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

  1. jakeeeef
    Joined: Sep 2009
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    jakeeeef Junior Member

    Flexi shaft drive- energy cost

    Hi,
    I'm still only a quarter of the way through this thread and am quite sure that using RC racing boat type flexi drives will have been discussed somewhere. I just haven't got there yet.

    I wish to put together a self contained drive unit that I can use on a selection of different hulls that I am working with.

    I don't like the underwater thickness required by a right angle gearbox at the prop, or the leg thickness required by a twisted chain drive. They both seem unacceptably draggy to me.

    Because the nature of the multiple designs I am working on require this propulsion to be fairly self contained at the rear of the boat, I don't want the additional length of an unsupported spring steel shaft behind- although these appear to be an elegant solution if you have the sort of boat that would allow it.

    I essentially want to make a human/ (occasionally electric) powered outboard motor that will clamp to anything like a transom. It will steer and hinge and lift at the top to provide both shallow water drive and complete lifting. Its general size, bulk etc, will be that of a small (say 4hp) gasoline outboard- but obviously lighter in weight. I'd also like the 'leg' that I'm making to have the durability to be able to take a higher, electrical power (36v lithium ion drill)- with different gearing and prop as I'm trying to build a piece of hardware that will do about three jobs at once! This will also mean that for human powered use it will have a reserve level of strength and should last well.

    I have worked out that a 1/4 inch flexi driveshaft, running in a teflon tube, lubricated by teflon grease can be curved into and entirely encapsulated within a thin (and already hydrodynamically correct) dinghy centreboard cut into two halves, routed out inside for the teflon tube, then epoxied or bolted back together again to encapsulate the tube. The centreboard will actually provide a skeg protecting the prop- yet more costly underwater foil- but I think on balance probably worth it as I don't want to constantly ground and snap props. These flexis are, in principle, how they do model boat outboard motors. I have one of these and it certainly feels easy to turn the prop by hand.

    The larger, higher speed (60+mph) radio control boats use inboard brushless 500 watt+ motors or more powerful nitro engines with the 1/4inch flexi drives so the hardware should take the power. BUT does anyone know the energy cost of such flexi drives when curved through 70 degrees over about 20 inches?
     
  2. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Hope above comments help.

    PC
     
  3. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    There is a thread on this forum called "efficient electric boat" (I think) where Jeremy Harris builds his electric low power outboard using only universal joints for a 90 deg. change of direction underwater. His setup looks to be extremely low drag and, according to him, lower friction than a cable drive. Also, the cable might not like the torque available from leg power - even with gearing. I have a couple of screwdriver flex drives and can apply enough torque by hand to twist them out of shape.
    Another CCC competitor, Dennis Adcock used a long spring steel shaft with a bush at the prop end on a strip steel bracket hinged above the waterline; he could lift it out of the water with a piece of string which seems very practical.
     
  4. jakeeeef
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    jakeeeef Junior Member

    Thanks Alan, I watched every CCC video on the watercraft site last night, still deciding whether to take the plunge. I'd want to either fully plane or foil as it's about time someone did, and it represents a lot of work. I enjoyed seeing your various iterations of Four Candles/ Fork 'Andles.
    I was disappointed the airscrew version didn't perform better. I guess a single large prop would have been better. I can see you were going for the Decavitator approach. I once built a copy of the Gossamer Condor prop (HPA) and seem to recall it being about 2 metres diameter. It worked really well- although not for long. I didn't have a clue on where to start on a HPA prop , until by chance someone sent me a copy of the book Gossamer Odyssey by Morton Grosser, and surprisingly he includes a handy little table of all the prop stations- along with lots of detailed diagrams (why don't people put these things in books any more?). Unfortunately the prop I built was destroyed in a crash, but I can send you a copy of the stations if you ever want to do one.
     
  5. alan craig
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    alan craig Senior Member

    jafeeeek, thanks - I might try to build an air prop again one day; I tried wire cutting foam but didn't have enough control to shape the fine tips, but I now have a big lump of nylon to machine a hub, and 12mm carbon tube to make the spars of a propeller so I may try again one day. As for the CCC entry there is lots of potential left in air props, my boat was simply underpropped and was using hardly any power; each drill uses about 65W no load and 260W static, with a propeller. Each battery is 54Wh energy which means they should be used up in less than 15 minutes but I could drive around for about 30 minutes, which means that they were putting a pitiful 60w into the prop while moving.

    Back to pedal power, here is a system which is ready made and very similar to what you describe:
    http://h2proped.com/technical-specs-h2pro-ped.htm
    Note the thickness of the flexible shaft!
     
  6. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    They can handle the power well enough, but not when the power input is much lower rpm. Power is torque * rpm. To maintain the same power, as rpm goes down, torque goes up. The RC boats are very high rpm, very low torque. A pedal drive is going to be very low rpm and very high torque. The strain on the flex shaft will be far greater when that power is transmitted at human pedaling speeds.
     
  7. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    That looks like a nice system, but it is not something that is going to win any efficiency awards.

    Initial timing belt reduction, a tight radius flex shaft bend with four feet total flex shaft, and finally two 90 degree gearboxes. That's a lot of losses.
     
  8. portacruise
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    portacruise Senior Member

  9. jakeeeef
    Joined: Sep 2009
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    jakeeeef Junior Member

    I had a good look a the flexi drive off a small petrol garden strimmer last night. It was just under 1/4 inch thickness at 6.1mm. It is much longer than I need at about 150cm. Grabbing one end of the flexi with a vice and turning the other with a spanner in side its bushed strimmer shaft- it is quite springy- although the spring does take up after a couple of increasingly difficult turns, but I wonder if this is a good thing in a boat drive- as the spring takes up the initial torque and any sudden loads. I guess because its a spring, any torque it steals from the system it then gives back at some point? I'll ask the H2Propped people about their flexi size, from the outside sheath diameter it looks like standard hand drill gauge which is normally about 4mm.

    The existence of the H2 prop demonstrates to me that this is an approach that should work- to an extent- but clearly there are losses in that system. As they are selling it commercially, They must have had to design for worst case scenario -load wise, that is a very powerful and big cyclist putting it in a hydrodynamically inefficient hull. I wonder if they have achieved this required level of durability in the system by under propping it?
     
  10. portacruise
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    portacruise Senior Member

    H2 may never have been on the market. H2 taking advance deposits only, and it was like that when I first saw it quite awhile back. Unless they are a small company, making one at a time?

    PC
     
  11. jakeeeef
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    jakeeeef Junior Member

    Well, they were around to answer my email about the flexi sizes. The front flexi is 5/16ths, the rear is 1/4 inch.
     
  12. jakeeeef
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    jakeeeef Junior Member

    I must admit that I have a concern that the unit uses a smaller flexi at the rear because by the time it's got through the front flexi the torque has been diminished enough to get away with it.
     
  13. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    There is a difference in losses between a constant torque motor and the lumpy toque of reciprocating legs or arms. The torsional spring of the thin shaft smooths the lumps but your muscles don't get back any energy -rather they see no difference between force even if no distance is moved -no work done.
     
  14. jakeeeef
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    jakeeeef Junior Member

    Anyone know where in the UK I'd get such a spring steel shaft? Any idea of thickness required/ unsupported length I'd get away with?
     

  15. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    I believe earlier - much earlier- posts had lots of information on the shaft size.
     
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