Extreme Dolfinism

[trux]

Ron will tell better, but from what I saw, I think it does not change the working angle. The blade stays in the same working angle during the entire motion (up or down). Well, it changes slightly because the angle of legs and feet is variable during the kick, which adds to the angle of the blade. Ideally the angle of the blade towards the direction of the movement should be constant (better told, depending on the forward velocity). So the angle of the legs adds some deviation from the ideal angle, but since the DOLfin is further from the body, you can reduce the amplitude of your kick (while maintaining the same amplitude at the blade), and hence you also reduce the angle deviation induced by the legs/feet. Not bending the knees is also important to reduce messing up with the working angle.

You are right, Connor, that the Aqueon changes the angle, because there is a spring trimming it, and keeping in position. Unfortunately, the Aqueon does it in the opposite way than is needed - the more you push, the bigger angle it has. In fact you'd rather need a variable angle depending on the speed, not on the force you use on the blade. The higher speed, the smaller angle you need. And Aqueon does not do it.

 

[noa]

Hi Trux,
Interesting explanations although somewhat confusing for me. If the angle of the hydrofoil does not change during the kicking stroke, how would it create forward propulsion ( especially at slower speeds) ?
The way I would understand that a foil like the one on a Dolfin works is the contrary to a classic "elastic" fin, it would be at an optimal angle of attack producing propulsion at almost every phase of the stroke. But it would still be working with an angle induced by the kicking.
Did I get anything right or am I completely off ?
Posted via Mobile Device

 

[trux]

What I wrote was that the angle does not change during the kick in one direction, and that's like it should be for achieving the optimal propulsion. But it reverses into the opposite position as soon as you change the direction of the kick (I speak about the upward and downward kick directions).

So the angle direction is induced by the kicking, but not the angle size. That's determined primarily by the angle delimiters (at least that's what I understood from the design, and what would make sense). At selfsustaining hydrofoils, the angle is also delimited by the forward velocity (simplified told: the streaming water pushes the blade back into lower angle with growing velocity), which is the ideal situation, but I do not know whether the hydrofoil of DOLfin is fully selfsustaining, partially only, or not at all.

Angle proportional to the force of the kick, as at the classical monofin, is the improper way, and it contributes to the loss of efficiency. You need bigger angle of attack at slow speeds, and smaller angle at high speeds, and not angle changing with the effort you put into the kick as at a monofin.

So from this point of view the hydrofoil has a big advantage in comparison to an elastic fin. The disatvantage are the dead ends of the kick. When the fin reverses its movement, and the blade flips over, there is practically no propulsive force for a short moment. At an elastic fin, there is always a smaller part of the blade in the working angle of attack, even during the kick direction reversal.

 

[REVAN]

I'll try to explain in words how it works, but the physical details of all the force interactions between the water, the fin blade and the foot support are somewhat complicated. So, I'll keep this focused on general details of the results.

The fin is not spring loaded, as I understand DAPRA's Powerswim (Aqueon) device functions. As Trux said, that actually provides the opposite desired action for fin rotation limit. It is also not a fixed angle relative the foot plate either.

The fin control action is not symmetric and in fact functions very differently on the two different stroke directions. On the down-stroke (power stroke using the quads and stomach muscles), the fin angle control functions similar to how Trux suggested where it rotates to an angle relative to the foot support structure. However, on the return stroke, the fin angle control is more like what Trux described as a "self sustaining hydrofoil". The foil finds it's own angle-of-attack (AOA) relative to the free stream flow. That AOA is a function of the foils speed though the water and other details of the fin suspension system that dynamically balance the foil.

This combination of hydrofoil angle control provides a very comfortable fit with the different muscle groups involved when a swimmer is stroking the monofin, and it also keeps the foil flying efficiently under all conceivable speed and power conditions. The narrow cord-line of the foil minimizes the dead-band at the reversal of the kick. Although, I should point out that the dead ends of the stroke are not really dead. There are forces at play involving the rotation of the fin blade and how that feeds the starting and stopping vortexes shed by the foil. DOL-Fin technology does a pretty good job of recovering those forces as thrust, particularly at the bottom of the power stroke when the foil reverses direction and starts back up again. You can feel the force transmission through the fin stroke at the point of the reversal.

Foils are finicky and need special AOA control if they are to have any advantage over elastic fin technology. That special AOA control can become quite complicated. The advantage of the DOL-Fin technology is getting good AOA control of the foil from a mechanical device that relatively simple to manufacture and maintain, even though the kinematics and dynamics of the device are doing something that is a complicated function of speed, power and motion.

 

[cdavis]

Answered my question.

Thanks

Connor

 

[neurodoc]

Those last few posts also answered one of my questions...a few months ago I started building a monofin with a pivoting rigid blade, with a nice organic curve to it like a whale tail. When finished, the fin had 30 degree angle, elastomeric pivots to store energy, and looked really nice. It didn't work...at all. Epic fail. Turns out that the time and distance spent on the end of the kick cycle where the blade changes directions used up most of my kick amplitude, because the fin was too long front to back. I proved the reason that the Dol-fin has a narrow foil and works well that way. It was fun and instructional though not successful in the end. I'm taking some of the parts and making an inexpensive fin in a completely different way now, since I have the materials on the workbench...

 

[sinkorswim]

wow that was very interesting, and although i am at the fetal stage as far as diving and monofin use, i can be sure to follow your progress reports and wish you the best of luck with your project.

 

[REVAN]

FYI: Just a few days left to bid on the Experimental Monofin for sale. This is a one-of-a-kind monofin and a low cost alternative to the DOL-Fin Orca monofin. It is basically a DOL-Fin Orca, but without the streamlining. Although it is a unique design, it is a good functioning monofin that was built very strong. I could not manufacture this design for what I'm selling it for. But, this one is used now and has served its purposes for me.

EXPERIMENTAL MONOFIN FOR SALE: Minimum Bid $585 US | Facebook

Also, If things go according to plan, I'll have a DOL-Fin Orca available on the Facebook Wall in about a week.

Ron

 

[REVAN]

The experimental monofin has sold, but DOL-Fin Orca SN #0002 is now for sale on the Smith Aerospace Corp. Facebook Wall:

Smith Aerospace Corp: DOL-Fin Monofins | Facebook

The DOL-Fin Orca production will begin slowly at about one per month, but will hopefully improve to double that rate as I get more experienced at making them and can devote more time to production. Presently, I am spread out between multiple tasks that limit the time I can invest in production activities. If I can find some good help and there is customer demand for more throughput, that rate could double again sometime later this year.

Ron

 

[sinkorswim]

great looking bit of kit , and to see it at its infancy stage through to production must make you feel very proud.
Hope it proves to be a sucsess and you produce more items that give us a means of exploring the deep at a faster pace.
Good luck.

 

[efattah]

Video from the 77.9m dive I did at Vertical Blue 2011 with the DOL-Fin Orca:

[ame=http://www.youtube.com/watch?v=J98Druc8Zjw]YouTube - ‪Eric Fattah 77.9m Dive with DOL-Fin Orca‬‏[/ame]

 

[jome]

Nice! At least on video it looked like you had pretty good propulsion from the bottom How did it feel, could you have gone deeper? What are the main challenges in CWT using the DOL-Fin?

 

[Spearo_the_Hero]

Cool vid. Eric. Can you describe your feeling of the unit? I mean are you ready to replace it hands down from your normal monofin?

 

[efattah]

On the 77.9m dive I felt I could have gone perhaps 30m deeper (107.9m).

I have been using the Orca exclusively since my return to Canada (April 19). I even caught fish while spearfishing with it. Every day my technique improves, and I am still far from the ideal technique. I have no plans to go back to my monofins at the moment.

I used to complain that the device had no power when overcoming negative buoyancy. I have changed this opinion and now routinely do 40-45m dives with my 7.5mm suit (getting very heavy at the bottom). When overcoming extreme positive buoyancy at the start of the dive (with 7.5mm suit) I still cover more than 2m per stroke without bending my knees at all.

It is not only a matter of learning the technique, but it takes a long time to make the technique automatic (so you can do it with panic/narcosis), and it may take months for the muscles to adapt to the technique and become efficient in the required motions.

The technique in the video is wrong, I am using it too much like a monofin.

I would mention that as I use the Orca all the time during recreational diving and spearfishing, it is getting a bit scratched, but Ron tells me the surface of the fairing can be easily re-finished to a smooth finish.

 

[Fondueset]

Eric - you seem to be saying that you are using the orca with more of a full-body undulation - and less reliance on knees. Am I right? In my monofin practice in the pool I try to avoid knee bending and find I cover a lot more territory.

 

[efattah]

Eric - you seem to be saying that you are using the orca with more of a full-body undulation - and less reliance on knees. Am I right? In my monofin practice in the pool I try to avoid knee bending and find I cover a lot more territory.

It's true with the Orca you need to keep your knees extremely straight, but unlike the monofin the powerful ankle action both backwards and forwards needs to be timed in sync with the rest of the stroke. My ankles actually get pretty tired especially during surface swims, but they are getting stronger.

The incredible thing is that with no knee bending, and minimal core bending, you still get a huge amplitude stroke, because the amplitude comes primarily from the ankle action.

 

[Fondueset]

Interesting. I'd not have guessed the orca wants straight legs. I focus on not bending my knees during pool sessions - so hopefully I'll have some decent groundwork for using the orca.

 

[yaffle]

You really put that safety to work Eric!

Yaffle

 

[REVAN]

Thanks for putting that video together Eric. It will be very interesting to see the techniques people will develop to optimize potential advantages of the device.

I just returned from a trip where I spent quite a bit of time exercising the DOL-Fin HP in open water. Most of my work with the HP to date has been doing performance evaluations in the pool. However, this was the first time I have really used it for what it was intended for; having fun in open water.

I did both scuba and freediving with the unit. I know that most of the readers of this forum are freedivers, but I have to say that I was blown away by how it performed for me when scuba diving. Freedivers have been taking advantage of monofin efficiency for years now, and so there is less room available for improvement; but to go from contemporary scuba fins to a DOL-Fin HP is such a significant technology leap for scuba diving that it is mind blowing!

The HP swam beautifully and I could feel the improvement in thrust efficiency over the Classic. Pool testing indicated a 10% speed gain over the Classic for the same input of effort. OW testing was fast, smooth and effortless. I love those foldable raked fin tips (thank you DOL-Fin Orca development). I can swim at 2 mph with an 80 CFT scuba tank on my back and be totally relaxed doing it. This fin is many years ahead of its time for the scuba industry.

It will be difficult to overcome the resistance scuba divers will have to this new technology (just look at how long it has taken scuba divers to accept that split-fin technology is a real and viable method to improve thrust efficiency). So, I will appreciate any word-of-mouth promoting freedivers are willing to offer to their bubble-blowing counterparts to help get the word out.

I have a lot of work to do here catching up on things, but I'll be providing more info and updates from my trip as I can.

Ron

 

[Mullins]

Monofins have been available for SCUBA divers to use for ages now, but I expect they prefer to have their feet free to move independently and just don't see much benefit in something that trades maneuverability for straight line speed. For the great majority of SCUBA applications divers would surely feel even less inclined to wear something as wide as a DOL fin? If you needed to cover lots of ground horizontally in open water it might be worthwhile.