Thanks for the input everyone, this is what I love about DB! Project for the next rainy days: Neck weight. Probably the 2.5 kg option, since this is pretty much what I also need in a pool [emoji4]
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Extreme Dolfinism G2
- Thread starterREVAN
- Start date
Thread Status: Hello
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It can take a long time to get an up-to-date response or contact with relevant users.
It can take a long time to get an up-to-date response or contact with relevant users.
Not sure exactly what you are including in "duck diving technique" In my experience, the actual surface dive is not much different from using bifins. Good technique with an arm pull should get you down 15 ft or more. After that, the Dol-fi gets tricky, primarily because it's negative and has so much less drag than bifins or normal monos. The fin wants to fall faster than you do and there is a strong tendency to fall sideways if your weight is on a belt. Makes it hard to relax on the way down. The Dolfin definitely needs a neck weight to counter the lack of drag. I've added floats to mine, several different designs. The wood ones improve the surface dive because of the added weight of the wood, getting that up in the air helps drive the body down. I'm now experimenting with non compressible, supposed to be closed cell, foam floats that will make the fin substantially more buoyant than water. If I'm right, the floats, the neck weight, plus a couple of extra pounds on the belt will make for a very relaxed decent, with no tendency for the fin to try to lead the dive.
Well, the issue I was a bit struggling with was that it took quite some effort to lift the fin out of the water, as it was well below the surface after when starting the duck dive after the breath-up. Thus, I had a bit of trouble to recreate the same fluid and efficient duck dive I'm used to from my bi-fins. But I think using a neck weight will already be a significant improvement.
Thanks for the pointer on free fall, I'll watch out for that you've described the next time I'll have the chance for a "serious" diving session. Which material are you using in your floats as an incompressible wood-substitute?
Thanks for the pointer on free fall, I'll watch out for that you've described the next time I'll have the chance for a "serious" diving session. Which material are you using in your floats as an incompressible wood-substitute?
You have to move forward a bit if the fin is not on the surface or very near it. Once the fin is on the surface, Try a surface dive technique where you are on the surface starting the dive, bend your knees,bringing your feet up to your butt, pivot your body so that you torso is straight down, then straighten your knees fast, shooting the heavy fin up into the air. This works pretty well with a Dolfin.
I'm now using some 8 lb density closed cell foam that I bought. Stuff is expensive, but, if you want some, let me know. The stuff I made wasn't closed cell enough. What I've got now is imcompressible at my kind of depth and not supposed to absorb water. We will find out. I've had it to 90 ft once and no sign of water intake yet.
I'm now using some 8 lb density closed cell foam that I bought. Stuff is expensive, but, if you want some, let me know. The stuff I made wasn't closed cell enough. What I've got now is imcompressible at my kind of depth and not supposed to absorb water. We will find out. I've had it to 90 ft once and no sign of water intake yet.
Wood is tempting, and so far is the best thing I've tried because of it durability under punishment. However,the wood available to me(white pine) is relatively heavy(about half the density of water) and requires a large float. I'm not wild about the increased inertia at the end of my toes and think it its just a bit draggy. Those things would be even more true for a float big enough to make the fin substantially positive, like I'm experimenting with now. So, I'm trying foam again. The first time didn't work out so good. This time, maybe better foam and I've incorporated a wood pad to absorb the punishment. Have to use it more to see what I think.
I did try the pipe. Thanks for the idea. It sounded real good, but I could not get it to work, buoyancy wasn't enough until the pipe got really large and draggy. PVC is surprisingly heavy. Attaching the pipe to the fin was also an issue. I did not want to put a screw hole in the pipe. Strapping it on wasn't stable enough. There is a surprising degree of torque on the pipe while stroking, everything I tried came loose. Maybe if I found some very thin walled pipe and glued it to the fin with 5200, it might work.
After all this experimenting, I have MUCH more sympathy and respect for Ron and his development process.
After all this experimenting, I have MUCH more sympathy and respect for Ron and his development process.
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Few people have any idea of how much work it is to actually make something that works well and that is somewhat affordable to reproduce. It's much easier to just build something pretty than it is to make something function on par or better than other monofins with 40 years of development history behind them.
It's definitely a great piece of work you've put together! I've great respect for the solid development process behind it, and for your efforts to share it with us [emoji4]
For the float, I was thinking about a combination of fiberglass supported by spray foam insulation. It should be rather easy to mold the fiberglass shape, which could get structural support from a sufficiently hard foam core.
For the float, I was thinking about a combination of fiberglass supported by spray foam insulation. It should be rather easy to mold the fiberglass shape, which could get structural support from a sufficiently hard foam core.
I tried something very similar to that, made some 4 lb density foam(supposed to be closed cell), shaped it and covered with epoxy impregnated glass cloth. I had two problems, first, the glass cloth wasn't near durable enough, came apart under hard use.
Second was water intrusion into the foam when I took it deep. It seemed to work fine in the pool, but after several 80-90 ft diving days, the foam was well on the way to soaked. It might have worked if I could have completely incased the foam in epoxy(no screw holes), but I needed some screws to hold the float on the fin.
Second was water intrusion into the foam when I took it deep. It seemed to work fine in the pool, but after several 80-90 ft diving days, the foam was well on the way to soaked. It might have worked if I could have completely incased the foam in epoxy(no screw holes), but I needed some screws to hold the float on the fin.
@REVAN Out of curiosity I tried searching for 'plasticised wood' but couldn't immediately find it as a product but rather some links to the process of doing it. Do you know of any suppliers of plasticised wood? There are some epoxy based wood treatments that are used in marine applications but it's not immediately clear that they would work at the depths being talked about here.
@Philip Fennell : It is just vinyl coated wood. I use a light wood like douglas fir and cut it to shape and drill the mounting holes. I pot the holes with resin and create a plastic outer surface on the float with spray-on vinyl truck bed liner. The wood creates the floatation and structure, the plastics seal it up and keep the water out. The wood is much stronger and less expensive than foams.
Other options are to seal the float with a wood stain and resin based clear-coat, which looks awesome, but is harder to apply and make it look good, and it is more disappointing when it gets scratched up.
The nice thin about the truck bed liner is that it is easy to re-apply the vinyl on top of the old coating if the old coating gets too damaged and needs repair. Just remove the floats and spray it again, or even just brush it on.
On the Orca2, the polycarbonate fairing protects the internal float, so its water resistant coating probably won't ever be damaged such that it will need to be recoated. That's just one more reason why I decided to drop the X-22 design and couple the floatation/buoyancy with the Orca2, and make the less expensive Pilot without flotation.
Other options are to seal the float with a wood stain and resin based clear-coat, which looks awesome, but is harder to apply and make it look good, and it is more disappointing when it gets scratched up.
The nice thin about the truck bed liner is that it is easy to re-apply the vinyl on top of the old coating if the old coating gets too damaged and needs repair. Just remove the floats and spray it again, or even just brush it on.
On the Orca2, the polycarbonate fairing protects the internal float, so its water resistant coating probably won't ever be damaged such that it will need to be recoated. That's just one more reason why I decided to drop the X-22 design and couple the floatation/buoyancy with the Orca2, and make the less expensive Pilot without flotation.
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Revan i'm interested to know how you selected the foil profile that you're using ,trial and error or theoretical understanding. i'm guessing that at some stage you've tried a symmetrical section fin ,if you have i'd like to know how it compared in both thrust and drag and was there a large difference between the section you decided was best compared to the profile you thought was the worst for your setup ,now I know that the up and down strokes aren't equal due to human physiology but i'm thinking this could help the fin track ,and another question didn't you find with I think it was the x 20 model with the vertical struts that it was helped by said struts to stabilise the fin, helping to counter the fins tendancy to overtake the diver.
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"...interested to know how you selected the foil profile that you're using ,trial and error or theoretical understanding."
All engineering is some combination of both. The more mature the technology, the less trial and error is needed to achieve an acceptable design. Without any theoretical understanding, design iterations are nothing more than random attempts at improvement and it can take forever to stumble onto something that works.
Even with my background in fluid mechanics, I had a lot to learn about unsteady fluid flow with oscillating propulsive foils. It's just not a well understood field in engineering (at least not when I started all this back in the '90s). So, I had a fair amount of trial and error involved in the early stages of development for the DOL-Fin. Now I know a lot more about it, so there is not so much error in my trials any more, but I can still be surprised more often than I'd like.
Along the way, I did build and test a symmetric hydrofoil. I found that was inferior to the cambered foil in the DOL-Fin application. It should be no surprise to anyone that the selection of the foil can have a huge impact in the overall performance of the monofin. The design and shape of the fin blade determines what the fin can do, and how it is used determines what it will do, within those constraints of what it can do. A good design requires balance between both capability and uses within the operational space.
"...didn't you find with I think it was the x 20 model with the vertical struts that it was helped by said struts to stabilise the fin, helping to counter the fins tendancy to overtake the diver."
The vertical fins on the X-20 can help some with stability in the freefall phase of a dive. However, the vertical fins have a fairly large deadband about them due to the wake from the shoe and required large ankle deflections to have an effect. I use the vertical fins with a kind of bang-bang steering control input, and with that I could use them effectively, but from my observations, most people don't seem to use appropriately aggressive steering inputs for it to work. So for most people, I think the vertical fins are ineffective and may as well not even be there. In that respect, there is no real difference between the X-20 and the Pilot, even though there could be a difference with proper training and use.
All right, got my attention. I'm "most people", don't feel any significant steering effect from the vertical struts, which surprised me. Ron, how exactly are you able to get some steering impact? A little "proper training" might be useful for me.
I use a lot of side-to-side deflection (about the most that I can get out of my ankles), and it has to be applied early, before there is any significant perturbation away from vertical. If you get too far behind the divergence, there won't be enough authority in the tailfins to correct the disturbance. The better your weight is trimmed the more error can be tolerated before recovery isn't possible without using fin strokes for more authority. If you are not diving on a line that gives you a good vertical reference, I don't think it will be possible estimate the error accurately enough to make it work without using neck weights to make yourself less unstable.
Using a camera boom with twin GoPros mounted to it (which creates a little extra drag), I documented the following performance with the Pilot2 on my first trial swim with the prototype as documented in the video below:
16.5 seconds with a push and 5 fin strokes per 20 meters, with KKG technique. Average speed is 1.25 m/s.
Extrapolating that performance to 25 meters predicts:
20 seconds with a push and 6 fin strokes per 25 meters, with KKG technique.
Does anyone have some comparable statistics for a Specialfins or WW Nemo, or other recreational flyer style monofins? I've seen a lot of useful videos for hyperfins, but I'm having trouble finding good videos for these recreational models. Most of the ones I've found are "first swim" videos and the swimmers don't have refined monofin technique to make it a good baseline for comparison.
16.5 seconds with a push and 5 fin strokes per 20 meters, with KKG technique. Average speed is 1.25 m/s.
Extrapolating that performance to 25 meters predicts:
20 seconds with a push and 6 fin strokes per 25 meters, with KKG technique.
Does anyone have some comparable statistics for a Specialfins or WW Nemo, or other recreational flyer style monofins? I've seen a lot of useful videos for hyperfins, but I'm having trouble finding good videos for these recreational models. Most of the ones I've found are "first swim" videos and the swimmers don't have refined monofin technique to make it a good baseline for comparison.
