Split fin hate?

[John C. Ratliff]

Do you notice a difference in how they kick? I assume so...

Scoop design, does it frog kick well?

What material do you use for the scoop?

I have used both a flexible sheet material of a neoprene-like material, and also a rubber-impregnated nylon fabric. It needs to be flexible, but not stretchy, if that makes sense. It also needs to start tight near the toes, and a simi-circle at the tip of the fins.

I used the material that I cut out of the fin as a cover material for the fabric, after cutting it into strips. I have enclosed a photo of the back side of my fins to show how I did it. I then used aluminum pop rivets to secure it, with a coating of Shoe-Goo on each underside.

It does work with a frog kick, but slightly differently. And, there is no difference if you use the top of the foot for the kick, or the sole (I've seen it both ways).

When I first started this with the Voit Viking fins (see the above blue fins with an orange interior material), the Vikings were so stiff that I could hardly kick them and bend them. With this modification they worked much better. During my U.S. Air Force career as a Pararescueman, took this design with the Avanti fins on parascuba jumps, jumps out of helicopters, and many, many dives.

They work quite well, without the huge stress on the leg muscles that stiff fins such as the Jet Fins have. Jet Fins work on the principal of having the 'jets" just below the toes, but that only (sorta) works on the downstroke. On the upstroke there is a lot of "dead area" in the middle of the fin. With the scoop design, there is much less "dead area" on the fin, as can be seen by the strings in my 1970 photos. (I cannot believe that those photos are 53 years old now, but they are!)

SeaRat

PS, it is very easy to make a set of split fins out of any flat blade without a middle rib; simply drill a hole below the toes, then use a knife (a filet knife with a thin blade works best) to cut the blade straight down the middle (along a pen or pencil line) to the end of the blade. The hole keeps the cut from traveling up the blade to the foot pocket. The split blade allows the water to flow down instead of off the side of the dead area of the blade. This produces a noticably easier stroke on the legs, and actually a more efficient water flow for the fin.

As shown above, there is an advantage of the split, but the scoop is even better as the water flow goes straight out of the blade, rather than running through the split.

 

[-JD-]

@John C. Ratliff thanks for posting up your experiments and development!

I'm curious what you think of the Exotec "Folding Fins" execution. Looking at the video, they don't have the "slack" ( as yours do) at the trailing edge when unloaded, but do seem to form a channel as the membrane loads up and pulls the ribs inward.

Since they promote the fin as being modular with repair parts, including the membrane replaceable, It look like it would be relatively simple for them to produce a membrane spanning a slightly wider angle which would enlarge the volume of the channel.

I guess the flipside question is if the slack results in dead-spots at the transition vs channels produced by stroke-force causing inward flexing of the ribs. What is the net effect in efficiency of lost energy in flop vs possibly less than sufficient channel volume/shape and also what is the tradeoff for precision/control.

ETA: first SB thread on the Folding Fins: Another Odd Fin

 

[lexvil]

The only fins I hate more than split fins are fins that are to stiff.

 

[John C. Ratliff]

@John C. Ratliff thanks for posting up your experiments and development!

I'm curious what you think of the Exotec "Folding Fins" execution. Looking at the video, they don't have the "slack" ( as yours do) at the trailing edge when unloaded, but do seem to form a channel as the membrane loads up and pulls the ribs inward.

Since they promote the fin as being modular with repair parts, including the membrane replaceable, It look like it would be relatively simple for them to produce a membrane spanning a slightly wider angle which would enlarge the volume of the channel.

I guess the flipside question is if the slack results in dead-spots at the transition vs channels produced by stroke-force causing inward flexing of the ribs. What is the net effect in efficiency of lost energy in flop vs possibly less than sufficient channel volume/shape and also what is the tradeoff for precision/control.

ETA: first SB thread on the Folding Fins: Another Odd Fin

Give me a little time to explore these unique fins. I'll let you know my opinion in a few days.

SeaRat

 

[boulderjohn]

The only fins I hate more than split fins are fins that are to stiff.

A couple decades ago, when my arthritic knee was a major factor in my life, and I only had a couple hundred dives, almost all in the tropics, I was on a liveaboard trip with a group of friends, and we swapped fins for a few dives. As a result, I got to compare several fins. One that I absolutely rejected was the Mares Avanti Quattro. I told the others that it was so stiff I felt like I had a sheet of plywood strapped to my feet. I bought a much easier kicking fin, the Mares Power Volo.

I am now a couple decades older. That formerly arthritic knee was replaced long ago, but the other one is now worse than the replaced knee was then, and it will be replaced in November. For the past probably 15 years, I have almost exclusively used stiff fins, including ScubaPro Jets, and Hollis F1s. For NDL reef diving today, my fin of choice is the Mares Avanti Quattro +, which I take to lighten the load for airplane weight limits. Yes, that is the very fin I absolutely rejected years ago. I don't even give its stiffness a second thought. It just feels like a fin to me now.

So my theory is that stiff fins are noticeably stiff, uncomfortably so, if you are accustomed to softer fins. You are judging them in comparison to what you are used to. If you use them a while, you get used to them, and you don't notice it.

 

[John C. Ratliff]

---

If you'll look at the photos of my Avanti Scoop Fins design, you'll see that the outside ribs are actually reinforced with a second layer, to house the scoop membrane. But because the blade is no longer there, the outside ribs are actually more flexible, even with this reinforcement, than the original Avanti blade. This takes a lot of the load off the leg, and the difference between the two is noticeable.

By the way, there is a vast difference between how the Avanti fin flexes in warm, tropical waters and in my Pacific Northwest water, where temperatures rarely get above 72 degrees F.

My initial experiment was with the Caravelle fins by U.S. Divers Company in 1968 when I was in Okinawa and in the USAF. Here's a photo of it, with a sheet metal reinforcement. The center membrane is from a tire inner tube.

SeaRat

 

[lexvil]

A couple decades ago, when my arthritic knee was a major factor in my life, and I only had a couple hundred dives, almost all in the tropics, I was on a liveaboard trip with a group of friends, and we swapped fins for a few dives. As a result, I got to compare several fins. One that I absolutely rejected was the Mares Avanti Quattro. I told the others that it was so stiff I felt like I had a sheet of plywood strapped to my feet. I bought a much easier kicking fin, the Mares Power Volo.

I am now a couple decades older. That formerly arthritic knee was replaced long ago, but the other one is now worse than the replaced knee was then, and it will be replaced in November. For the past probably 15 years, I have almost exclusively used stiff fins, including ScubaPro Jets, and Hollis F1s. For NDL reef diving today, my fin of choice is the Mares Avanti Quattro +, which I take to lighten the load for airplane weight limits. Yes, that is the very fin I absolutely rejected years ago. I don't even give its stiffness a second thought. It just feels like a fin to me now.

So my theory is that stiff fins are noticeably stiff, uncomfortably so, if you are accustomed to softer fins. You are judging them in comparison to what you are used to. If you use them a while, you get used to them, and you don't notice it.

Hate to tell you this but I try different fins all the time, I don’t consider jets to be stiff, the are actually quite lively. The Mares are some of the worst for me, unstable.

it’s good that there are so many choices, we can all find our Goldilocks fin, to bad so many don’t try other fins.

 

[John C. Ratliff]

There are many different types of Jet Fins. I was issued a pair of Lightning Jet Fins, with the lightning bolt in the logo. Those are longer, and more flexible, than the earlier Jets. So I think it depends upon which Jet Fin, and which era, you’re looking at as to stiffness.

 

[Bob Evans]

I wish I had more time to bring some light to the " Split Fin" name, which I coined years before others came into the marketplace. Foil Force Fin - Force Fins | Worlds Finest Fins for Swimming, Diving, Bellyboating

Susanne and I received the DEMA award for the most likely product to succeed at the show. Over the years I pushed the concept into some fantastic designs.

 

[BoltSnap]

I wish I had more time to bring some light to the " Split Fin" name, which I coined years before others came into the marketplace. Foil Force Fin - Force Fins | Worlds Finest Fins for Swimming, Diving, Bellyboating View attachment 793113View attachment 793114Susanne and I received the DEMA award for the most likely product to succeed at the show. Over the years I pushed the concept into some fantastic designs. View attachment 793110View attachment 793111View attachment 793112

I have known you since I owned my dive store in NY in 1994, I believe that you should get a "Best Creative/Innovative mind in the dive industry" award! Very few, if any, people have your talent and the boldness to do what you have been doing in the dive industry.