4 hp outboard torque

Discussion in 'Boat Design' started by jakeeeef, Jun 30, 2024.

  1. jakeeeef
    Joined: Sep 2009
    Posts: 198
    Likes: 36, Points: 28, Legacy Rep: 16
    Location: Hamble

    jakeeeef Senior Member

    I'm converting an old Suzuki DT4 2 stroke outboard to electric. I bought the engine(s) (2 of them non runners for less than I later sold the power heads, carbs, flywheels etc for. So my engine legs were less than free (although they will both need a good grit blast and repaint).
    I've got a long shaft and a short shaft leg with gearboxes and props, so the electric go-cart/ pit bike 72V 3000w motor and controller once suitably mounted will bolt straight on with a coupling piece to the splined driveshaft. My electric motor Rpm is 4900 constant or 6600 max, so not a million miles off what the gasoline motor would have been. The motor comes with a reverse capable controller, but I'll use the legs mechanical reverse. Might as well- as I've got it.

    Anyway, my motor is rated at 5.44 Nnm. Anybody know roughly what torque the gasoline power head would have been? I wonder how it compares? 3000w is almost exactly 4hp so that aspect compares well.

    I think this is a cheap way of getting a decent electric outboard. The battery was £400, the motor kit with controller and throttle £200. The engine bits free, as I said. Will have to spend a few quid at the engineers to weld flanges onto my propshafts, will need a battery lead and a few connectors. Bit of paint and prep. But I'll be ending up with a motor with a proper marinized aluminum prop, leg, gearbox, with a choice of leg lengths (will take about 10 mins to swap legs over). Strikes me as a better bet long term than the flimsy plastic Torqeedo and Epropulsion crap. And the 4 HP versions of those are serious money...

    I'm guessing I might need to cool the motor with a big fan under the cowl, and/ or use the legs impellers to circulate seawater round brass tubing spiralled round the motor... I'm hoping to remove the impellers though as they rob a fair bit of power I'd imagine.
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    1 Horsepower = Torque x R.P.M. / 5252
    You need to know at what RPM you had maximum power, and that will give you the torque.
     
  3. jakeeeef
    Joined: Sep 2009
    Posts: 198
    Likes: 36, Points: 28, Legacy Rep: 16
    Location: Hamble

    jakeeeef Senior Member

    Does an electric motor produce its max torque at its rated sustained RPM? This would be 3000w at 4900 rpm.

    The seller quotes two rpm figures 4900rpm and 6600(MAX)rpm.
     
  4. alan craig
    Joined: Jul 2012
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    Location: s.e. england

    alan craig Senior Member

    Well, I just accidently deleted my carefully worded reply, not writing all that again. So; you need a wattmeter for testing; here is my conversion without its stainless steel can "lid".
    DSC03806.JPG
     
  5. gonzo
    Joined: Aug 2002
    Posts: 17,481
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    Location: Milwaukee, WI

    gonzo Senior Member

    Electric motors, like gas motors have different torque curves. You can sometimes get those from the manufacturer. However, your question was for maximum, so if they give you power and you have RPM the formula will give you torque.
     
  6. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    @jakeeeef
    "Does an electric motor produce its max torque at its rated sustained RPM?"

    On the contrary: a (typical) BLDC motor shows the lowest torque at its highest RPM.
    Example diagram:
    Electric motor types
     

  7. alan craig
    Joined: Jul 2012
    Posts: 389
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    Location: s.e. england

    alan craig Senior Member

    Hold one of those little brushed motors for r/c cars in one hand and connect a 12v battery to it. You will feel an instant pulse of maximum torque when you make the connection because at zero rpm there is no back EMF. Within less than a second you will feel no more torque and the motor will be spinning at max. speed because the back EMF now equals the battery voltage minus a bit to overcome bearing friction. If you now grip the shaft with mole grips and hold them against something so that the motor doesn't turn and connect the battery again you will experience maximum torque continuously, and zero efficiency because there is no rotation. Your hand will also get warm very quickly.
     
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