Electric outboard drive for small cruisers

  • 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.



  • 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)


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



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


    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. [...]


    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...

  • 25 May 2020 15:01
    Reply # 8991391 on 8809939

    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?

    I also took aboard a 100A ammeter. I chose this one because the readout is waterproof, and just needs a 29mm holesaw through the cockpit side to install it. That part was easy. The shunt took a little bit of wrestling with the heavy cables, but I got it done. However, it's supposed to have a "bright, waterproof, daylight readable OLED screen" but I can barely see it on a sunny day, even with some shade over it.

    Anyway, I was able to get a reading on the mooring, not going right up to full throttle for more than a few seconds. It would seem that 50A - 60A is enough for all normal purposes of moving in and out of harbour. That's 1250 - 1500W, or up to about 2HP. Quick bursts of full throttle for manoeuvres should be safe enough, as the ammeter is rated at 110A intermittent and 175A for 30 seconds. 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. So why did a 150A breaker trip??

  • 24 May 2020 20:50
    Reply # 8990425 on 8809939

    While in the process of removing the backstay mounts it dawned on me that they should definitely be converted to outboard brackets!

    They should be strong enough (even though they're designed for upward pull) and are quite optimally placed for an outboard mount. With the motor completely lowered the propeller slightly touches the rudder when the tiller is all the way to port, so will add some sort of stopper to prevent accidents. I can comfortably reach the throttle (in its extended position) when sitting at the tiller. With my Fingal I had to hang off the transom to reach the controls so this is kind of a luxury. 

    I just need to figure out how to attach a vertical plank down from the aftmost support. If it's easy enough I'll make a mirror of it for the other side. Maybe a (double) wooden plank attached to (each side of) the aftmost bar with a stainless rod/tube attached from the bottom of the plank to the bottom of the middle bar. It would also be nice if the plank could be easily removed as it's not particularly efficient hydrodynamically speaking, when sailing on her ear like these boats like doing.

    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. Will have a look at this tomorrow.

    Once again David I'm glad you're out there making mistakes so I don't have to - I will get a 150A breaker as well. 

  • 20 May 2020 12:58
    Reply # 8981400 on 8809939

    Motor trials, on a nice sunny day, almost calm, with almost flat water in Ravenglass harbour. Aboard at HW. I lowered the motor to its fullest extent, as if for a heeled or pitching situation. Then I motored up and down the harbour at full throttle. That gave me 4.2 knots in a straight line, decreasing to 3 knots in a tight turn. The motor hum was quite loud, but not unpleasantly so.

    But 10 minutes of full throttle caused the 100A circuit breaker to trip. Clearly, the motor must have been drawing the full 105A, as advertised. So I had to drift for a few minutes before the breaker cooled enough to re-engage. Not good. I will have to change it for a 150A breaker.

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

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