Who dares to suggest a reason for this?

[ADR]

maybe it's spiritual.........

[Walrus]

That type of design with "vents" near the footpockets has been used quite a bit in Bi-fins. Started at least 30 years back with the original "jet fins" and is even used with some of the newish hybrid Jet fin / split fin types as well as other wierd designs. If my memory serves me correct there was a long bladed freediving fin with holes like that used in the "The Big Blue" movie.

Good or bad, I don't know. My thinking is that the part of the blade nearest the footpocket doesn't provide much thrust so perhaps the holes reduce the resistance but not reducing the overall thrust as much. It would effect the flow dynamics a lot, I'm not the expert on that.

I would be a bit worried if it makes the fin weaker. If after paying 400 - 500 Euro for fin and delivery, months of waiting... A broken fin would cause some pretty severe hair loss

[ADR]

...some of my older monofins just called me to say they are feeling very nervous

[ADR]

My fins rightly felt nervous as I pulled out the hole saw yesterday and tore into one of my fins. The fin is a hybrid that i put together from my Waterway glide footpockets and a standard blade from a waterway model 1 (medium stiffness)

Strangely the holes had less impact than I thought they would. The blade stiffness did not seem to really change and the number of kicks per 25m went from 5 to 5.5(single kick and glide) The only noticable thing to me was a feeling of increased drag. Not sure why the holes appear in some production fins. I imagine it's to do with setting up a certain type of flow across the blade and have asked someone with a training in fluid mechanics to have a look at it.

....a very strange feeling indeed taking a hole saw to a monofin

[trux]

Yes, as Walrus wrote, the holes should reduce the transversal resistance of the fin - in the place near the footpocket, the blade moves water only perpendicularly to the swimming direction, hence you spend a lot of energy with no gain of forward propulsion.

I suspect your holes are too small to have an effect you could sense when kicking, and only increase the drag in the glide phase. The plain round holes will also likely create more turbulences than the opening as we can see in jet fins - those channel the water more efficiently and with less turbulences.

[Mullins]

Go on Andy, take a great big chunk out and see what happens

[ADR]

comparisons to jetfins I think are a little tenuous. I had a bunch of pairs in my DIR tech diving days and they effectively have one way vents rather than holes that do nothing in the up kick but channel the water in the down kick. I have attched a jetfin pic so you can see what I mean. While the shape of the holes will make a difference the jetfin comparison doesnt really help us with knowing what to do next as they effectively have two surfaces to play with. Think of a yacht with two sails and the vent/hole effectively the gap between. We (to continue the analogy) have only one sail and are cutting holes in it....sorry but i'm from a sailing family

[HJ]

Originally Posted by Mullins

Go on Andy, take a great big chunk out and see what happens

Hey guys, I couldn't resist. Howz this for a big chunk cut out? ;-) See below pic.

[I hoping my photo uploads and attaches here... don't really know how to do it]

I'm the newcomer/beginner here and have been posting on another thread. So I don't have much mono experience, but it's very intriguing to me, especially the hydromechanics of it all. I've never understood why monofins were designed like spatulas! All that drag in the middle! Seems like a lunate design could work well, don't you think? The photo shows a crude mono prototype that Bob Evans (Force Fin in Santa Barbara) made for me back in 1993-94 when I remarked on the fact that all fast-swimming fish have high-aspect-ratio lunate-shaped caudal fins (e.g., swordfish, blue fin tuna), and most monofins did not. Bob patented his design in 1994 (you can see his patent on that "Weird Fin Collection" link). As I said, the prototype shown is crude... needs more gradients and things (it's basically a flat piece of fiberglass). Anyway, I took it on a pool test drive recently, and it worked for me (again: not experienced), although clearly room for improvement. The bungie/web strap foot pocket looses some energy transfer (and probably creates drag), but I like the comfort and toe-in design. I have a broad forefoot and have never liked closed foot pockets much. Bob also has different foot pockets with a simple strap in the back; I use the swim-training ones all the time. FYI: Bob has approved my showing and talking about this. I did some marketing for him many years ago, but I have no financial connection to him. He's not currently in production with it so you couldn't buy one in any case. Just for curiosity sake.

What do you think about monos going in this lunate direction? (see also Ted C's "Lunocet")

Harald Johnson

[BennyB]

Andy - you're a brave man mate!

Harold - looks like someone's attached a pair of crocs to a boomerang

[trux]

Originally Posted by harald johnson

Hey guys, I couldn't resist. Howz this for a big chunk cut out? ;-) See below pic....

Interesting, I'll need to add all these photos to the weird collection too
I am afraid though that this last design cannot be really efficient, because you can't really get the blade into the right angle of attack in the upward and downward kick. The angle seem to be pretty constant and the blade seems to bend only from the foot-pocket to the extremity, hence in practically 90 degrees to the swimming direction (well, it is little bit less due to the angle of the demi-lune - around 60 degrees at the extremity). On my mind, it does not really help with the propulsion a lot.

Well, of course, I believe that you can swim with it, but I am persuaded the efficiency is far from the ideal. On the other hand, on my mind the design of current monofins are certainly not ideal either. Lunocet may be a good way to go - the blade halves are actively inclined against the kick, and I believe if properly tuned up and with a trained kick, they may be more efficient. Time will show.

In our club, we have an university professor working in department of physics of fluids. We already spoke that he could let his students research some freediving fins related topics, we just did not get yet to define the jobs. Modeling and comparing different shapes of fins like these ones may be one possibility.

[HJ]

Originally Posted by trux

Interesting, I'll need to add all these photos to the weird collection too

Hey, Trux... talk to me before you add this photo to your collection; I'd like to be able to give you the correct information. Thanks.

I am afraid though that this last design cannot be really efficient, because you can't really get the blade into the right angle of attack in the upward and downward kick. The angle seem to be pretty constant and the blade seems to bend only from the foot-pocket to the extremity, hence in practically 90 degrees to the swimming direction (well, it is little bit less due to the angle of the demi-lune - around 60 degrees at the extremity). On my mind, it does not really help with the propulsion a lot.

Regarding angle of attack, I don't see the problem. The whole blade changes angle as the body undulates. The blade is pretty stiff so there's not much (although some) bending or "folding."

I guess it depends on the purpose of the monofin. If we're adding a tail to the human body, and if the purpose is endurance speed, then why not copy the champions in the natural world: blue fin tuna and swordfish? Take a look at their tail fins:

http://indian-river.fl.us/fishing/fish/sword.html
http://en.wikipedia.org/wiki/Northern_bluefin_tuna

They have a classic high-aspect ratio (span to length). As Ted C. says about his Lunocet's: "- high aspect ratio... the higher the ratio of span to chord length (chord length is the distance from the leading edge to the trailing edge), the greater the efficiency. Classic monofin designs have poor aspect ratio." I agree and my prototype reflects this. Compare my experimental fin to the drawings of the tuna and swordfish.

Now, if the purpose is slower movement and conservation of energy, maybe a lower-aspect ratio is in order. More like a whale or dolphin. Google "dolphin tail" and see top, left photo. Much lower aspect ratio. Or how about slower and lower yet: like a Grouper, see:
http://en.wikipedia.org/wiki/Goliath_grouper
The tail on this guy actually looks most like a traditional monofin.

In our club, we have an university professor working in department of physics of fluids. We already spoke that he could let his students research some freediving fins related topics, we just did not get yet to define the jobs. Modeling and comparing different shapes of fins like these ones may be one possibility.

Cool. What club is this? And what university? Where?

HJ
(who is not an engineer nor a physicist but just likes to think about this stuff)

[Mullins]

Tails need angle of attack in order to create any forwards thrust. Humans wearing flat blades have 0 degrees angle of attack. In order to get any angle, a blade needs to flex. Otherwise you'd just be pushing a flat surface sideways through the water, for no benefit.

Of course the lunocet gains angle of attack a different way, by varying the angle of the entire blade surfaces.

[HJ]

Originally Posted by Mullins

Tails need angle of attack in order to create any forwards thrust. Humans wearing flat blades have 0 degrees angle of attack. In order to get any angle, a blade needs to flex. Otherwise you'd just be pushing a flat surface sideways through the water, for no benefit....

Hi Dave -- I just checked the fin again, and it does have flex front-to-back. More flex in that direction than lateral (folding).

But I'm not sure I agree with you and flat blade thing. I've worn very stiff, basically flat fins, and there's still plenty of thrust (maybe not the most efficient). What you have is a foil moving through water at different angles depending on where your legs and feet are. And also depending on the shape of that foil. I'd love to see a diagram or a computer animation showing lift and drag forces of various fins in motion. Anyone have that???

[Mullins]

I think if you looked at a video of yourself wearing those stiff fins, you'd find they were achieving an angle somehow. Probably through either the blades or the footpockets flexing.

I get the impression the blades on the lunate mono would flex sideways quite a lot, which would give you no benefit.

[Bill]

'I guess it depends on the purpose of the monofin. If we're adding a tail to the human body, and if the purpose is endurance speed, then why not copy the champions in the natural world: blue fin tuna and swordfish?'
The very simple reason is speed (power to drag). Sailfish and Wahoo are usually considered the fastest fish and at 30-40 knots they have a P/D ratio about ten times the best human. If you'd really like to feel puny calculate the speed required to lift the centre of gravity 3-4 meters out of the water in a dolphin show and then consider that no human has been able to get to 1M or even clear of the water.
The grouper and large whale comparisons are closer to what we need. When you see a freediver leaving a greasy trail in the water, you'll know that someone has done his homework on fish that reduce drag.