Electric outboard drive for small cruisers

  • 27 May 2020 16:12
    Reply # 8995638 on 8995174
    Anonymous member (Administrator)
    Anonymous wrote:
    Anonymous wrote:

    Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop.

    I had been thinking about this and it doesn't surprise me at all. I was thinking about the scenario where the prop gets out of the water while at full throttle. If the motor were to spin unrestrictedly it would probably go up to 20.000 rpm or something (surely not good for its health). So there has to be an rpm restriction, meaning if the prop is allowed to turn freely it wouldn't use that many amps at all.

    Now I looked up David’s posting (9.3.) about that 5hp Haswing Protruar 24V outboard. I am quite impressed. Since it is said to be brushless, I am pretty sure it is an AC motor, either of a synchronous or asynchronous type. To make it run on 24V DC, there must be  a built-in DC-AC inverter. This both regulates the AC voltage and the frequency of it before going to the motor. Therefore, the motor cannot run away since the rotating field created by the stator coils will lock the rotor’s speed to the same speed (or a little lower if it is an asynchronous motor).

    The great things with these modern switch-mode supplies of later years (AC-DC or DC-AC), is that they have very little energy loss, and they are very compact.

    Arne


  • 27 May 2020 15:16
    Reply # 8995461 on 8990425
    I wrote:


    I didn't actually check how much the motor can safely turn with the rudder centered, I just (wrongly) assumed I'd always keep the outboard centered. When maneuvering in harbours though this would be very welcome as Tua-Tua reverses like a dog. 

    Got the inboard engine lifted out today by Hiab for €50 (I wouldn't even have had to pay that but wanted to pay something) by a colleague of my brother's. While there I checked how much the outboard can move with the rudder centered and it goes clear 360 degrees by a very comfortable margin, so that's going to improve tight space maneuvering by a lot! A "stern thruster". :)

  • 27 May 2020 12:58
    Reply # 8995174 on 8995144
    Anonymous wrote:

    Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop.

    I had been thinking about this and it doesn't surprise me at all. I was thinking about the scenario where the prop gets out of the water while at full throttle. If the motor were to spin unrestrictedly it would probably go up to 20.000 rpm or something (surely not good for its health). So there has to be an rpm restriction, meaning if the prop is allowed to turn freely it wouldn't use that many amps at all.

  • 27 May 2020 12:41
    Reply # 8995144 on 8809939

    The shunt has two little screws for the meter cables, as well as M6 studs for the current carrying cables, so that's OK.

    It was LW this morning, so Weaverbird was dried out. Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop. The voltage across the shunt corresponded fairly closely with this - 0.00022V against the theoretical 0.00024V - so I'm gaining confidence in the ammeter.

    So this afternoon, I'll go back aboard at HW and move the mooring strop to the stern so that I can carry out a static pull against it. That should result in maximum prop load and maximum current draw.

  • 27 May 2020 09:24
    Reply # 8994883 on 8809939
    Anonymous member (Administrator)

    Yes, of course, I asked because there were so many numbers flying around. With a shunt resistance of only 750MicroOhm (the lowest I have seen), any small voltage drops in the connectors could give errors. Therefore, make sure that the current meter is connected directly over the shunt.
    Personally, I would have liked to have a big bypass switch over the shunt. One does not have to monitor the current constantly.

    Arne

     


  • 27 May 2020 08:26
    Reply # 8994831 on 8809939

    Surely if at 100A the voltage drop across the shunt is 75mV, the resistance has to be 0.00075 ?

  • 27 May 2020 08:17
    Reply # 8994829 on 8809939
    Anonymous member (Administrator)

    David,

    I wonder what the specs of the shunt resistance says, 0.001, 0.005 or 0.01 Ω ?  My guess is 0.001Ω....

    Arne

     


  • 27 May 2020 07:17
    Reply # 8994780 on 8809939

    Scott,

    The current, the markings on the throttle and the RPM all seemed to be progressing in linear fashion, so I thought it reasonable to extrapolate from the situation at 90A.

    The suggestion of putting a multimeter across the shunt is a good one. I'll try it today.

  • 26 May 2020 21:37
    Reply # 8993954 on 8991391
    David wrote:

    Now I'm confused.

    [...] The readout goes up to 99.9A and then a meaningless reading, but I'm convinced that the full throttle current is near enough to 105A. [...]

    David,

    If you can't measure the difference between 99.9A and 160A then how did you convince yourself that the full throttle current is near 105A?

    I looked at the ammeter spec briefly. If you have a DMM then you might want to try putting that on the shunt. If it is really 100A then you should see 75mV. At 150A it should be more like 112mV. I think most DMMs will have enough resolution to see the difference between 0.075V and 0.112V.

    Last modified: 26 May 2020 21:52 | Anonymous member
  • 26 May 2020 16:10
    Reply # 8993246 on 8935375
    I wrote:

    4x Flexible ETFE 100W solar panels €345.11

    Somewhat unsurprisingly it turns out one of the panels doesn't provide any amps. It does however provide the correct voltage so not sure what's wrong with it. Have messaged the supplier, suspect they will just send a new one. Last batch came in three weeks so no reason to panic just yet.

    David wrote:

    Now I'm confused.

    I got another breaker, rated at 150A and took it aboard - only to find the one that I'd already installed was rated at 150A, when I could get a good look at it. So why did it trip?

    Color me confused as well. Since it's (presumably) tripped by amperage (and not heat) that rules out some initial thoughts. Weird that it'd happen after 10 minutes, a breaker is slower than a fuse but not that slow. Maybe a spike (but why)? I don't speak fluent electricity so can only guess here. Perhaps the breaker is faulty? Are you able to reproduce the issue? If so then replace the breaker with the other one and try again? A bit annoying to only be able to measure up to 99.9A draw though, otherwise you could get a better picture of what's going on...

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software