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Thread: Electric dinghy

  1. #1
    Join Date
    May 2015
    Location
    Port Elizabeth, South Africa
    Posts
    337

    Default Electric dinghy

    I got myself a used Minn Kota 40. It is a hefty beast, almost as heavy as my 2hp Yamaha. I might yet strap it to my kayak but in the meantime I did a few test runs on a Miracle dinghy.

    First outing was in a 15kt wind. With my weight in the stern and no tiller-extension the boat trimmed bow completely out of the water. The dink was impossible to steer at anything less than full power.

    Second outing was in ideal conditions and I managed to get some good speed and power measurements. A pvc pipe sufficed as throttle/tiller extension.

    Power was from two 22Ah LiFePo4 batteries, hooked up in parallel to three 50W solar panels. With the sun not very high and no mppt or buck converter, the panels only provided 70W.

    Speed and power results:
    2.2kts at 125W (speed setting 2)
    2.6kts at 170W
    3.1kts at 200W
    4.2kts at 360W (speed setting 5)

    The top speed is already on par with my Yammie's performance at fast cruise throttle, i.e. noise just above being tolerable!

    Next up will be a series of modifications to improve the motor's efficiency. Shaft fairing, prop spinner, new prop etc. I have not yet looked at the throttle electronics, perhaps a pwm speed control might be needed.



  2. #2
    Join Date
    Dec 2003
    Location
    Sound Beach, NY
    Posts
    4,549

    Default Re: Electric dinghy

    Cool. Years ago I bought an electric trolling motor to power my 20' sloop in calms. Since then I have used it in a 24' plywood ketch and on two dinghies.

  3. #3
    Join Date
    Oct 2009
    Location
    South Puget Sound/summer Eastern carib./winter
    Posts
    19,567

    Default Re: Electric dinghy

    aw bulwinkle, that trick never works

  4. #4
    Join Date
    Nov 2010
    Location
    Chesapeake Bay
    Posts
    230

    Default Re: Electric dinghy

    I'm looking forward to seeing what you come up with... your experiments have been really interesting. You've got me looking at trolling motors and batteries now...

  5. #5
    Join Date
    May 2015
    Location
    Port Elizabeth, South Africa
    Posts
    337

    Default Re: Electric dinghy

    I turned a prop spinner from a block of meranti and made a shaft fairing by simply bending a sheet of thin plastic around the shaft.

    Wind on the day of the test made speed measurements difficult. For what it is worth, the spinner on its own made no noticeable difference in speed or power.
    The shaft fairing could show a 0.1kt gain in speed and some 5W reduction in power draw.
    The reduction in power could simply be because the battery was at a slightly lower voltage.

    The fairing certainly helps in steering, especially straightening out after a sharp turn. It also allowed me to raise the motor by 30-40mm without the prop slurping down air.

    The shiny puppy in the foreground is my new 24V brushless beastie. A few more tests with the Minn Kota and model plane props then I will take it out to play.

    As a side note, I also tested the range on the LiFePo battery (13V, 44Ah) to a 90% discharge. Range was 7.1nm at an average of 2.4kts. No solar was used for this test.

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    Last edited by whiskeyfox; 04-15-2021 at 03:31 PM.

  6. #6
    Join Date
    May 2015
    Location
    Port Elizabeth, South Africa
    Posts
    337

    Default Re: Electric dinghy

    To fit the model aircraft props, I turned an adaptor from a stump of Pohutukawa. As far as turning goes, it is really just a lidded box with a hole in the bottom and slots in the sides!

    The shaft diameter means that the model props need to be drilled out to a much larger diameter than what it was designed for, leaving the hub vulnerable to possible failure.
    The shear-pin fits in a slot in the wood and the torque is transmitted to the prop only at the outer rim of the spinner.

    The only pusher prop I had on hand was an 11x5.5 knockoff of an APC electric. Even at the first speed setting the blades folded back alarmingly. I trimmed it down to 9", then 8", 7.5" and finally 6.8".

    The 7.5" size performed the best at speed settings 1 - 4.
    At each setting the speed was up roughly 7% and power draw down by 5%.

    It flexed too much at full power for further gains.

    The stubby 6.8" prop matched the stock Minn kota prop for speed up to the 4th setting but required much less power.

    Only afterwards did I measure the blade thickness near the hub, 2.5mm. An APC branded 10x5 electric prop measured 4mm.
    More tests to follow.

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    20210414_180415-1.jpg
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  7. #7
    Join Date
    May 2015
    Location
    Port Elizabeth, South Africa
    Posts
    337

    Default Re: Electric dinghy

    Some footage of the shaft fairing with the stock propeller.


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