Pedal Powered Boats

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

  1. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Hi, Jeremy:

    There are some advantages to having the prop fold back as far as possible which are traded away for a folding reverse. Hitting bottom is less traumatic on the drive train, water resistance is less when coasting, and pedal crank intertia is less with a full fold prop. Deployment might be more problematic, though.

    Do your folding blades deploy symmetrically every time they are engaged for forward drive? My RC prop versions sometimes stick in lopsided partially open positions. Usually due to scum algae or weed interference around the hinge area. Perhaps your hinges are better protected.... Some folding sailboat props blades are linked with gears to allow proportional deployment. Wouldn't they be more likely to seize as scum, weeds, build up with use?

    There are commercial HP flip up legs, but I have not seen any that are steerable. Do you think a flex cable U joint would work at the top end to permit a flip up/steering leg?

    Porta
     
  2. Jeremy Harris
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    Jeremy Harris Senior Member

    I've got around the prop hitting the bottom issue by adding a fin at the base of the leg to protect the prop. It increases drag a little but both protects the prop from damage and also deflects weed a bit, I think.

    My prop always spins out very hard, even at low rpm. The force on the blades from both their mass (being stainless they're pretty weighty) and thrust drives the blades hard to the stops very quickly. I've never noticed an asymmetric deployment and suspect it'd be pretty obvious if this happened, from the gross imbalance in the prop that would result.

    I did look at using gears to couple the blades early on, but that increases the hub diameter a lot. In the end I just pretty much copied the design Rick used and haven't found a problem with the hinges binding (yet!).

    With a bit of ingenuity another double UJ could be used at the top to couple a shaft to a flip up leg, I'm sure. To save having to incorporate sliding joints the leg would have to pivot about hinges centred on the UJ pivot pins. You can get rubber concertina bellows type seals for these UJs, so it should be possible to keep the leg sealed and yet still allow it to tilt and turn. The flip up hinges could be an external yoke arrangement fixed to the boat, with perhaps a teleflex cable for steering.
     
  3. portacruise
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    portacruise Senior Member

    Thanks, Jeremy.

    Yes, I was thinking weight of the blades would make a difference in the deployment hang up. Keeping fine clinging debris out of the stop area appears to be an issue with my more open design, as well. My lighter blades rely more on water bite for deployment, and the lateral force on the hinge may be causing a pinning effect...

    Here's a pinless U joint design which might be of interest. http://www.sears.com/craftsman-impa...nt-socket-adapter-3-8-in/p-00923766000P#specs One of the diagrams below the pic shows how it works.
    May be too short and expensive for your application, though it is reputed to be smoother and more flexible. I have also seen pics of cable versions somewhere I can't seem to find now. Maybe not strong enough for small boat applications.

    Porta



     
  4. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    Jeremy
    From an earlier post of yours:- "If a way could be found to transmit drive to a steerable leg and allow it to flip up if it hit an underwater obstacle then I think it might well be a pretty good system overall."
    I tried something like this on my old boat a while ago, a steerable flexible shaft. I was using an Involute gearbox with an inline flexible shaft driving a stainless steel folding prop. The shaft was fixed to a skeg which could slide up and down, the slot for this was on a steering arm controlled by a rod and lever at the seat. This did work to a certain extent but took a lot of force to move the shaft, turning was not that good, ok for cruising but not any tight manouvres. Also increased the stress on the flex shaft, steering arm had to be strong which increased weight. A good experiment at the time, if I hadn't done it would have always wondered if it would work.

    Ian
     

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  5. portacruise
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    portacruise Senior Member

    Ian, this can be done with a pivoting flex shaft which allows for flip up and precision steering within boat's own length:

    See film clips in post #53 and #54

    http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-4.html

    A cable drive or some other flexible method of power transfer would be required in the case of HP.

    Porta

     
  6. Scheny
    Joined: Feb 2012
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    Scheny Junior Member

    Thanks to everyone for the quick replies.

    I tried to sum up the requirements for a competition boat:
    • 650Watt @ 5m/s + 104Newton drag
    • 150Watt @ 3m/s + 35Newton drag

    My calculation shows, that my new propeller design (extra small, extra robust, cheap AND weed shedding) has 83% efficiency at sprint speed and up to 95% efficiency if not under full load (calculated by 100% - (P_losses/P_input*100) @ zero pushing force and 5m/s) like when used at low speeds/power.

    The maximum pushing force for different diameters is:
    • 150N @ 18cm
    • 170N @ 20cm
    • 190N @ 25cm

    I think, that at only 20cm diameter / 25cm total draft, the prop would not need to be lifted up at all when protected with a fin.

    How much force for acceleration (all props have 104N @ 650W, 5m/s) would you guys consider as necessary?

    I will post the prop design soon, at the time I have to finish my tax information ;)
     
  7. portacruise
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    portacruise Senior Member

    If the competition is a sprint, you would want to go with the maximum force of acceleration possible, and hope the drive train will hold together under the stress (even the heavy mitre boxes have failed!). If the competition is a 24 hour endurance event, acceleration is not important unless you are reversing course. Seems like even a small 2% increase in efficiency is more significant in an endurance event.

    Porta
     
  8. Jeremy Harris
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    Jeremy Harris Senior Member

    Very true! I raced against DennisA in the Cordless Canoe Challenge this year and although his boat was definitely a fair bit faster than mine in a straight line I gave him a reasonable run for his money, largely from having faster initial acceleration and a better turning ability. Ultimately the higher wetted area of my boat was my downfall, as Dennis was able to pass me and get a couple of boat lengths ahead by the finish line. We were both running similar size home made folding props, the main difference was that my boat was a fair bit lighter than his and, being flat bottomed and driven by a steerable drive leg had a very tight turning radius.
     
  9. Scheny
    Joined: Feb 2012
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    Scheny Junior Member

    This is the first calculation of the new prop. I calculated the efficiency for different rpm-settings (I have added the required power below the rpm) against varying speeds from 0-5 m/s.

    The prop is only 20cm in diameter. The second one has the same dimensions, but the blade area is increased by 50%. This should improve efficiency for lower speeds and make it worse for higher, because of the higher wetted area.

    It's interesting, that more area improves cruise by 2%. Sprint is improved by 6%, so there seems to be no drawback.

    The bold fields are the "power required for a certain speed" axle. The prop is optimized for a 5-6m high speed cruise boat like a V15. So, the bold fields are what you need for steady cruise at the corresponding speed.

    Everything above 83% is green, but as I said before:
    • it's weed shedding
    • it's extra tough
    • it's cheap to manufacture
    so, I didn't plan to get above 90% efficiency.

    As the "bigger" version has better values, I will carry on calculating on this one. This is really cool, as I called the 150% area prop the "undestructible".

    With the high skew and 5mm thickness you can make fish&chips out of everything in your way :cool: ;)

    (left attachment normal version, right one the 150% area version)
     

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  10. portacruise
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    portacruise Senior Member

    Scheny:

    Thanks for the attached tables.

    I have also found that some wide versions of model props (Zinger XW wood for example) are more efficient that the regular widths, even of other brands. Tests have been done by trial and error over years, in a non dedicated way, during leisure times on water. I use a prop adapter that allows almost instantaneous change out while on water. But my applications are under 50 watts, and not HP, not super low resistance hulls. RC props are cheap, but not weed shedding or tough. Weed shedding props are notorious for being inefficient.

    If your props are not proprietary, I'd be interested to see what they look like.

    Porta



     
  11. Dennis A
    Joined: Aug 2009
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    Dennis A Junior Member

    Wider Props

    Scheny

    When I beat Jeremy in the Cordless CC I had increased the width of the Prop by 45% to virtually eliminate slippage which gave me my higher top speed. This extra width I think partly reduce my acceleration combined with having 50 % less power than Jeremy.
    Does the Prop design programme that you are using consider slippage as this I find is a major factor when making props for pedal boats
    Dennis
     
  12. Scheny
    Joined: Feb 2012
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    Scheny Junior Member

    I use Javaprop (mh-aerotools.de), but I take over the values and further evaluate it with my own Excel-sheets.

    As for slippage, I let Javaprop calculate the pushing force over speed for a given wattage. Therefore it should be included.

    In fact, the efficiency chart I posted does nothing else, than comparing the pushing force to the force which comes out of the equation force=power/speed. The lost power is slippage+drag.

    This prop with the extended chord has more viscuous drag, but less slippage. As far as my charts show, the lower slippage seems to cause less induced drag, which eliminates the drawbacks of the higher viscuous drag for speeds below the designpoint.

    As I have to calculate a 3-dimensional field, it takes a lot of work in order to calculate the force over speed for all wattage/rpm scenarios independently (Javaprop does not feature such a function, only 2-dimensional fields). This is why it takes so long for me to iterate the design.

    I give you pictures of the prop and I am curious about your thoughts. I think that I can still improve it by 3%. I will post the 3D data for milling, as soon as I will freeze the design process.

    Andreas
     

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  13. Jeremy Harris
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    Jeremy Harris Senior Member

    Not sure that's at all right, Dennis. Your boat was inherently faster to start with, by design, lower wetted area, better hull shape, for example, whereas mine was a flat bottomed punt!

    Also, the power difference wasn't anything like that great at the prop, if anything I think you may well have ended up with more effective propulsive power than I did, just because I had my prop geared for a far too optimistic maximum speed. I was trying to drive my prop at more than double the best rpm for the actual boat speed I achieved, which is why I ended up hammering the drill batteries so much. The prop should have been turning at around 600 rpm for the speed I actually got up to, but I was trying to turn it at around 1400 rpm, so it was just bogging down the drill very heavily. With hindsight (what a wonderful thing...........!) I should have switched to the finer pitch prop before the race, but ever the optimist I decided to go for broke........... (and failed!).

    The prop I used normally runs at a very much lower power level, and rpm, and doesn't have much slip at all. The slip ratio goes up as the power goes up, or more correctly as the blade loading goes up, and had I operated it at its design loading it would have been a lot more efficient.
     
  14. Dennis A
    Joined: Aug 2009
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    Location: Amersham bucks uk

    Dennis A Junior Member

    Prop Design

    Scheny

    Due to the weed shedding function, your latest prop is starting to look like a Torqeedo Ultralight.

    Jeremy
    In the first Cordless CC I tried a similar overloading of the drill and burnt out the motor.
    This year I was running the prop at 350 rpm ( no load) but can not measure this when the boat is drill powered. I believe that your prop would be subject to 30% slippage based on the figures you stated.
     

  15. portacruise
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    portacruise Senior Member

    Scheny:

    Thanks for the post.

    Interesting shape, with the blunt nose hub serving as a front fairing? Is there a way for the program to make allowance for strut shape and placement behind/ in front?

    What is the indestructible material for the milling machine, aluminum, polymer, etc. ?

    Is there significant efficiency loss by raking back the blades in order to get some weed shedding? There are other methods of weed shedding, if that be the case...

    Porta

     
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