Bought long fins, did not get much more kick!

[dumpsterDiver]

There is a reason the military divers use jet fins.... Just saying. If you want to swim against currents, you need stiff paddles, long fins are designed to move the most water with the least effort, but they are inherently bad at providing torque. Think diesel engine vs. F1 car. F1's use little tiny engines that rev at super high speeds to provide the max horsepower, which is work over time. More HP=more work done with less effort, they also have horrible torque ratings which means they don't have a lot of power. Diesel engines have very low HP numbers, but astronomical torque numbers, so they require a lot of energy to do the work and can't do as much for the effort required, but if you need to pull something, you need high torque.

Jet fins are like diesel engines, you're not going to be as fast in perfect conditions as the freediving fins and you will have to exert more effort to do it, but if you have to kick through the current, you need short and stuff, not long and floppy. OMS Slipstreams are actually better than jetfins for this since they are a bit stiffer and a lot lighter, but the same basic fin design is the same

What is the "the" reason that military divers use jet fins? Is it because they are the best at propelling a diver through the water?

Freedivers select their fins for one reason, to move through the water as efficiently as possible. I think the general consensus is that a single mono-fin IS the best fin for that. Spearfishing freedivers desire more mobility than a single fin provides and they almost universally select long freedive bi-fins. The best performance is derived from carbon fiber and/or fiberglass composite blade material. This material has a high degree of elasticity and they return the energy delivered to them by springing back from the deformed position and this elastic return helps to push the water.

If jet fins pushed a diver through the water 'better" freedivers would be using them and they most definitely are not.

I have no military background, but from what i see in movies etc... the military divers may be jumping from high platforms, might be scrambling over rocks or obstacles as they approach shore and these types of applications are not going to work with a very long freedive fin. I imagine that a rugged dive boot is essential for them to scramble onto shore and venture onto land. freedive fins don't accommodate a heavy soled boot, you must use a neoprene sock or a lycra sock or go barefoot. I suspect that they use a particular type(s) of fin because it is the best compromise for the varied challenges that they are presented with.

In any regard, the fin choice(s) of the military may not be applicable to my personal diving situations.

I've tried a lot of fins and freedive fins with a well fitting, full foot pocket and CF or Fiberglass blades move me through the water "better" than any other fin. This doesn't mean they are faster, just more easily and more efficiently. Freedive fin blades are manufactured with a range of stiffnesses, so they can be used by an 80 lb child or a 250-lb athlete.

They also provide another benefit, you can kick slowly and steady and move along at a good clip. A diver using small, weak bladed fins, is going to be kicking faster and in my experience, this is more likely to scare fish away.

 

[tbone1004]

has nothing to do with scrambling around, they are just as awkward on land and they aren't used to walk around in. They have torque, flexible fins do not, no freediving fin can take a diver with full technical gear through current, it doesn't have the power. They are not as efficient, but they just don't have the oomph, a blade fin has torque and that's what you need to go through current and to go fast. Jet fins can kick faster and accelerate faster than any freediving fin, they are designed for acceleration and power, not maximum efficiency, it's a tradeoff, but required for things like any sort of technical diving or any rescue diving where maximum speed and acceleration could be the point between life or death.

You'd also be surprised at how stealthily you can move in a jet fin, if done properly they are far superior for fish surveying

 

[Kharon]

So technically, Jets with spring straps are STILL the best. :cool2:


Sorry, couldn't resist...

You may be right. Still I'll stick with my very rugged Aeris Accel fins. They fit into my carry-on and propel me just fine.

 

[dumpsterDiver]

has nothing to do with scrambling around, they are just as awkward on land and they aren't used to walk around in. They have torque, flexible fins do not, no freediving fin can take a diver with full technical gear through current, it doesn't have the power. They are not as efficient, but they just don't have the oomph, a blade fin has torque and that's what you need to go through current and to go fast. Jet fins can kick faster and accelerate faster than any freediving fin, they are designed for acceleration and power, not maximum efficiency, it's a tradeoff, but required for things like any sort of technical diving or any rescue diving where maximum speed and acceleration could be the point between life or death.

You'd also be surprised at how stealthily you can move in a jet fin, if done properly they are far superior for fish surveying

I used jet fins for many years. Until one dive when another diver was just kicking my butt, slowly cruising together, but i was burning way more air and working much harder. I realized then that he was not in that much better shape than me.

I don't have "full technical gear", but i sometimes dive with a big steel tank, a pony bottle, a stage bottle, a catch bag, a speargun, lift bags and a marine radio (among other things) in currents that are over 2-3 knots, and the freedive fins work best for me.

I never made time trials with different fins, but I realized a long time ago that i could swim very fast with jet fins, if i used a super fast short kick cycle.. i could not sustain it for any duration, but it was fast. That time of racing application is not applicable to most recreational scuba diving.

 

[tbone1004]

never said it was, long strokes if you can sustain them with the stiff blade fins are the fastest way through the water, but you have to have some serious legs to be able to do that.

Best analogy is to think about the warm blooded sharks and hunting whales. They all have relatively large, very stiff fins that are short and stocky compared to the rest of them. They are built for speed and acceleration. Think tuna, great whites, and the toothed whales

Now look at creatures like the humpback, the whale shark, and most of the smaller fish. They have relatively large fins compared to their body size, and they are relatively flexible. They use less energy to cruise at higher speeds, but they can't accelerate or hit the same top speeds. For things like diving in stiff currents, surges, and doing things like swift water rescue or rescue swimming, there is no alternative to stiff bladed fins, they take a lot more energy to move, but nothing moves faster or has the power when you need it.

 

[dumpsterDiver]

never said it was, long strokes if you can sustain them with the stiff blade fins are the fastest way through the water, but you have to have some serious legs to be able to do that.

Best analogy is to think about the warm blooded sharks and hunting whales. They all have relatively large, very stiff fins that are short and stocky compared to the rest of them. They are built for speed and acceleration. Think tuna, great whites, and the toothed whales

Now look at creatures like the humpback, the whale shark, and most of the smaller fish. They have relatively large fins compared to their body size, and they are relatively flexible. They use less energy to cruise at higher speeds, but they can't accelerate or hit the same top speeds. For things like diving in stiff currents, surges, and doing things like swift water rescue or rescue swimming, there is no alternative to stiff bladed fins, they take a lot more energy to move, but nothing moves faster or has the power when you need it.

nope, now you are mistaken... do some time trials if you don't believe me. short amplitude, very fast kicks are the fastest sprint times..

 

[tbone1004]

sprint times yes, for 50 yards sure, but as you get closer to top speeds, the frequency slows down a bit and the amplitude goes up quite a bit, in short sprints you're still getting up to speed by the time you hit the end. Run 500+yards and the first 50 or so is short and fast get up to speed then to stay there you have to have the legs, but you get about 24 inches of amplitude and you can maintain that top speed for quite a ways, it's fast as hell, lots and lots of energy required though

 

[Eric Sedletzky]

I use freediving fins for both freediving and scuba.
One of the things I can say about the efficiency of freediving fins and where they drop off is: When you freedive you are basically clutter free, light, unencumbered, and your body can "slip" through the water a lot more efficiently and quickly than when you have full scuba gear on.
Freedive fins work very well when they are not met with a lot of resistance from clutter that affects your slipstream. Start adding more scuba gear, and not just any scuba gear, but gear that has inherently lousy hydrodynamic qualities and all of a sudden freediving fins are a lot of work. It would be similar to driving a car with all sorts of stuff sticking out making it much less aerodynamic (poodle jacket BC or BP/W with way too big a wing, etc), then on top of that loading more weight into the car (a heavy tank) and then climbing a hill (going against current). The freediving fins would be like lugging it and using a lot more effort (gas) and would be less efficient than dropping a gear (using shorter fins) and getting a better RPM (leg kick cycle) for the amount of resistance, thus making it easier.
I've found that I can use freediving fins for scuba, but They work best when I am as minimal as possible. That means many times diving absolute minimalist with no BC, all hoses - one second stage hose and an SPG are tucked against my body as tightly as possible, nothing dangling or sticking out causing unneeded resistance. I am also very aware of my slipstream and constantly adjusting body position, my head, keeping my arms back, essentially trying to make as small a hole in the water as I glide forward. Freedivers do this to get as much out of one breath as possible, I just transfered this theory to scuba, so in a way It's like I'm freediving but I have a tank on my back and I can breathe underwater.

If I don't use long fins then I use Jets or Turtles. A lot of times I am hunting so I'm packing a spear gun, game bag, hook, light, scallop bars, big knife, sometimes an SMB, I'd be using a wing on my plate, etc. All this stuff adds up to more resistance. In that case it makes no sense to use freediving fins, that's what Jets and Turtles are for. The one thing I don't do that often is the feet up frog kick. I don't see the point where I dive because there isn't anything to silt up. It's all rocky boulder bottom which is kept silt free from the ever moving Pacific ocean, plus I generally don't ride the bottom (belly scratch). I prefer to keep both feet straight back and use various other kicking techniques to maximize my slipstream and efficiency.

So there's definitely a trade off and a line where long fins aren't always the answer.

 

[REVAN]

Bothered by my inability to swim against the surge last month, I bought new long fins, 2'4" long. But the improvement in my swimming speed was marginal. I'm wondering:
Am I using the wrong technique in swimming with fins?
Or did I get too large fins for my leg strength, I need to be in better shape?
What do you guys think? Thanks.

There are many variables in fin function, and without knowing anything about what you are using or the blade stiffness or construction, I cannot determine if there is a poor match of hardware to how it is being used or not. However, you should be aware of some basic physics that will help you to understand how things work and what to expect and to look for regarding performance.

There is a cube relationship between speed and power required. Even if the long blade fins are 50% more efficient, meaning that 1.5 times more of your physical output is being effectively converted to into forward thrust, the achieved swimming speed will only increase by the cube root of 1.5, or by about 14%. It is better, but it is not going to be spectacularly better. Where you should notice a big difference is when swimming at the same speed that you used to swim with your old fins. At that speed, you should only be exerting about 2/3, or about 66% the effort previously required, or looking at it the other way, 33% less physical exertion.

The bottom line is that if you want to actually swim significantly faster, you need to do more than just get better fins. Going fast requires drag reduction more than anything else, because every pound of drag eliminated is a pound of thrust you don't need to make. Combine an efficient long blade fin, with a streamlined rig and well balanced weight and trim, and then you will start to see some big differences in speed.