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 Post subject: Mirage drive performance
PostPosted: Mon Jul 21, 2014 3:49 pm 
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Hobie Cat Engineering Team

Joined: Fri Mar 21, 2014 3:24 pm
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Dear Mirage Drive user,
We have developed this last months a dedicated instrumentation to measure the performance of the Mirage Drive. This instrumentation measure simultaneously the load applied by the driver in the pedals, the strokes amplitude and frequency, the speed of the boat and the hydrodynamic loading on the fins.
This study allows us to better understand the physics of this unique propulsion system, to separate the parameters acting on the efficiency and to compare the performance of different set up or pedaling motion.

The aim of this post is to let you participate to this study by sharing your experiences of the Mirage Drive and found here an answer to your old after-ride performance dispute!

To illustrate the type of questions you could ask, here is an example. Back from the Everglade Challenge 2014, Jim Czarnowski, our director of engineering, asked this question:
" I was in shallow water so I had to pedal with the fins closed to the hull. I was keeping short strokes and the fins were never crossing. It seems to work well! Do you think the performance could be the same than vertical short strokes with the fins always crossing?" (The "vertical" and "horizontal" short strokes are illustrated on the figure bellow

http://static.hobiecat.com/web_uploads/2014/07/21/HobieForumOscillation.pdf

Very interesting question for different reasons:
    The experience has shown that vertical short strokes can be very efficient, specially when you're sailing upwind.
    The fluid mechanisms involved are complex in both case with a potential ground effect with the proximity of the hull in the case of the horizontal fins' oscillation and periodic interaction of the fins in the vertical oscillation.
    The cadence or strokes' frequency plays a significant role on the system performance.
Few tests at sea with the instrumented kayak has answered this question (illustrated in the figure bellow), showing a better efficiency for the case of vertical oscillation (fins crossing). The efficiency is defined here as the ratio of work out/Work in.
    Work outdepends of the speed and the drag of the boat
    Work in depends of the load, the amplitude and frequency of the driver's stroke


http://static.hobiecat.com/web_uploads/2014/07/09/Fig_eff_Forum.pdf

We'll be happy to hear your comments and questions.

Best,

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Benoit Augier
Ph.D. Fluid Mechanics
Research and development
Hobie Cat Company
Oceanside, CA, USA


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PostPosted: Mon Jul 21, 2014 5:00 pm 
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Not sure, but if Im like other Hobie owners we have fitness trackers. Maybe if we 'shared' our results with Hobie along with what we were doing at the time (using mirage, not using mirage, fluttering, not fluttering, etc) that would help with designing an even more efficient drive somehow? Maybe too complicated, but with todays technology maybe not? Why aren't fitness trackers built into our Hobie's LOL?


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PostPosted: Mon Jul 21, 2014 6:30 pm 
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Very interesting graphical results.....it basically shows, what I discovered with my 2010 Outback that to obtain quicker rudder response, when moving from a stop or very slow movement, short quick cross strokes got the kayak moving and responding to the rudder.....same for my 2013 PA-14.
Re efficiency.....as an example, today I covered 10.4 miles, in 5 hours and 23 minutes.....trolling with my down rigger, 2.5 pound weight at a depth of 35 feet, pulling a quick fish K15, 5" long lure.....this has a reasonable amount of drag...and I am 72...not a spring chicken.....I really feel the Mirage Drive provides a significant advantage to the Hobie kayak owner because of the overall efficiency it provides Vs using a traditional paddle.......on my off fishing days, I ride my stationary Exercycle and I can confirm, fishing in my PA-14 is much better and I occasionally bring home a nice fish for my efforts.
Staring with my Outback and now with my PA-14, I discarded the traditional pedal straps in favor of some home made, 1" wide heel straps.....my size 12 Keen water shoes fit very tight into the Hobie straps.....the heel straps work fine for me as I do not "crank on the power" using a "pull back stroke".....I am long past trying to break the hull speed design limitations. 8)

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Hood River, OR


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PostPosted: Mon Jul 21, 2014 6:32 pm 
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Think you are outside of my engineer knowledge base :mrgreen:

One thing I have learned is that when in deep water, rather than a continuous pedaling motion, if I do one long thrust of the pedals then stop before pressing the other pedal, my speed is equal to a constant pedal motion.


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PostPosted: Tue Jul 22, 2014 12:37 pm 
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Location: Naples, Italy
Personally, I'd love to see just how much fin rotation produces the most efficient drive with the ST Turbo fins, at what speeds and with how much effort. See Roadrunner and JimL’s work on this here: http://www.hobiecat.com/forums/viewtopic.php?f=11&t=10176&p=53038

Maybe this would lead to longer clew adjustment screws becoming available, complete with rubber rings :-)

I can always dream :-)

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PostPosted: Tue Jul 22, 2014 9:43 pm 
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Good study...thanks. As a former national ranked athlete (XC/Track/Bodyboard surfing), lifelong physical educator, full time bicycle commuter, and recreational "mirage drive" kayaker, I'm stoked you are investigating this manner and am looking forward to your final results and conclusion.

As for me, I've done enough "pushing the envelope" physical challenges and have choose to "Enjoy the Ride." I'm finding plenty kine enjoyment just pedal-sailing my boat and becoming one with her in all aquatic locations and situations.

My primary cadence is long and steady, using the full range of mirage movement (even in heavy wind). This way, I don't even think about pedaling. It's automatic and I can focus my brain power on other matters. However, when turning fast, I do switch to short, fast, vertical movements.

Enjoy,

Longbikermike
sent by iPad using TapaTalk

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Valle Vista, CA


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PostPosted: Mon Jul 28, 2014 2:13 pm 
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I have been an avid long distance cyclist for many years and one learns that there is a limited portion of the pedal stroke that produces the power and even cleats and straps do not change the dynamics when needing maximum thrust or power. When paddling a kayak the most efficient stroke is one that is in effect a half stroke that ends when the paddle is in line with the body and then brought up for the next stroke.

My first time out on a Hobie boat with the Mirage drive I could feel that the amount of resistance from the pedals as the greatest in the second and third quarters of the stroke. I can keep up a much greater rate of speed over a period of hours with no more exertion on my part by doing a half stroke with the pedals.

When I am out on the water with other people with the same Hobie model they get frustrated as they see me going faster and not working as hard as they feel the are in trying to keep up with me. I believe it is because they are wasting time and energy with the last portion of the stroke.

I cannot see how the last 25% of the stroke when the fins are nearly flat against the hull is going to be as effective as the portion of the stroke when the fins are nearly 180 degrees from the bottom of the hull. There are probably multiple factors in play in terms of the hydrodynamic forces around the fins and under the hull of the boat. I can also see there there could less efficiency with the rear drive unit operating in the wake turbulence of the front drive in a Hobie tandem boat.

It would be valuable to put multiple pressure sensors on each side of the fins and measure the forces across the fin during each point in the pedal stroke. This would provide some of the empirical data needed to understand their output profile. Adding a paddle wheel device to measure speed and distance would be a lot more accurate than relying on a GPS to provide this information. It would also automatically compensate for current for the data gathered which GPS derived data cannot.

There is also the aspect of the Hobie being a displacement hull and so its speed is limited and applying more power past a certain point is wasted. In terms of human power expended per meter of distance there is going to be a point of rapidly diminishing returns.


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PostPosted: Mon Jul 28, 2014 2:16 pm 
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islandspeed2001 wrote:
Think you are outside of my engineer knowledge base :mrgreen:

One thing I have learned is that when in deep water, rather than a continuous pedaling motion, if I do one long thrust of the pedals then stop before pressing the other pedal, my speed is equal to a constant pedal motion.


You are benefiting from the glide capabilities of the kayak which is done by speed skaters and cross country skiers as well to maximize the distance covered with their physically limited efforts.

There may also be a residual turbulence effect that is minimized with the pause as you are going into "cleaner" water.


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PostPosted: Mon Jul 28, 2014 5:58 pm 
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Location: Auckland NZ
There is another factor in that the fins are arranged one behind the other so that as the fins cross at the bottom of their travel, additional power is generated via a squeezing effect as the rear fin picks up and works upon water that is already being propelled backwards by the front fin coming down in the other direction.

I have a suspicion that there may be some merit in having the rear fin angled "flatter" than the front fin and using a short stroke around the centre of the full travel so that the rear fin is working almost continuously on the front fin's slipstream water.

I would be really impressed if anyone can detect this effect scientifically and advise accordingly!


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PostPosted: Mon Jul 28, 2014 11:39 pm 
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stobbo wrote:
There is another factor in that the fins are arranged one behind the other so that as the fins cross at the bottom of their travel, additional power is generated via a squeezing effect as the rear fin picks up and works upon water that is already being propelled backwards by the front fin coming down in the other direction.

I have a suspicion that there may be some merit in having the rear fin angled "flatter" than the front fin and using a short stroke around the centre of the full travel so that the rear fin is working almost continuously on the front fin's slipstream water.
You're absolutely right. "Contra-rotating" blades are not new. Typically they get about 5 to 15% efficiency gain. Hobie fins are, in fact, highly modified (bi-directional) prop blades.
ImageImageImage

The Mirage Drive takes advantage of this concept when the fins rapidly cross each other. There is a small potential gain by tightening the clew outhaul on the rear fin (thus flattening the leech), but is is offset by a loss in performance in normal pedaling and an imbalance in the fins that begins to cause the boat to rock. So in reality there is no effective gain (yes I tried it).

Wintersun wrote:
My first time out on a Hobie boat with the Mirage drive I could feel that the amount of resistance from the pedals as the greatest in the second and third quarters of the stroke. I can keep up a much greater rate of speed over a period of hours with no more exertion on my part by doing a half stroke with the pedals.

When I am out on the water with other people with the same Hobie model they get frustrated as they see me going faster and not working as hard as they feel the are in trying to keep up with me. I believe it is because they are wasting time and energy with the last portion of the stroke.
A couple of things are happening here. 1) Due to increasing bio-mechanical efficiency, legs generate greater power as they straighten. 2) In bicycling, pedals generate their greatest leverage when perpendicular to the axis of the leg (close to the 3 o'clock position). Similarly (but to a far lesser extent), the Mirage Drive crank arms deliver their best mechanical advantage when nearly straight up, so the effect sort of depends on the length of your legs; they deliver more thrust in the #4 and 5 positions than the 6 and 7 positions. IMO, between the two, factor #1 is the more important when it comes to thrust and fatigue, but the net result is a combination of the two. There is also an unrelated but very important influence of Drive friction, commonly present, but easily minimized by proper tuning and lube.

Quote:
I cannot see how the last 25% of the stroke when the fins are nearly flat against the hull is going to be as effective as the portion of the stroke when the fins are nearly 180 degrees from the bottom of the hull. There are probably multiple factors in play in terms of the hydrodynamic forces around the fins and under the hull of the boat.
You're right. There is a small negative effect called interference drag as the fins get next to the hull, but there is a much bigger gain called "ground effect" where the fins become more efficient. Very similar to an airplane floating over the runway just before landing or a pelican gliding very close to the water. Dr. Ben tells us rapid fin crossing is the more significant of the two.

Quote:
I can also see there there could less efficiency with the rear drive unit operating in the wake turbulence of the front drive in a Hobie tandem boat.
Normally both fins have clear water except in the narrow zone where they cross (see above).

Quote:
It would be valuable to put multiple pressure sensors on each side of the fins and measure the forces across the fin during each point in the pedal stroke. This would provide some of the empirical data needed to understand their output profile.
The best thrust zone in a fin changes as the boat accelerates, depending at any moment on hull speed, pedaling speed and fin adjustment. In addition as you already suggested, fins continually accelerate and decelerate within each stroke. It may be impossible to identify an optimal output profile that would apply in all these conditions. I think there is as much art as science that goes in to the design of these blades! The fins have improved a lot over time and I suspect it has mostly been by trial and error, and intuition. The neat thing about these fins is, unlike boat props, they don't have to be changed out for different boats and speeds -- they have the plasticity to adapt within a wide range of conditions.

Quote:
Adding a paddle wheel device to measure speed and distance would be a lot more accurate than relying on a GPS to provide this information. It would also automatically compensate for current for the data gathered which GPS derived data cannot.
Great idea! The older Drives have a pitot tube speedometer insert that also accomplishes this -- has to be calibrated for boat and load though so it's not quite as good.

Quote:
There is also the aspect of the Hobie being a displacement hull and so its speed is limited and applying more power past a certain point is wasted. In terms of human power expended per meter of distance there is going to be a point of rapidly diminishing returns.
Yes, but as a "semi-displacement" hull, conventional hull speed formulas don't strictly apply with kayaks. When and how the effect takes place depends on the length and width of each model.

No doubt, Dr. Ben will be able to answer these excellent questions and observations much better when he is able to address these matters himself. Great topics!! 8)


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PostPosted: Wed Aug 27, 2014 4:20 pm 
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I'm glad I came across this thread/topic because I recently purchased my 2014 Outback about two months ago and to be honest this Outback is a slow barge in the water. I got this kayak to go offshore with a group and we only head out about 2 to 3 miles, deep water and when I come back I'm totally exshauted, can't even pull my kayak off the beach back to my car.

Now in deep water its hard to visually see your speed because there's nothing to reference to and my legs are craping up and that shore is not closing fast enough. At that point my stroke is set on #4, #5 and about 36 per min and using my arms to assist. When I read above that this 76 year guy is trolling a deep lure for 12 mile and I can't even do 3 mile it makes me wonder, was he under sail! or just peddling LOL. I'm only 68 and thinking of getting a trolling motor if I'm going to get some decent fish from this barge.

But I will try some of what I've read here in these post and hope it helps. When I first saw this method of propulsion and people were telling me it was mimicking the penguin summing under water, I guess I thought it was their way of flying in a more viscos fluid. So I feel loose trailing edge of the fin should work the best. Their has to be a little cupping effect in the fin to accumulate some pressure on one side and a negative pressure on the other. So the quicker you can keep the principle going the more speed you will get. But whether it less exhausting for the legs I will have to see


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PostPosted: Wed Aug 27, 2014 9:57 pm 
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gavinfla....I hope you invest in Turbo fins and the sailing rudder for your Outback.
I am 72 and had no problem lake trolling with my down rigger covering 8 miles in 4.5 hours and not being pooped out.
I now have a 2013 PA -14 and routinely cover 9 - 10 miles in less than 6.0 hours.
It helps to have a GPS feature with your fish finder so you know exactly how much progress you are making....time/distance/speed/points of interest you mark for future reference/track, etc, etc.
Lastly, you also need to "work up" to the longer distances if your leg muscles are out of shape.....practice improves distance and will keep your waist line in check :wink:

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PostPosted: Fri Aug 29, 2014 7:36 am 
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I'm almost 68 and I trolled for five hours yesterday with very little fatigue. Getting my PA14 on and off the roof of my Highlander is harder than pedaling it. I also use a half-stroke, with each pedal going only a third to half of its full travel from center to stop. To me, the speed I achieve is almost exactly what I'd get walking at that "stride speed." Except that the Mirage requires much shorter "steps" to achieve it. My speed yesterday was a "stroll" and it was effortless, even into the wind.

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PostPosted: Fri Aug 29, 2014 9:53 am 
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This is a petty cool discussion. A few yrs ago I made a drive that mounts on the mirage drive that converts the back and forth motion to circular motion basically. I then mounted a cordless drill to the mirage and it actually worked pretty well. However the design required the pedals to operate in their normal manner (crossing thru center). I too have always been impressed with the mirage drives ability to shallow or butterfly pedal (like butterfly wings). If I ever get around to picking this up again my plan is design the mechanics so the pedals only operate in a butterfly fashion. I think the efficiency would be very close yet the danger of damaging the fins in shallow water would be greatly lessened ( yea you guessed it I messed up a set of fins hitting the bottom with my mechanical drive, that's the reason I dropped it). In practice when pedaling a mirage drive manually you develop a special feel for when you mirage hits bottom, at that moment you have a choice of ether plowing thru it (usually damaging the drive and fins) or stopping and reversing the stroke to clear the fin ( this happens to me all the time, knock on wood I've never wrecked a drive or fin. With the mechanical motor setup there is no feel so the design is pretty useless unless I convert to a butterfly stroke.
Actually I'm a little surprised a rotary pedal motion was not used in the original design, with the flippers working like butterfly wings underwater (might have been mechanically a simpler design, and they could have designed much longer flippers). Thing of the flippers going thru the water in a dolphin swim fashion (very efficient)
Cool stuff
Bob


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PostPosted: Sun Aug 31, 2014 8:47 pm 
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Dr.SteelheadCatcher wrote:
gavinfla....I hope you invest in Turbo fins and the sailing rudder for your Outback.
I am 72 and had no problem lake trolling with my down rigger covering 8 miles in 4.5 hours and not being pooped out.
I now have a 2013 PA -14 and routinely cover 9 - 10 miles in less than 6.0 hours.
It helps to have a GPS feature with your fish finder so you know exactly how much progress you are making....time/distance/speed/points of interest you mark for future reference/track, etc, etc.
Lastly, you also need to "work up" to the longer distances if your leg muscles are out of shape.....practice improves distance and will keep your waist line in check :wink:



Hey Dr SteelheadCatcher.. Yepe I have all that equipment I guess I have to build up my legs than if I'm going to keep up with you guys. But I'm in the ocean so the wind, tide and current can be tough but not making excusise I'm out of shape. I'm going to try those half strokes (vertical) on my next outing.


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