Mirage drive performance

I went out yesterday out into sarasota bay, there had to be 400 sail boats out there for some huge regatta at the Sarasota sailing squadron. The winds were about 7-8 mph out of the east (very unusual) so I'm sure everyone had a good time. I tried out my mirage drive with the adjusting screws turned way out. Boy what a difference that made pedaling was very easy and I didn't experience the fluttering and violent shaking that I do with my other set of pedals, so obviously my fins on the other set of pedals must be shot. I really have to thank roadrunner for helping us understand how to tune our mirage drives.
I also got a chance to try out my new wing sail. The original wing was a conservative design (not high lift based on the naca 0012-300 airfoil (per tom spears designs), the thickness was around 10 percent of the cord. After two yrs of heavy use the sail was pretty ratty, I had used steel tape measure for the battons covered with Mylar, bad idea the salt water reached the steel eventually and they all rusted out (not one of my best ideas lol). The new battons are 3 layers of PVC covered cloth they seem to be working ok and will hopefully hold up to the salt water. The original wing was designed to furl up tight just like the mainsail does but I ended up not needing that feature (big wasted effort lol). The new wing is based on the eppler 420 wing shape which is high lift low Reynolds airfoil (way thicker). The new wing creates much more lift.
By constantly pedaling, and with my motors set a the normal 1/4 throttle I noticed an improvement in fuel economy, I covered 10 miles and used 1/2 liter if gas for the day ($.44 cents worth of fuel for the day (measured). My speed average was 8mph with a top speed of 9.7 mph for the day according to my gps.
Definitely the properly adjusted mirage drives had a significant impact. I was able to maintain a higher cadence without getting tired.
Fuel economy ended up around 75 mpg, if I had been willing to sacrifice a little speed I'm pretty sure I could have got 150mpg with an average speed of 6.5 mph (150 mpg averaging 8mph is my ultimate goal).
Thanks to roadrunner I'm well on my way, just a few more improvements then I'm done.
Next test will be with two strong peddlers (with the loose fins) without the motors, we will attempt to go ten miles averaging 8 mph without becoming exhausted in 5-6 mph winds (upwind of course)), not bad for a pedal boat.
Bob

"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."

This is incorrect. The travel distance for the arms is the same at every position as is the degrees of arc that they move. I adjust the setting for the length of my legs which is not different than adjusting the seat height on a bicycle. If the seat is too low or too high I lose muscle efficiency due to under or over extension but that is not to say the seat height makes the bicycle more efficient.

I get most of the power from the fins during 80% of their range of movement. If I spend 100% of my pedal time and effort within this range (with about 10 inches of pedal travel instead of 12 inches), I am getting more power from the fins per unit of time.

The gearing setup with the Mirage drive unit is very good and requires a cadence that is within the range of normal walking. It does not require great leg strength or speed to provide maximum thrust for the average person. With a bicycle one can change the gear ratios depending upon one's physiology and conditioning but with the Hobie Mirage drive it is like a fixed speed track bicycle where on has to choose a fixed gear ratio and in this area Hobie was successful with its design.

Thanks for your comments. The better we understand our equipment the better we can use it to its best advantage. Perhaps my comment was not entirely clear so let me try again.

I agree that the traveling distance of the pedals is the same in all positions -- wasn't referring to that at all. On a bike you move the seat up and down to optimize your leg position on the pedals. On a Mirage Drive you can't move the seat (with exceptions), OR the Mirage Drive, so you have to move the crank arms forward or back on the Drive. Since you can't move the axis of rotation, this makes a significant difference.

On a bike, leg force peaks at about the 3 o'clock position (90 degrees) as you can see on the stroke chart below. The Mirage Drive is similar in many ways, yet there are some huge differences: the stroke length is only about 45 degrees (or about 10.5 inches), which drives the fins about 180 degrees. Each pedal position (1 - 7) advances the pedals about 8.33 degrees along that axis (or about 2 inches). This gives us our leg adjustment. If we overlay this information on the power curve, you can see that positions # 4 and 5 coincide with the maximum force available relative to the fixed axis. Therefore positions #4 and 5 do in fact have the most thrust available. I've laid out the midpoint of each pedal position along the curve for clarification.



As you can see, the # 1,2 and 7 positions are at inferior power curve points. 3 and 6 are better, but 4 and 5 are best. If you're short enough to use the #1 and 2 positions, you can simply move the seat to the front pegs or add a back cushion. If you're in the #7, you're going to have less power available.

Near the bottom of the chart, the position range representing 45 degrees or 10.5" is shown for each Mirage pedal position. As you can see, they are all the same length, but at different points along the axis of rotation.

If you spend much time in the various boats you can feel the difference easily. On the PA I get better cruise times using a boat cushion to push me to the #4 position from the #3; On the Adventure I use the #5; the Revo 11 I need the #6 and the Sport I use the #7. All have the same stroke length; all are selected for optimal leg extension.

To put it all in perspective, IMO the three most important items for efficient operation (provided your Drive is properly adjusted and lubed) are
1. leg position is by far the most important -- should be almost straight at the end of the stroke without bouncing the fins against the hull or thiir limits.
2. pedal position (to the extent possible) -- adjusted for leg position, but modifiable by seat back thickness and design, seat pegs, footwear, back position and boat selection.
3. balancing the boat (front to back) so it runs on its lines -- the bow should be in the water at cruise speed -- modifiable by weights and weight management.

I hope this clears it up.

I read a study a few weeks back ref a different pedal configuration developed by another kayak manufacturer... someone I've never heard of .... and basically went on about the position and set up of the Mirage drive is so different from a bicycle that it gives very little benefit as far as exercise and is a huge waste of energy..., their's was, of course better, also no reverse on the MD, etc.,..... whatever.... I do know I'm 61 years old with a knee that should have been replaced 5 years ago, a another knee that I'm not even letting them look at and a back that comes and goes and I pedal consistently 4-5 miles a trip and we have some screaming a$$ currents here depending on moon and tide... I HAVE gotten cramps but have been able to work through them (just keep pushing and favor the hurting leg by upping the work from the other)... and am usually ahead of the guys I'm with who are paddling conventional yaks, especially if we're fishing and cruising...I own a PA, OB and Revo and have a lot of time in all three...

I have a dodgy right knee and I just love how it's faster when you're peddling hard to do very short strokes with your legs almost straight. I can (just) outsprint my kayaking buddy like that, but only with the seat inclined way back so I'm resting my elbows on the rear deck and so that my body is as supine (laid back) as possible. It feels like a combination of being able to breathe deeply more easily and having noticeably better biomechanics (reminds me of my old recumbent bike on the flat)

Anyone else notice this?

I only do it in calmer seas - when it gets rougher and I don't have the sail on, I prefer to be more upright to be able to keep the boat steadier in boat wakes by holding onto / pulling on the carry handles (RIP )

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Simon

This is how I have my custom seat set up on my Outback (though not so far back, I think). I couldn't stand the stock seat at all, killed my back. I got a Skwoosh seat that has the same back height as my recumbent bike (Rans V-Rex) then built a PVC frame that sits in the well behind the seat for the back to lean against. It gives the back the same angle as the seat on my recumbent. Rock solid seat, and VERY comfy.

I can sit in that seat and pedal all day without any trouble, and I feel like I have a very solid connection to the yak - no concerns at all in waves or wakes. I even look forward to the high-wind days to go out on the local "sailing lake" and splash about in the bigger waves!

I'm buying an AI or TI in the spring, was planning to build another seat frame for it (the Outback one won't fit) but hopefully the new seats will work for me as-is.

What has not been measured is the thrust provided by the blades at different angles of arc from the bottom plane of the boat. Close to the boat they are getting interference from the flow of water past the hull and forcing water against the hull which is going to provide for less forward propulsion than when the blades are going from 90 degrees or perpendicular to the hull and 45 degrees to the bottom. There is no "ground" effect as with an airplane close to the ground. The mechanics are completely different.

The more reclined position possible when pedaling as compared to having to sit upright when paddling makes a very noticeable difference when there is a strong headwind. There is less surface area presented to the wind and less air drag as a result.

Recumbent bicycles also present less surface are and reduced air drag though this is largely offset by the less efficient position of the rider as they cannot use their entire body, in particular the torso muscles as effectively. It is why the current speed record involved a rider in the prone position but leaning forward with their chest down and close to parallel to the ground.

I think the OP was fairly clear that overall, short vertical strokes are better, without, as Wintersun says, specifying just how vertical:


While I totally agree about the advantage of reducing windage, the supine vs. prone HPV analogy has little bearing here in my opinion as no matter how laid back or upright you are seated in your Hobie, you are still seated in a semi supine position and not using your back/shoulder/arm muscles like you can on an upright bicycle when sprinting or hill climbing. Also, I noticed the effect in 0 wind conditions (otherwise I'd've been sailing ), so I still think there may well be a biomechanical advantage to leaning back further (with your upper body weight taken by your elbows resting on the yak), in addition to the more obvious respiratory advantage of a more open, more relaxed chest for easier breathing. Has non one else tried this?

BTW Getting way off topic here but while It's generally accepted that ALL recumbents suck at hill climbing due to not being able to use your upper body muscles to help out, for HPV record breaking attempts there have been both prone (head first) and supine (feet first) racers for years and generally the supine ones have won out. (Edit:) Supine HPVs have taken the world record for at least the last 14 years.

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Simon

You seem to be a veritable treasure chest of misinformed pontification! Some fish species use hydrodynamic ground effect to move efficiently near the bottom. The poacher fish is an example, as noted by B. N. Nowroozi et al in the following study:
"The fluid flow shown in Fig. 5B indicates that the poacher is able to use the ground to augment its lift and thrust. In addition, the mechanical efficiency of the pectoral fins increases as the fish approaches the bottom (Fig. 6). . Moreover, it has been shown that swimming or flying in close proximity to a substrate decreases drag (Vogel, 1981). This decrease in drag would add to the increase in locomotor efficiency for a benthic fish." file:///Users/jrc/Downloads/0fcfd50e1f39649bd3000000.pdf

I haven't ridden a bike in a long time but I do remember going uphill we would stand up and pedal to get up hills using our body weight with our legs locked straight (using no muscles at all just gravity and our weight to propel the bike. I suspect this is the reason recumbent bikes suck at going up hill.
In my experience with mirage drives I'm using pure leg muscle force to propel my mirage drive (no free gravity) weather I'm sitting up or laying down it's all the same muscles. You could possible steal one of those sliding seats from the rowing guys (in the dead of night), fit it to a kayak along with two hand grip posts to hold onto. By doing this in a lay down position you are adding more muscle groups to the mix possibly increasing your speed and endurance without increasing wind resistance.
It's too bad you can't use free energy (ie gravity and your own body weight (ie.. The elephant in th room in my case (lol)) to propel the kayak. The only way I can think of doing this while sitting down would be to sit on a stair master and shift your weight from one side to the other, let's call it the mirage but drive. You might have something that propels the boat just by shifting your weight back an forth, much less pure muscle energy used. It might sound silly but but I think about that kind of stuff (free energy). That's the reason all my kayaks are equipped with wing sails so I can amplify every pedal stroke into more energy than if I didn't have the wing (the wing amplifies your forward movement energy just like an air conditioner works). This energy is independent of whatever natural wind is available (so it still works even if no natural wind is present, basically you are creating your own apparent wind by your forward motion), that's the basis for my whole setup.
Of course I don't think like anyone else lol.
Actually I have always wanted to try SUP but have no desire whatsoever to stand up and paddle the thing. So I went ahead and designed a mirage drive system that propels the sup just by using gravity and shifting your body weight to propel the sup. Basically it looks like a stair master mounted on the sup, you straighten your leg on one letting your weight propel you forward, then alternate to the other leg going back and forth like on a stair master. Of course you still have the paddle in your hand for balance and steering but most of the propulsion is from gravity, I'm calling it my gravity drive, or gravity propulsion. I would be using the standard mirage turbo fins and posts. The whole works weighs 5 lbs and is removable. I also designed one that simulated cross country skiing with a back and forth sliding motion where you shuffle forward and back to propel the sup. I will probably build both and see which I like more.
Actually the whole idea came to me after designing a different type of mirage drive that I could put on a wave and pedal sitting sideways in the sailing position with the pedals mounted sideways in the center under the sail and a mirage drive mounted on each hull which would double as center boards. The drives would click into external brackets mounted to the inside of each hull 2/3 of the way back on the hull. Obviously I want a wave, but have no desire to have a boat without mirage drives, so I'm just designing my own mirage drive system for it for my own use only knowing Hobie will never build any of this stuff, and nobody else can or will because of their patents. Beside it's only a couple hundred bucks in materials and a couple weekends of labor in the garage to build anything I want or need as long as I'm just using it for myself. A quad mirage drive setup on a getaway with two mirage drives on each side would be way cool with 4 people pedaling (two on each side pumping the two sets of pedals (one on each side all facing inward)). Making everyone earn their keep on those no wind days lol. When not needed they simply flip up out of the way ( that's why they are mounted on external strap on brackets on the inside of the hull.
In my opinion Hobie has just touched the tip of the iceberg with their own invention (the mirage propulsion system). I personally would like to see them replace the need for any gas or electric propulsion system, and replace it all with wind and/or human power. Something you don't have to be a world class athlete to use, and expand on their theme of no wind no problem. Currently on windless days there are no waves or getaways out on the water, but plenty of TI's, they have the power to change all that. Plus everyone gets a little exercise which doesn't hurt any of us.
Ok that's my 2 cents.
Bob

My solution was to use a trolling motor mounted on one of my rod holders I got tiered of getting exhausted peddling my OB