Pedal Powered Boats

Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. I57
    Joined: Feb 2008
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    I57 Senior Member

    Catamaran

    The mirage drive boats I have seen all are on a beamy hull. If the drive were fitted to a long slender hull with outriggers or a catamarn it would have to be faster. I'm no technical expert like Rick so can't say why they are less efficient. I use a large 1:3 ratio gearbox that is fitted with pedals and attaches directly to the flexible drive shaft. The shaft can be lifted out of the water for beaching and the gearbox is unbolted for transport. The other difference with the mirage drive is the push pull action which some say is harder than a pedal action.

    Ian
     
  2. papawoodie
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    papawoodie Junior Member

    RE: Mirage - Catamaran

    Choosethisday,

    I'm not an engineer, nor am I an expert.... (but it has never stopped me from sharing my opinions).

    It's my understanding that the primary deficiency of the Mirage drive is in the drag factors related to lift (or drive).

    The area of the submerged parts in the Mirage are significantly larger than those of a better designed prop unit. At idle, drifting from an equivalent "push", you can readily see that the Mirage will cause a lot more drag than an identical hull with a thin prop drive leg.

    The other, perhaps more monumental, factor relates to the 'push', or the power, delivered by the two different drive units.

    The top speed and motive power of the Mirage fins is quickly reached. Beyond that level, it is difficult increase speed or power. Not a good analogy, but imagine a 10-speed bike limited to 1st gear -- you start off easy and rapidly reach optimal pedaling rate... but you'd have difficulty moving beyond that pace. The fins, by necessity must move thru the water in two directions -- once to deliver power or drive, and again to recover (reset back to the original position). All of the recovery motion is powered by the opposing leg pedal cycle. As with kayaking, the blade that moves thru the water uses that motion to bring the other blade into position. Not only does the Mirage fin need to change direction to reset, it does so while using power as opposed to delivering drive.

    [I realize that each fin pushes as it moves in either direction, but the leg that is applying force is counteracting with the other leg. Unlike the circular revolution of a sprocket wheel, where the power is continuously delivered, the Mirage uses lever arms, which move back and forth.]

    As I said, I'm no expert, but it seems that a good chunk of your applied force (from your legs) is used just to reset the blades. Certainly, purely from a bio-mechanics perspective, this is less efficient. Unless I missed it, all the Tour de France bikes took advantage of fully revolving cranks, chosen for efficiency over ratcheting lever arms.

    Beyond that, there is the consideration of the optimal power range of the drive fin.

    When up against the hull, in its starting position, it resides in the flow that 'hugs' the hull. To disrupt this flow, creates additional drag, as the flow must re-establish itself.

    From that position, at rest against the hull, some movement is required for the fin to initiate its power stroke. It must start moving before it starts delivering....

    (And when it starts moving, it creates additional drag as it further disrupts laminar flow along the hull)

    The fin begins to propel thru the water, for some optimal range of motion... yet eventually, towards the end of the stroke, it reaches a 'compressive' point, where it is pressuring water against the hull. At this point, where it is squeezing out the water between the fin and the hull, the opposing blade is doing the same thing, compounding the issue. [On the other hand, this might be the peak of power delivered in the fin's range of motion...]

    Altho' I'm not able to define it, there seems to be another problem area where the blades cross each other. Certainly, the displaced water from the leading fin has to cause some interference to the trailing blade as they approach and depart from one another. I suspect this 'conflict' for undisturbed water, represents an arc of maybe thirty to thirty-five degrees of the potential stroke.

    Combining all the 'problematic' areas, it might represent as much as half of the swept area of each fin. Perhaps 10 to 15 percent at the start, 20 to 25% thru the crossover, and 15 to 20 percent at the end of the stroke against the hull.

    A prop, sweeping in a full circle, had the advantage of 'lift'. Its foil shape creates pressure differences that assist it in supplying motive power. The foil shape actually helps, in essence, to 'pull' the prop forward.

    You can put this to the test the next time you ride in your car. Put your hand out the window with the palm flat, rigid and facing forward. The 'drag' induced by your hand will 'pull' (feels like pushing) your hand backwards quite forcefully. Now try again with you palm held parallel to the road... its much easier to hold it steady in the wind. Move you thumb in slightly and arch your palm (creating a foil shape) and you'll feel your hand suddenly feel lighter. The 'lift' you create can, to some extent, overcome the drag and make your hand and arm 'sail' in the wind (without any application of 'power').

    A prop, when chosen well for the task, will harvest 'lift' in a similar fashion. Only this time, when combined with applied power (from whatever drives it to rotate -- your legs, a motor, etc.), it will pull thru the water.

    In contrast, the Mirage lacks the 'lift' and relies solely of 'push' and it is severely hampered by its 180 degree range of motion. It uses power (from your legs) to reset (change direction), and while doing so is providing no forward motion and is creating significant (and slowing) drag.

    Under power, a prop moves thru a full 360 degrees, and never has to 'reset'. While in motion, it generates lift to overcome its own drag. And the blades do not cross over one another, disrupting the water flow, and robbing potential.

    This is all just speculation and intuition on my part... for whatever it's worth...

    It would be interesting (and far more enlightening) to find a comparative report from a knowledgeable and reputable unbiased study.

    David
     
    Last edited: Jul 24, 2010
  3. Choosethisday
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    Choosethisday Junior Member

    Papawoodie, thanks for getting back to me. I really do appreciate the post and it gives me quite a bit to think about. Also, I took a look at the 2001 Hyrofest results for comparision. The tandem Hobie did OK against the tandem Seacycles. However, as your post implied, this may be largely due to the fact that this was an all out competition and not representive of what I could expect for my purpose in that in these competitions there is likely not going to be anything but full out use of the Mirage. From what I have read here this is when the Mirage actually reaches it's highest efficiency. So it may be that the Mirage is not ideal for my purpose. However, since the fins and spindles would not be near a hull that might help my application. Also, I did plan to make the wetted part of the extension as hydrodynamic as reasonable, so that might help as well. And actually, I could also not use the pedal part of the drive and simply modify a bicycle crank to drive the fins. And this would not only make the drive cyclical to retain the momentum but would also allow changing the ratio of the dirive. Mmmmmmmmmmm, much to think about.
     
  4. papawoodie
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    papawoodie Junior Member

    RE: Mirage - Catamaran

    ChooseThisDay,

    I'm glad my rambled thoughts made some sense...

    Not sure if the back and forth power input could be converted to a wheeled sprocket... Not having seen the Mirage unit in person, it would appear that the lever action is required to swish the fins from side to side... Maybe you have more insight on that than I've gained.

    I'm still convinced, from everything I've read in this thread, that a flex shafted slow turning an efficient prop makes for the best propulsion.

    Seems you intend to center your drive between the hulls, which give lots of design flexibility. I'd consider using a simple pivoting support skeg (off the aft cross member) to allow 'kick-up' for protection in shallow water. A line from the top of the skeg to your seat would allow you to raise and lower the shaft for transport and for beach launchings and landings.

    Are you starting your design from a Mirage unit on hand, or do you have previous positive experience with it? Do they have some sort of reversing ability?

    Back to your questions of efficiency inherent in the Mirage, the only supportive comments I've ever run across were all sourced from the Hobie website or literature. This seems to be in direct contrast to reasonable logic and knowledgeable nautical research and testing.

    Frankly, I'd feel that the 38% rating is optimal and probably not the average sustainable output over extended time and varying conditions.

    Hopefully someone with more insights will chime in...

    Would love to follow along with you as this proceeds... Keep us posted!

    David
     
  5. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    Catamaran

    The mirage drives I have seen are fitted in monohulls through a slot in the bottom of the boat. If fitted between the cat hulls it might be too high above the water to be effective. The problem with props is turning the pedalling action through 90 deg, either using a gearbox or a twisted chain drive. On the other hand why not rotate yourself 90 deg as with 'Sidewinder'. All that is required is a standard bike setup with crank and chain to a smaller crank fitted to the shaft, the cat gives you this option. Only drawback is a cricked neck.

    Ian
     
  6. Choosethisday
    Joined: May 2010
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    Choosethisday Junior Member

    First, I absolutly agree that a slow turning prop is the best developed design for coupling to the power of a human. But as mentioned there are some downsides. And there are many ways to convert from rotary to linear motion, some of which are easisly doable using even my limited skills and equipment. As far as being too high that is true, hense my making an extension to bring the pedals to the proper height. I do intend to center the drive between the hulls, and if I go with a bicycle crank/s I can also reasonably easily rig a dual pedal setup to the one unit so both riders can pedal. And once again, from all evidences the rotary prop is better but for ease of building, reliability, and draft, the oscillating prop (Mirage drive), has it's advantages. And if one looks at the results from the Hydrofest competition, the Mirage did not do all that bad

    http://www.humanpoweredboats.com/HydroFest/2001/HydroFest2001Results.htm

    .
     
  7. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Hi, Choose. The Mirage drive can be a good compromise, depending on what the parameters are for your boating. It is probably better in applications like weedy water and where you don't need too much sustained high speed or extreme distance or reverse capability. If you are building for competition in speed, distance or endurance records, especially in excellent water conditions, the underwater prop is still the way to go. Air props would probably be next, if there are draft or weed considerations, but at some sacrifice to being high profile in air resistance and with greater weight and complexity.

    Hope this helps.

    Porta

     
  8. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    I've often thought that a way to create a linear, under the hull, paddling motion might be reasonably effective, as a way of avoiding the problems associated with rotary drive. I envisaged a pair of front hinged paddles, each connected to a linkage that reverses pedal movement (so pushing forward moves the paddle backwards. The forward moving paddle would be pulled back into place by being connected to the other side of the pedal that was moving the other paddle backwards.

    To avoid drag when moving forward, each paddle would fold upwards on the forward stroke, aided by buoyancy from being made of foam cored composite. Each paddle would be restrained by a limit cable, so that when 'open' it is at around 90 degrees to the direction of travel. Such a system could be fairly simply driven by inter-linked cables. I doubt it would be super-efficient, but it would probably be better than some other paddle systems, as the paddles would always be driving in the direction of travel, rather than an arc.

    No doubt someone has already tried such a scheme and found snags I've not thought of..........

    Jeremy
     
  9. tinhorn
    Joined: Jan 2008
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    tinhorn Senior Member

    Kinda. This is from Human-Power Boat Patents which I found at my favorite online bookstore.
     

    Attached Files:

  10. portacruise
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    portacruise Senior Member

    There are specialized racing kayak paddles used with special racing hulls that come very close to matching anything that can be done with leg power on water. Once the proper techniques are mastered, they easily outperform the Mirage system and are a close challenge to any of the records set by leg power on water. Plus hand paddles can operate under more extreme conditions of draft, weeds, rocks and are a much lighter.

    http://www.youtube.com/watch?v=bQRtgEIs--k

    Porta

     
  11. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    They must have sold a lot of these. I see them everywhere. :rolleyes:
     
  12. Choosethisday
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    Choosethisday Junior Member

    Portacruise, I know there are some very good paddles out there but some of these can be $400-$800, too rich for my blood. Also, with a cat a normal paddle doesn't work well at all. And Jeremy, if you want a really neat way to do what you are talking about take a look at this.

    http://www.youtube.com/watch?v=mlnz-7YjWqc

    And Tinhorn, I did look into the paddle wheel idea. But beyond the wind drag and the splashing issues one has go with a really big wheel to be efficient. And even if you go with the auto adjusting blade angle system you still have the issue of the differing distances between the blades depending on where they are in the rotation. So while something in the 2 meter size would likely work well this would not be practical for what I want to do. And because a wheel is so depth sensitive rough water could be an issue. My goal with this boat is to come up with something that my wife and I can pedal for seveal hours at 4 mph and for bursts of perhaps 6 mph. At this point, mainly because of the simplicity, I am leaning toward to Mirage. However, please, if anyone can think of something that will work about as well and be even simpler, I would want to hear about it.
     
  13. papawoodie
    Joined: Jan 2003
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    papawoodie Junior Member


    Choosethisday,

    For simplicity and easy 4-6 mph speeds, you might look into an in-line fish tail arrangement.

    Harry Byran recently did an article on his version, the Thistle [Boat Design Quarterly #36].

    You can see a vid on YouTube {Pedal-powered fin-driven decked canoe. Built to Harry's plans by Steven Bauer in 2003}

    The advantages, if any, over the Mirage are that it is utilizes a vertical fin that is larger is size and less rapidly 'swished'. Seems you'd get more power and less drag from an inline vertical fin, with more responsive steering. Still no reverse, tho'.

    With your cat you could easily rig dual fins, one off each hull. They look to be beachable, weedfree, shoal draft, and moderately powered. No doubt the fins could be made to kick-up for obstructions and launching. And with two hulls, there's probably no need for the 'dorsal' fins.

    Once the basic drive system is installed, you could readily swap out a set of fins depending on desired performance (larger, smaller, faster, slower, softer, etc..). It wouldn't take too many experiments to zero in towards more optimal parameters.

    I'll have to go look up the article, but I believe the fin is an easily sourced piece of Plexiglas or polyurethane. And the mechanics are all homemade from basic materials.

    No doubt, there's loads of room to improve the operational aspects for optimizing efficiencies. I'd be tempted to focus on the eliminating the wave making at the waterline... perhaps either moving the fin lower (to the bottom of the hull) or by putting a kind of 'cavitation plate' on the aft section (beyond the stern). Or both...

    Multiple pedaling stations ought not be problematic. With three 'engagable' positions, you could set up for solo use or tandem inputs. Seating and foot pedals would be installed, as desired, for the afternoon's intended purpose. The main drive elements remain in position and the foot bars are 'connected' as needed.

    Just thoughts, but it looks doable and cheap and fun to experiment with... and probably makes for a very leisurely time on the water.

    Certainly meets the quick and dirty criteria....

    David
     
  14. Choosethisday
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    Choosethisday Junior Member

    I had seen that video in the past but was concerned about a couple of possible issues. The first is that he was only getting 4.2 mph. In my tandem canoe I can't quite maintain that speed for an hour but I can for a time. From the video I can't be sure how hard he is working but I would guess at least at a moderate level. Also, the boat was wagging quite a bit. This wastes energy and while using his legs is certainly better than his arms it still implies a loss of efficiency. I do think that on the cat it would wag less but it might still wag some. I had started to build a similar system using two symetrical airfoils made of aluminum with spray foam cores. The foils actually turned out better than I thought they would. My idea was to have the foils at the ends of overlapping arms where one arm was enough longer than the other that they could oscillate in opposite directions, one in front of the other the full width between the hulls. From some prelimenary testing of the foils I think this would work well. But as I started to bear down and draw up a parts list and the machine work and such and think about all of the hassle of the rigging and actually setting a boat up after transport the whole thing just seemed to be too much of a pain. I hope it doesn't look as though I am whinning but my life is already busy enough as it is so if this thing turns out to be too big of a hassle to use I likely will not use if often which defeats my whole purpose in building it. That's why I keep coming back to the Mirage. It is a descrete unit with reasonable weight and size and the parts are readily available and wouldn't have to be custom made every time I bent something. I will see if I can post a pic of the cat and maybe the foils also.
     

  15. papawoodie
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    papawoodie Junior Member

    Choosethisday,

    Well, I encourage you to get the Mirage unit, rig it up and put it to use.

    Get out on the water and enjoy it....

    It'll probably be a wonderful boat and you'll have lots of fun with it.

    Better to be on the water with something.... than hard ashore with the unsatisfied urge to be out boating.

    You'll not be stuck with some useless boat and a huge 'lost' investment in any event. There'll be several people who'll want a boat like yours, if you decide to build something else. Or if you'd like to eventually try another option, the drive unit will value on the re-sale market.

    Best of all, you can keep us all up-to-date and provide loads of valuable feedback!

    I've never, in more than 20 years of building boats, ever seen everyone agree on any single design. There will always be something better for this, something better at that, and something else that 'promises' to be the best at whatever...

    But you won't be bothered by all that hype... You'll be taking your lovely wife out in the beautiful afternoon sun for yet another wonderful day together, sharing good times and making memories, on the water!

    So.... Go for it!

    And take us along the journey!

    David
     
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