Do split fins have less umph than blades?

[Rob9000]

After reading all of your input, here is another way I can explain what I am thinking: blades (including all fins that are not split, like Aqualung models) give the diver greater ability to move more water per fin thrust with maximum thrust effort from the diver, while split fins have more limited ability to move as much water per max thrust kick, but when used at more moderate thrust input the split fins are more efficient than blades. So if just cruising at a slow to moderate pace, then splits have an advantage. When the diver needs to push a lot of water to get through a current or catch up to someone, then blades have an advantage of being able to turn a sprinting input in to more water moved and more forward propulsion. Blades can have more power, but will take more power from the diver to generate it. At lower power inputs, the splits convert the thrust input in to more forward propulsion.

 

[Charlie99]

You're contradicting yourself. If split fins "don't deliver a strong thrust from just one kick" then they don't deliver strong thrust from a bunch of kicks either. The fin that delivers the strongest thrust per kick is the winner.

That's not a contradiction. I suspect that you haven't tried out both types of fins. Most people that have, will immediately recognize what I mean.

Test routinely show that divers using split fins can achieve and sustain a faster speed through the water. The thrust per each kick may be less, but you do many more kick cycles per minute. With blade fins you can deliver a big push from one or two kicks -- for instance when propelling yourself up over the tube of a RIB. That sort of extreme effort or exertion, though, can't be maintained while continously swimming. With most splits, if you try to do a couple of big, hard kicks to get way up out of the water to get over the side of a RIB, you just don't get anywhere.

 

[Charlie99]

After reading all of your input, here is another way I can explain what I am thinking: blades (including all fins that are not split, like Aqualung models) give the diver greater ability to move more water per fin thrust with maximum thrust effort from the diver, while split fins have more limited ability to move as much water per max thrust kick, but when used at more moderate thrust input the split fins are more efficient than blades. So if just cruising at a slow to moderate pace, then splits have an advantage. When the diver needs to push a lot of water to get through a current or catch up to someone, then blades have an advantage of being able to turn a sprinting input in to more water moved and more forward propulsion. Blades can have more power, but will take more power from the diver to generate it. At lower power inputs, the splits convert the thrust input in to more forward propulsion.

Actually, if you have the discipline to maintain the proper small amplitude kick while increasing the frequency of kicking, then the splits are still more efficient even at the highest speeds. Several tests, by different magazines and groups have consistently shown that divers using split fins can achieve the highest sustained speed. If you keep kicking the same way, whether or not you are in a current makes no difference. It's just speed through the water. If you panic and start kicking harder and wider, though, then the splits perform horribly.

To use your term, the split fins have less "umph".

On various threads about split fins, some posters will claim that the tests are invalid, that they were rigged, etc. The proponents on each side sometimes get very close to calling the others liars.

There are too many independent reports of "split fins don't work in currents" to just ignore them. At the same time, physics says high speed through water means that you should also get high speed going against currents, and that is indeed the experience of many other divers.

IMNSHO, this indicates that split fins are very sensitive to the kicking style, and that some divers cannot maintain the proper style of kicking when faced with a situation where they MUST make progress against a current to get back to a boat or exit point.

Charlie Allen

 

[DeepSeaExplorer]

That's not a contradiction. I suspect that you haven't tried out both types of fins. Most people that have, will immediately recognize what I mean.

Test routinely show that divers using split fins can achieve and sustain a faster speed through the water. The thrust per each kick may be less, but you do many more kick cycles per minute. With blade fins you can deliver a big push from one or two kicks -- for instance when propelling yourself up over the tube of a RIB. That sort of extreme effort or exertion, though, can't be maintained while continously swimming. With most splits, if you try to do a couple of big, hard kicks to get way up out of the water to get over the side of a RIB, you just don't get anywhere.

Doing more kicks per minute to get the same amount of thrust is a decrease in efficiency, not an increase.

As to whether it can be maintained, that comes with experience and time in the water to fully develop kicking technique. I routinely do dives where I'm swimming continuously for up to 5 hours with no break.

 

[mdb]

Doing more kicks per minute to get the same amount of thrust is a decrease in efficiency, not an increase.

DeepSeaExplorer: Doing a higher amplitude kick with far less effort is, obviously, way more efficient. Speed is speed, if you can do 2.6mph in no current, you can do the same in current. Just your kick style. For those who prefer a stiffer fin, a wider scissor kick, there are many versions of split fins that do so. You may prefer paddle fins, ducks and frogs sure do, others have adopted a style of fin that seems to be the choice of many Marine mammals, and fast moving pelagic fish. My garage is still full of SP jets, Voit Duck feet, Imperial Turtles, Mares-Planas, Avantis, and various others. These days I prefer my Apollo XT's, since I am used to a wider scissor style kick and like the "feel" of the stiffer fin. My kids think I am nuts and they dive with the bio-fin pro's. All individual diver preference.

 

[Charlie99]

Doing more kicks per minute to get the same amount of thrust is a decrease in efficiency, not an increase.

As to whether it can be maintained, that comes with experience and time in the water to fully develop kicking technique. I routinely do dives where I'm swimming continuously for up to 5 hours with no break.

Energy or work is the product of distance and force. With split fins, you have very little resistance. Even with a very rapid flutter kick, you aren't doing a lot of work.

You can see the same sort of thing in car engines. Horsepower is a function of both torque and RPM. Looking at either just torque or just RPM doesn't give an accurate picture of how much power the engine is delivering.

 

[art.chick]

The way I look at split fins is that they are most appropriate for people with injuries or who are new to the sport and lack leg strength at the moment. If I were assisting a student who knew he was just going to boat dive in mellow, tropical waters, and he had had knee surgery (like me), I would say, "get split fins" to get him going in the sport. I don't see the advantage for a person who has the capacity to get his leg strength up enough to use real fins, though. I only dive with Jets.

 

[tceylan]

During a dive, you can always switch to split fins in an emergency by cutting conventional fins using diver's knife if you need more speed/efficiency.

 

[John_B]

During a dive, you can always switch to split fins in an emergency by cutting conventional fins using diver's knife if you need more speed/efficiency.

When I first saw this thread I wondered if the OP asked the question after reading this thread in the Accidents and Incidents forum, where the diver who did the fatality analysis for the ME points out that the deceased diver was diving with conventional fins that had been intentionally cut down the center to create a sort of DIY split fin:

4. Perhaps most significantly, he had taken his fins and cut them down the middle to mimic a split fin design. By doing so, he significantly reduced the amount of thrust he could get from the fins.

John

 

[tceylan]

My apologies. If I did know about that incident, then I wouldn't make jokes about cutting conventional fins to turn them into split fins.