Efficient electric boat

Discussion in 'Boat Design' started by Jeremy Harris, Jun 22, 2009.

  1. bapou
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    bapou Junior Member

    For my solar-boat hull I modified the GA-boat design by putting glass fiber coated thin plywood in the regions below the waterline that may be more vulnerable for touching the ground. It didn't strongly affect the weight but was much more work.
    The hull is therefore more robust than a classical GA boat but still more sensitive (but also much lighter) than a traditional glass/plywood construction.
     

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  2. bapou
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    bapou Junior Member

    Hi Jeremy and Rick,

    after the calculations of Rick, I would like to try to fit an aircraft prop
    on my drive pod for first tries.

    Unfortunatly, my drive pod is far from optimal in the hydrodynamic sense and
    is not prepared for aircraft props.

    Therefore some questions:
    - How would you fit an aircraft prop on the axis (the axis is 14mm at this position in the moment but I can reduce the diameter). Are there already standard solutions to this problem ?

    - The hub of the aircraft prop is much smaller than the diameter of the motor and casing. How far away from the motor should I mount the propeller. I could add pieces on both side of the pod to improve the shape (see the rough sketch). Are there any standard shapes
    that I can use ?

    Thanks
    Thomas
     

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  3. Guest625101138

    Guest625101138 Previous Member

    I have attached the drag curves for the two loading conditions.

    The foil section attached can be used to take nose and tail cones from - it is not disimilar to what you have drawn. Leave the long section parallel. The tail cone should incorporate the propeller. You can get aluminium spinners from HobbyCity.

    Rick W
     

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  4. portacruise
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    portacruise Senior Member

    Thanks Tiny, I hadn't thought of a possible oscillation flexing like that. If only one blade root was damaged and not both, maybe there could be some unusual flexing due to imbalance? On a model prop the root area is much thicker, with a gradual thinning out to a very thin section at the very tip. So I was expecting only the tip sections to distort with a pull forward under a heavy load such as acceleration. This would cause an increased loss of efficiency as long as the prop remains distorted like that, was my guess. It is also possible the very end of tips remain distorted even under moderate loads. I know from measurements that much of the thrust comes from the tip sections, because cutting even 1 inch off the tip reduces the efficiency considerably.

    There have been at least a couple of well made commercial products that used model air props (in water) with capability of maybe 500 watt short bursts possible with human pedal power. The last one I recall was a very lightweight and compact twisted chain unit called the spinfin made by Bob Stuart of HPB.... Don't know of any that are presently in production, though it's a good guess that some of the Stuart units are probably still in operation.

    I have used fiberglass model props on my one man portable electric boats for decades without durability problems- but they are not racers and top out at 50 watt maximums. Model props measure to be more efficient than the assortment of commercial props (including the seacycle and other HBP units) I have tested at 40 watts. The low cost and great variety of pitch and sizes have made it possible to select the exact pitch for a particular motor and gearing which gives the lowest power draw. In my case, the best combination can vary over a several pitch and diameter sizes for the dozen different commercial motor and gearing setups used. I usually set a reasonable target speed at 40 watts and work backwards to get me the motor, gearing and prop with the least draw.

    Most of the power levels I read about here are way above what I use. So I hope some part of this information may be of use, even if just in construction of prototypes.

    Porta



    Porta
     
  5. Jeremy Harris
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    Jeremy Harris Senior Member

    Thomas,

    I agree with Rick, although I wouldn't bother changing the front of your pod, just the aft end where the propeller fits. Form drag is dominated by the shape of the rear of bodies like this, the front can be surprisingly bluff yet still have little overall effect on drag. If you look at the shape of my drive pod you can see that I traded off the viscous drag from the extra wetted area from extending the front against the small improvement in form drag such a shape would give. This was driven by the need to fit the motor as far forward as possible.

    You could use a spinner as Rick suggests, or better still make one with a better shape. Spinners tend to be ogive, whereas you can make a small improvement by opting for what is often called an "energy recovery" afterbody shape. This old report has a picture in it of a long spinner with the sort of shape that gives a useful gain in performance: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930093333_1993093333.pdf

    As an aside, the NASA Technical Reports Server is a very useful resource, although it is an easy way to lose a few tens of hours if you're not careful! The link to the server home page is: http://ntrs.nasa.gov/search.jsp

    Jeremy
     
  6. bapou
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    bapou Junior Member

    Hi Rick and Jeremy,

    thank you very much for the drag calculation and the assistance with
    the pod shape. Now it's time to go to
    the workshop and try things out.
    I was a bit surprised that the drag curve looks so linear above 5 knots (for the
    low displacement case).
    Can one trust in this part of the curve or is it unrealistic ? This would mean that in theory one could achieve 10knots with reasonable thrust on this displacment hull ?

    Thomas
     
  7. Guest625101138

    Guest625101138 Previous Member

    The analysis does not allow for trim changes or sinkage. In the case of such a light boat with large flattish area it is likely to be on the plane so the drag might flatten out a little rather than continuing to climb as the graph shows.

    I push my pedal boats to 10kts with around 600W and the displacement is 100kg plus or minus a few kg. The latest one has a mild tendency to plane but I would need to go a lot faster than 10kts to get fully on the plane.

    The boat on the link is not a classic planing hull and is not dissimilar to yours:
    http://www.steamlaunch.co.nz/Whio.html

    A ball will plane if moving fast enough.

    Rick W
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    bapou

    "...Can one trust in this part of the curve or is it unrealistic ?.."

    The method of producing that curve has more holes in it than a colander. But if you wish to believe the numbers, that is your prerogative.
     
  9. Tiny Turnip
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    Tiny Turnip Senior Member

    electric assisted pedal powered boat patent

    An interesting patent on an electric assist pedal powered boat here.

    http://www.freepatentsonline.com/7549902.html

    PDF with the drawings attached.

    Torn between two threads, I thought this probably belonged here. I'll link from the Pedal powered boats thread.

    TT
     

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  10. portacruise
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    portacruise Senior Member

    This patent may very well have some merit. But for those that are not aware, a issued patent itself does not mean that the device itself works or ever will work....

    Some interesting info and a standing cash award for the proof that certain machines work:

    http://www.phact.org/e/realfree.htm

    Porta

     
  11. Jeremy Harris
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    Jeremy Harris Senior Member

    The most interesting thing about this patent seems to be that the patent office actually granted it. I can't see anything in it that's truly novel, it seems to be entirely prior art, mostly a description of a hybrid drive system that is already well established. Back when I last patented anything a great deal of effort (and money!) went into searching for prior art before the patent office would grant a patent; now they seem happy to grant them for things with no novel content.

    Jeremy
     
  12. Jeremy Harris
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    Jeremy Harris Senior Member

    An update on progress. I've finished building a unit using my original TowerPro RC model motor, connected to a 2:1 Mitrpak gearbox. Overall it's working fairly well on the bench, perhaps a little noisier than I would like, but the gearbox is still running in.

    Losses are looking reasonable so far, and, judging from the heat produced, are mainly in the gearbox seals. After about an hours running on the bench, the power consumption with no load (just the motor drive train and gearbox losses) is around 14 watts. It started out at around 19 watts, but is dropping as the gearbox runs in, I think. This loss includes the electrical loss in the motor and controller.

    The figures are:

    Input voltage = 12.8V (equivalent to 4S LiFePO4 cells)
    Input power = 14W (full throttle, no load)
    Motor RPM = 1382 (full throttle, no load)
    Prop shaft RPM = 691 (full throttle, no load)

    I'm planning on aiming for a design rpm for the propeller of 550 at a cruise speed of 4mph (1.8m/S). At this rpm, the motor, controller and gearbox use around 10 watts, which isn't too bad, although I'd like it to be lower. I estimate that the total electrical power input for cruise speed should be around 85 watts, so the electrical/mechanical transmission losses will be around 12%, probably as good as I'm likely to be able to get.

    Most of the noise seems to come from the gearbox, so I'm hopeful that it will be muffled a bit by the water.

    As a secondary experiment, I've been trying to rewind a spare motor for a lower Kv, so that I can use it as a direct drive to a prop. This has been pretty time consuming! I'll try and post some more photos tomorrow.

    Jeremy
     
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  13. portacruise
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    portacruise Senior Member

    Torqueedo appears to be using outrunner motors and a different prop (cruise model) in their new and expanded lines:

    http://www.torqeedo.com/us/hn/products/produktuebersicht.html

    Can't tell if they still use the noisy gearbox in the newer models. Their video is claiming to be the world's most efficient boat motors at 55% all the way to the water- including prop.

    Porta
     
  14. Jeremy Harris
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    Jeremy Harris Senior Member

    55% is reasonable, I think, for a general product aimed at being used on a wide range of craft. So far it looks like my electrical/mechanical efficiency will be around 85% or so. I think I should be able to get 75 to 80% efficiency from the prop, so hopefully my overall system efficiency could be around 65%.

    This is a fair bit better than the Torqeedo, but has some compromises that it looks like they weren't willing to accept (bigger prop diameter, thinner high aspect ratio prop blades, lower prop RPM). I'm also designing for a single point, a boat that needs around 30N of thrust to propel it in still water at cruise speed. The Torqeedo will be more of a general design, suited to use on a broader range of boats, so I would guess that it's probably sub-optimal for most boats it endes up being fitted to.

    Jeremy
     

  15. portacruise
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    portacruise Senior Member

    I thought some of us tinkerers could beat that efficiency. Their prices seem high, but then we have a large number of hours invested. Their newest product for kayaks only weighs about 15#, including battery. Wonder what the target weight of your system will be Jeremy... Some pretty neat technology linking GPS to give a good fuel guage, and linked steering to existing rudder system, plus quick on-off mounting:

    http://www.torqeedo.com/us/hn/products/ultralight/product-description.html

    They point out that top speed improves with the better hulls in the video.

    Porta



     
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