Efficient solar powered (electric) kayak

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

  1. portacruise
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    portacruise Senior Member

    Thanks for the pics of a great setup, EK! Can you pull the motor head out of water from a sitting position? How is the setup done on your inflatable?

    That's a really good showing with your modified MK!

    Hard to know what "peak total efficiency" includes, so 45% may be quite good when they state a regular troll is about half that. Battery losses, hull losses, etc????

    There is no fairing on the TQ shaft and their prop looks to have a lot of surface area w/ 3 blades, looks more like a lossy weed prop than the efficient props I have seen.

    Weight/size are important to me, wouldn't be any use to have a 100% efficient boat propulsion if it requires infinite weight to do it :).

    PC

     
  2. ElectricKayak
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    ElectricKayak Junior Member

    Yes, the motor pivots and pulling it out of the water is the same approach as raising the rudder. They are independantly deployable from the sitting position. The inflatable setup is similar. I've attached some pics.

    I think 45% for the maximum system efficiency for the TQ is believable as is being double a typical trolling motor. My system more than doubled the trolling motor performance. The TQ would be even better with a high efficiency prop.

    I agree about the weight! Don't really notice the weight on the water but sure do packing it. I'd like lighter AND more efficient. :)
     

    Attached Files:

  3. ElectricKayak
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    ElectricKayak Junior Member

    Well, after a couple weeks of the most useless correspondence with HobbyKing, I've given up. They don't seem to have any idea about this motor and are not able/willing to get basic info as internal resistance and no load current (even though listing those specs for other similar motors). I suppose one has to buy and test themselves when dealing with these people but I'm not sure it's worth it given the poor Rm values listed for the 90kv motor Alan referenced earlier...this one would have to be more than 10x better to be viable.

    For example, the Kt on these motors is around 0.1Nm/amp. In a direct drive configuration that might be 10-15 amps when pushing a large, slow spinning efficient prop at a decent cruise speed. The I^2R losses dominate unless the Rm values are very low. I'm not sure these motors are designed with that level of efficiency in mind, the main focus seems to be getting the helicopter off the ground! :)

    If belt drive losses are really as low as indicated in the JH thread, then a more common higher spinning motor with good specs might be better for efficiency. Something like the SK3-6374-192kv...although that is missing key specs also (has Rm, missing noload current draw).
     
  4. ElectricKayak
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    ElectricKayak Junior Member

    As a followup to the Torqeedo discussion on efficiency and after looking at the Hobbyking brushless motors I decided to put the MK motor on the test bench and measure its parameters. Some may be interested in the results so I post them here.

    I measured a Kv of 170. A Kt of 0.56Nm/amp. Internal resistance of 0.099 ohms. And no load current of 3.3 amps at 5 volts.

    Crunching the numbers, the motor's peak efficiency 70%. This occurs at 12.5 volts and 20 amp draw. Motor efficiency increases with higher voltage. Also for any given voltage efficiency varies between 0% and some maximum depending on current. At my full throttle current draw the MK is 65% efficient. Efficiency would be better if the prop has less pitch at full throttle but it is within 6% so not too bad given full throttle isn't the point of design optimization.

    At my cruise speed the motor is working around 5 volts and drawing around 13 amps. This results in an efficiency of 55%. 12.85 amps would be the point of peak efficiency at this voltage so the prop is well matched to the motor at cruise speeds. Not surprising since I tried several and this one was the best.

    This helps explain why the "bigger" props all produced lesser results. Even though they were gaining efficiency (turning slower) the motor was losing efficiency at a faster rate. The lower voltage and increased torque were moving away from the point of peak efficiency for this motor/boat/speed combination.

    All these numbers suggest the system efficiency at cruise is around 38% (0.95x0.55x0.73). The peak system efficiency is around 45% (0.95x0.65x0.73).

    Finally, if I currently need 67 watts input to the system then increasing the motor/drive efficiency from 55% to 80% would reduce input power requirements to only 46 watts. A worthwhile gain and, with the potential of weight savings, reason enough to keep plugging away on Version 2 using an efficient outrunner and driveshaft solution. :)

    PS. I noticed on the bench the MK shaft and seal area quickly becomes quite warm. I didn't dare run it long. The overall casing remained cool however. It's also pretty obvious the seals introduce a lot of drag. I wouldn't be surprised if they were consuming 10 watts alone! If I was serious about pulling it topside I would remove the seals.
     
  5. ElectricKayak
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    ElectricKayak Junior Member

    I'm posting up a storm here...but have one question I'm hoping the more mechanically inclined/engineering types can answer...

    I checked into getting a "double universal joint" as pictured in the JH thread. But am being quoted around $200 for it!

    However, I can buy a single 3/8" joint for a socket set for $3.93. Two would be less than $8 and if I put them together doesn't that make the same "double universal joint"? What am I missing?

    $8 versus $200...:confused:
     
  6. kerosene
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    kerosene Senior Member

    lubrication...
    The socket set joint is not designed to work in continuous use. But $200 is quite high.
     
  7. kerosene
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    kerosene Senior Member

  8. ElectricKayak
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    ElectricKayak Junior Member

    Ok thanks. Yes the link works, but they are all single joints (quite a price variation) ! The $200 price i mentioned is for a double joint.

    If two single joints are the same as the double then I can just get two singles locally and use those (not the $4 hand tool ones). It might cost $50 for two but that is a lot more reasonable than $ 200 if they work the same.
     
  9. kerosene
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    kerosene Senior Member

    You can use two identical singles, just align them like the double joint.
     
  10. Builderjeff
    Joined: Apr 2015
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    Builderjeff Junior Member

    I have been dreaming about a solar powered kayak. A friend of mine runs a motorized kayak company here in south Florida. Powered by battery, and his rig is super fun and reliable. With the technology advances in battery's I think there is a market for solar powered personal water crafts. Thank you for doing what I was to ignorant to try(= i will have fun following this thread. keep up the good work!

    Jeff Warner,
    E-Boat Inc. boatbuildercentral.com
     
  11. portacruise
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    portacruise Senior Member

    Thanks for posting this message, this is interesting. Some comments in the body of your message.

    PC

    PC
     
  12. alan craig
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    alan craig Senior Member

    Electrickayak, here in UK a major supplier of industrial bits is RS Components. They do a steel double UJ with 10mm bores for about £35+tax stock no. 790-6744. Might be of some help.
    Your numbers for power and efficiency are very interesting; I have some numbers for my electric o/b, not quite so comprehensive. I'll put them here when I next have them to hand.
     
  13. ElectricKayak
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    ElectricKayak Junior Member

    Thanks for the input everyone!

    I've been looking into universal joints trying to understand the essential difference between my $3 UJ and a $50 one.

    It seems there are three basic groupings:

    1) $50-$500. Expensive for machine use with roller bearing joints and hardened steel materials suitable for high torque and speeds. These are expensive and perhaps my $200 quotes were for that type (I didn't know enough to ask the right questions at the time).

    2) $20-$50. Inexpensive for machine use with plain bearings and hardened steel materials. Low speeds (1000rpm) and lower torque. Need lubrication for extended use.

    3) $3-$20. Cheap for hand cranking. These are similar to the inexpensive machine joints in that they use plain bearings. However their materials are not as hardened. They would need lubrication for any extended use. Perhaps more interesting and relevant, they all use offset pins in the center block for ease of manufacturer. This has a higher tendency to bind under load/angle than if the center block pins were in the same plane. The increased articulation of the center block tends toward more vibration versus the other types.

    I note actual prices are all over the place within these classes. If one spends any more than the minimum for a given class they should make sure there is a real quality difference (ie better lubrication, higher quality journals, tighter manufacturing tolerances, stronger materials, user replaceable components or whatever).

    Conclusion: The category #2 inexpensive plain bearing type should be fine for an application like mine. It must be permanently lubricated. Two singles are as good as one double. Cheap $3 hand crank joints may be suitable for quick prototyping.
     
  14. ElectricKayak
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    ElectricKayak Junior Member

    By the way, despite the previous post, I still haven't settled on a driveshaft approach. The JH thread had good success with the double UJ. But there are too many bearings, seals and fabrication issues for my taste. I'd prefer something simpler.

    Things like:
    - a 3-6mm spring steel rod in a free flooding bent copper tube. Or similar idea with perhaps different driveshaft materials such as fibreglass rod or plastic PEX tube or flexible coil spring?

    - two straight rods coupled with a flex hose rather than UJ...without joints and bearings this could be free flooding and simple.

    - a 5mm RC boat flex driveshaft? I've never actually seen one...how suitable or strong are they?

    - At this point I'm avoiding right angle gear drives, twisted belt and twisted chain drives as too lossy, complicated or noisy. While a free flooding straight shaft may be the simplest I suspect it is too inefficient given the angle of thrust and the unequal loading on the propeller blades.

    Anything else simple and readily available?

    :idea:practical ideas anyone?:idea:
     

  15. alan craig
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    alan craig Senior Member

    Here are a few numbers for my electric outboard, which is a conversion of an Evinrude 3hp and powered by a Turnigy c80-100:

    No load, without prop fitted 130w absorbed by motor, ESC and bevel gearbox together (23.7v x 5.5A) at 1530 prop rpm.
    Adding the standard prop adds 10w in air.
    Power consumed in use depends on the boat - at nominal 24v:
    600w on a 16ft lightweight skiff
    800w on a 11.5ft heavy grp dinghy
    Very roughly half the losses are in the motor, and half in the gearbox.

    Here is an off-the-cuff suggestion for a drive method: plastic or brass bevel gears flooded by water. No seals needed but bushes must be water lubricated. Easy to make if you don't think too hard about protecting the gears from weed.
     
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