You can do it in a better way ( MirageDrive )

Discussion in 'Boat Design' started by VladZenin, Aug 12, 2006.

  1. VladZenin
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    VladZenin Senior Member

    Screw propellers are one of the oldest forms of propulsion, relatively simple, and still one of the most efficient and widespread including human-powered boats. The efficiency of the screw propeller is improved, if we can increase propeller’s blades and turn it more slowly. But there are many obstacles to realizing this ideal form. The Hobie MirageDrive is the first very bold challenge to propeller drives. The sweeping action of it (versus a complete revolution as with a standard propeller) allows utilizing larger blades in a shallower configuration. The drive is quiet and creates no splash. It is easily removable for transport and storage. Its fins "feather" into the flow when not pedaling and create very little drag; a propeller creates significant drag when it is not spinning. The blades can be "tucked up" against the hull for beach landings and so on. The detailed description of the Hobie MirageDrive you can find on website Hobie Cat Company is very proud of the MirageDrive patented worldwide. It is really an irreproachable drive for human-powered boat. Nevertheless all things have merits and demerits.
    I see the following disadvantages of this drive:

    • We can’t enlarge Hobie Mirage flippers more because of 180º sweeping (it will not be a shallower configuration);
    • Sweeping action is not the best way to get an efficient stroke under water;
    • Hobie Mirage two underwater flippers work much like a penguin’s fins to propel a boat. Penguins can’t swim back to front and don’t need to do this. They tern all body back instantly. We need make boat to swim back to front. Mirage drive can’t do it because it entirely simulates a penguin underwater flying.

    Picture 1 shows a kinematics scheme of the full-size traveling wave’s propulsion device. Every part of the hydrofoil assembly is in a sinusoidal motion and we can use 1 - 2 hydrofoils to propel a boat.
    My first model uses two phased-stroking hydrofoils. Hydrofoils made using torsion springs or a swimming flippers technology. This design demands of 180º turn of the hydrofoil axes for a reverse movement of a boat.
    The second model provides for a forward and back movement if we use just one hydrofoil.

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  2. Toot
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    Toot Senior Member

    I toyed around for awhile with the idea of creating a "fishtail" for propulsion. In a nutshell, build it out of foam rubber, in the shape of a fish... mimic a fish's motion through the use of pulleys and wires which limit each section of the tail's movement to a given angle which would permit laminar flow to largely be maintained through the entire length of the tail for minimum drag.

    I am convinced it could be done and I believe it could be very efficient... but the design knowledge needed would be greater than I could learn in a reasonable period of time.

    Then, of course, there's the small matter of a shark seeing a flapping tail above him and mistaking it for a tasty snack. You think surfers look like a meal? This would be waayyy worse! LOL
  3. VladZenin
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    VladZenin Senior Member

  4. ThomD
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    ThomD Senior Member

    Toot Harry Bryant has had such a device for a long time. I haven't seen it but the reviews have been good. If might have been better known if he wasn't an off the grid wooden boat builder.

    Yeah I know this is an old thread but it was the first one to come up in my mirage search
  5. VladZenin
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    VladZenin Senior Member

  6. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I posted while back about using existing kayak rudder gear for this.

    Beef up the cables, pulleys and foot slide and attach a longer tail.

    That way you could propel and steer hands free, which would be great for kayak fishing. Would also work if very shallow water.

    But I think the wing as shown in youtube would be better for cruising.
  7. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I'm happy to see this thread.

    A propeller on a sailboat is an ugly thing. It either need feathering blades, or needs some kind of engine to keep it turning, otherwise it acts as a brake.

    A fish (vertical axis) or whale (horizontal axis) tail would not need twisted blades set at a pitch, so could be oriented to the boat's line of travel, when not in use.

    I've even thought of bolting SCUBA fins to the trailing edge of a rudder, in desperation.

    Even if a propeller is theoretically superior on a straight powerboat, the matter is still worth pursuing.
  8. portacruise
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    portacruise Senior Member

    See Robo tuna?


  9. Jeremy Harris
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    Jeremy Harris Senior Member

    Flapping fin systems are OK for fairly low efficiency drives, and if arranged like the Hobie system so that the yawing (or pitching) moment is cancelled out work OK on small boats. The problem with all the single flapping rear fin systems I've seen (and there have been a few built over the years) is that they either induce a fairly strong yawing moment (for vertical fin systems) or a fairly strong pitching moment (for horizontal fin systems). Both of these moments cause the boat to move, either side to side or in pitch, wasting a significant amount of energy. There are various videos around on the web of some of these systems and they all show the same trait.

    There was a dual, transom mounted, vertical fin system built (I think it's way back in the pedal boat thread somewhere) that got around this problem in a similar way to the Hobie system, by having a pair of fins moving in opposition on long arms. I believe the efficiency was pretty reasonable, maybe as high as 50 to 60%, which is on a par with an "ordinary" type of boat propeller (if still well behind the performance of a narrow aspect ratio propeller as normally discussed here).

    The problem is that the pedal boat thread has got so long that finding all the many useful snippets in there is getting to be difficult, especially if you can't even remember which year the relevant post was made.
  10. portacruise
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    portacruise Senior Member

    Here's one:


  11. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Well no, to be pedantic, screw propellers are quite recent.

    "In 1784, J. P. Paucton proposed a gyrocopter-like aircraft using similar screws for both lift and propulsion. At about the same time, James Watt proposed using screws to propel boats, although he did not use them for his steam engines. This was not his own invention, though; Toogood and Hays had patented it a century earlier, and it had become a common use as a means of propelling boats since that time."

    Obviously, screws themselves had been around since Archimedes time (~300 bc) , but that was not for propelling boats.

    Much more common are oars of all types, for perhaps over 40,000 years.

    To be direct, I see your 'invention' as remarkably convoluted with its individual levers and associated connections.

    What would be wrong with linked 'cam' shaped devices to drive the flexible mat ? This is the most common method for achieving this effect in industrial applications.

    Do you have any experimental evidence that there is an increase of efficiency?

    In practice also, the tasks of reversing and directional control would seem to be somewhat limited.
  12. Jeremy Harris
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    Jeremy Harris Senior Member

    Yes, thanks, their video illustrates the yawing you get from a single moving vertical fin pretty well. Somewhere I've seen a system with two vertical fins moving in opposition, but I can't seem to find it at the moment. I believe it gets around the yawing problem.

  13. SaltOntheBrain
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    SaltOntheBrain Senior Member

    I think the problem is these people are imitating the wrong fish.
    Huge, slow sweeping tails give you the side-to-side motion of a slowly swimming shark.
    Think tuna. Low aspect ratio, rapidly oscillating tail. Maybe linked to a rapidly turning flywheel. The tail only needs to move an inch or so side-to-side, but as rapidly as possible.
    Gives me ideas.
    Where's my pencil?

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