a pool test and time their speed, if they cannot swim with spits at or over 2 mph....they don't know how to use them, or are in not very good shape (or both). Then time them with jets.. if they are around 1.7, then the issue is operator related. If they are proportionally slower, then it is physical strength.

Regarding side, angled or any other current.. it is fairly simple physics..where vectors and direction of force are the issue.

There is a bit of an issue with who uses what fin. For the most part, new divers tend to have split fins... "real" divers tend to use jets...

Over the years, I have had to help a lot of divers, but it has been because of their skill and strength, not what fin they used.

 

[FishOutUvH2O]

I also think Dan covered it pretty well. However, I would debate the "split fins are to flexible" statement. Split fins, like paddle fins are not all created equal. That is, they come in a variety of different materials, some flexible, some stiff. Even among the same manufacturer. Take for example the Scubapro Twin-Jet series (only because that's what I use). The difference in stiffness between the Twin-Jet and the Twin-Jet Max is night and day. The Twin-Jet's are incredibly flexible. Kick too hard and most of the force is lost in the flex. The Twin-Jet Max on the other hand could be used to drive a nail into an Oak 2x4 and not flex much*. There are of course trade-offs. Stiffer fins may wear a person out quicker.




*Please don't actually try it, the example was fictitious and for illustration purposes only.

 

[meesier42]

current has ZERO effect on the performance of a fin. If a fin can produce a relative velocity of 2 knots, then it can do 2 knots. Period end of story.

The ability of the diver to maintain a kick is also independent current. If the diver can only swim 2 knots then that's all they can do, if they can maintain it for 5 minutes or 60 minutes has nothing to do with current. Speed and endurance have lots to do with the FIN that an individual diver, so the DIVER has to find the fin that works best for them.

One of the more interesting speed records for fin testing is that a pair of split fins has the fastest speed doing the frog kick (also every other type of kick, but that should be expected)... even after seeing a video of it, I cannot do that kick, but clearly at least one person could.

Unlike what most people think, split fins are not modified force fins

although I agree with most of what you said, these two points that you made are simply wrong. First- speed test of fins have conclusively shown that monofins, freediving fins, and a few Force Fins are all faster than the fastest splits. Second all splits on the markets are based on a common design, most people know this as the "Natures Wing" design, but what most people don't know is that the "Natures Wing" patent is in fact a direct rip off of the Foil Force Fin

 

[Walter]

Have used the latter in 5+ knot currents

No matter what fins you use, if you swim into a 5 knot current, you'll be traveling backward.

 

[diver 85]

No matter what fins you use, if you swim into a 5 knot current, you'll be traveling backward.

----not with tag lines & doing a drift dive.....It's always straight ahead for me......


You ever dive the Turbine Channel of Toledo Bend with both generators going----excuse me, a 8+ knot current----we call it white water scuba diving----literally.....I'll take some video next time out, & you be the judge....OK???