Efficient solar powered (electric) kayak

Discussion in 'Boat Design' started by ElectricKayak, Mar 20, 2015.

  1. fredrosse
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    fredrosse USACE Steam

    Block of wood

    Your use of the woodblock is perfect, and re-sizing to get any desired load is easy too. Looks like rotation direction has virtually no bias, good news for you.

    Now, you might consider a focusing high temperature solar collector running a small steam plant rather than electric drive..................OK, just kidding
     
  2. ElectricKayak
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    ElectricKayak Junior Member

    PC: thanks for the comments. I'm near a lake and only test when it is glassy calm to avoid the wind/current bias. I also monitor voltage and current during the run and log the total watt hours consumed at the end of each run as a sanity check. Voltage droop hasn't been an issue although I do use a fresh AGM battery that is quite oversized for the test lengths. Also my runs are 500m long and I use the GPS average function for each run. I've post processed on the computer to verify the results and check for end-effects and found 500m a good stable run length. Instantaneous GPS speeds are erratic at these slow speeds so I don't use them. Overall this test setup seems pretty accurate and stable so if test results are inconclusive I consider the differences immaterial!

    A pool is shallow which adds a nonlinear drag component to the tests. I know from paddling one feels increased drag from shallow water. It's minor at these speeds but something to be aware of. I'm also not sure they are large enough to get enough steady state run conditions so it might be tricky interpreting the data. Having said that, if I had a pool in my back yard I'd probably be trying to use it! LOL

    Fred: yes the wood block does work well. I started with a 6" piece but it loaded to over the rated 42 amps at 100% so I cut it down 1/2" at a time. I actually sized one to draw 5.7 amps at 37% throttle. The reason may be little obscure but this matches my APC 10x6 prop test and allowed me to estimate the rpm. Actually, since I don't have a tachometer I measured the rpm by taking a picture with my cell phone (at 120 fps) and counting the number of frames in one revolution.

    (...and in the process discovered my Note 4 eight times slow motion doesn't really record at 30x8=240fps but cheats by recording at 120fps and then playing back at half speed of 15fps. Four times slow motion records at the same 120fps but plays back at normal speed of 30fps. Tricky!).
     
  3. ElectricKayak
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    ElectricKayak Junior Member

    Attached is a picture of the mods I've made. The fairing is wood in three pieces about 3.5 inches high each (about 11 inches total). The top two will rotate on the shaft but the bottom is fixed. Three inch hobby aircraft spinner installed and various props to test assuming it stops raining in my lifetime. ..

    Question: How deep to put the motor? Is there an optimum spot for efficiency?
     

    Attached Files:

  4. portacruise
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    portacruise Senior Member

    My thinking is you want to avoid interference generated by the boat or ground in considering depth.

    Thanks for the details on how you measure speed, which seems quite thoroughly done! One additional thing I do is to measure over the same course going in the opposite direction. Usually get discrepancies, some up to 20%- even with glassy smooth looking water. But that may be a peculiarity of my particular location...

    I ran a 12v MK on 24V using an external series/parallel switch for many years without any damage in extended use at 24v. The key is not to exceed the maximum amps by using a smaller prop diameter and/or pitch. You can get 2x power with a pair of batteries that way with a much cheaper motor (burst power- handy for emergencies), or you can just use in parallel 12v for slow extended cruising...


    PC
     
  5. alan craig
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    alan craig Senior Member

    I think the propeller should be as deep as possible for efficiency and as shallow as possible for practicality!
    I attach a picture of my own electric outboard; if you ignore the model 'plane prop you can see that the original prop goes under the flat plate, which is there to stop air being sucked down into it. You could try making a plate like this if you get problems with sucking air into the prop but I have a hunch that you won't have any problems as long as the prop is maybe an inch under.
     

    Attached Files:

  6. ElectricKayak
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    ElectricKayak Junior Member

    The rains stopped so I did a test. Unfortunately I was by myself so the numbers aren't comparable with earlier. Regardless 5.1km/h with stock motor tube drew 4.9 amps (at 12.5v) versus 4.1 amps when faired. 12.5 volts * 0.8 amps = 10 watt reduction. So about 7N of drag removed...basically as predicted...cool. :)

    Sadly all the other props performed the same or worse (by up to 10%) than the first 10x6P that I tried. So much for Javaprop! LOL Obviously there are other factors perhaps due to a lower efficiency of the motor when it is being spun slower or providing more torque (all the other props were slower spinning and higher pitched).

    Overall the results are in the ball park, but marginal given the current state of solar panels. I'm not sure much more can be done without changing out the motor. The idea of a brushless motor on a small flex drive sounds intriguing...are there good candidates for these without getting too complicated?
     
  7. portacruise
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    portacruise Senior Member

    As regards the MK:
    More of the blade area would be exposed to undisturbed water when going to a larger diameter and dropping to a lower pitch/faster spinning prop. APC has pitch of 4 available in various diameters up to 17". Would take some more tweaking, and maybe not much gains.

    As regards flex shaft drive:
    A much smaller, lighter motor will be sufficient with the proper gearing. Here's a link give you a general idea:

    http://www.rickwill.bigpondhosting.com/V4_Electric.htm

    There is a link to a video at the bottom of the page. With an optimal setup, 30 watts might be enough to get 3 knots.

    PC
     
  8. Joakim
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    Joakim Senior Member

    7N is at 100% efficiency. Motor, propeller and possibly cables (where did you measure the voltage?) probably have lower than 80% total efficiency, thus more like 5 N. Your fairing seems clearly thicker than the shaft, but it is hard to see from the picture. That obviously increases drag.
     
  9. Trent hink
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    Trent hink Junior Member

  10. alan craig
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    alan craig Senior Member

    Electrickayak
    you asked about a brushless motor on a flexi shaft. HobbyKing are currently selling some fairly large motors for quadcopter. If you go to multi rotor/motor then search by weight you can find a 90rpm/volt 1200w capable brushless which looks ideal to me. My own electric outboard has a model 'plane motor and controller and I have not had any problems.
     
  11. ElectricKayak
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    ElectricKayak Junior Member

    Ah yes, good point. I measured before the speed controller. The stock tube is 1.125inch and the fairing is 1.5inch.

    Total system efficiency is probably more like 50%:
    1) Digital speed controller and system wiring (90-95%)
    2) MK motor (70-80%)
    3) Propeller (65-75%)

    Taking (0.9, 0.75, and 0.7 as the most probables) yields 47% system efficiency. So 3-3.5N actual drag reduction.

    My sense is getting more than 60% system efficiency is not so easy.
     
  12. ElectricKayak
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    ElectricKayak Junior Member

    Yes, that is what I was thinking about (although I see the rated voltage is 45volts). I didn't know if anyone made them...thanks! This kind of motor shouldn't need a gearbox then but I see it only has a 3mm shaft. Do you have any idea of the torque output of these motors? Also I assume the 1200W is for the typical 3 minute flight so wonder what they can sustain (there was no listing stating average current). It's probably OK based on looking at other motors on that site that do show an average current capability.

    I'm very interested in your setup, especially how you go from the motor to the propeller. Actually anything else you think might be relevant. If I can put together a brushless system cheap enough I'd like to experiment in that direction also.

    Part of all this is an excuse to learn something and so far I'm learning tons in an area I know nothing about (propellers, drag, motors etc).
     
  13. ElectricKayak
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    ElectricKayak Junior Member

    As a general comment, since I now have a working system my plan is to use it for a while to get a practical feel for the variables (amount of power we really need, real world solar panel behaviour, speed desires, portability, etc) then I'll look at what problems really need fixing for version 2.

    At some point I'd like to migrate something to my sea kayak but ideally with less weight, more compactness and and higher speed. I already know that my two 50w flexible solar panels are the most I will be using on the sea kayak.

    In parallel however I'd like to understand options such as brushless motor drive. Or anything else people think I need to be considering given the goals.
     
  14. ElectricKayak
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    ElectricKayak Junior Member

    The first thing that struck me is the noise. Yet another consideration. As a kayaker I'm used to silence, nothing but wind and waves. Noise may make the topside brushless motor approach a non-starter for me.
     

  15. portacruise
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    portacruise Senior Member

    Yes, I hate noise also. Using belts and a low frequency control may help. Some discussion here:

    http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-24.html picture post #358.

    Direct drive will be difficult without replicating the massive metal troll motor concept. Then comes the related efficiency issues due the torque required to run an efficient prop. Perhaps a large diameter high torque printed circuit pancake motor (claim quiet, 92% efficient) would be the lightest compromise.

    http://coreoutdoorpower.com/technology/

    Main power drive considerations for me have been portability, simplicity, efficiency, quietness. Speed of 3 knots is enough for the protected rivers and creeks which are my chief application. Rarely go to open water or against river currents, but I would feel better with short surge power which is useful for wind, waves, wakes, rapids in such cases. When mounted on a stable one man inflatable pontoon, flex shaft provides ability to bounce off bottom and pack in capability.

    PC
     
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