Can a too large prop really stall a gas motor?

Discussion in 'Electric Propulsion' started by canton33, May 3, 2018.

  1. canton33
    Joined: May 2018
    Posts: 1
    Likes: 0, Points: 1
    Location: Easton, MD

    canton33 New Member

    So all these electric motor companies seem to claim that their motors can swing more efficient props than a gas or diesel motor. I know it has been beaten to death here that this is false. But one of the central claims that I keep seeing and don’t full understand is that an electric motor can move a larger propeller than a gas engine, even with the same shaft rpm, because the gas engine would stall if you tried to swing it. Since electric motors produce 100% torque immediately, it theoretically makes sense, but my intuition is that it wouldn’t be a practical difference until you reached a ridiculous prop diameter. So what’s the truth here? If the claim of these electric motor companies is false, how do you mathematically show it to be false?
     
  2. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 48, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    There probably is a mathematical formula for failure but it's also easy to conceptualize.

    Run different pitch props and see how much it affects the topout rpm. Every inch of pitch can have 200-300 rpm effect. Diameter also affects the rpm obviously. The guys who run flats boats are constantly playing with the balance of pitch, diameter and cupping.

    So imagine at low rpms where the gas motor develops the least horsepower the effect is going to be that of lugging the motor and shortening its life.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 14,735
    Likes: 755, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    First, there are many types of electric motors, with different torque curves. Also, even though the stall torque is high, it lowers very rapidly as the RPMs increase. However, propellers need more or equal torque as the RPMs increase. Check the following torque/RPM chart
     

    Attached Files:

  4. alan craig
    Joined: Jul 2012
    Posts: 271
    Likes: 50, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    Any engine will swing any size propeller provided that the overall gearing matches the engine output power, at whatever rpm to whatever rpm the propeller absorbs the same amount of power. But a I.C. engine would stall if fitted with a propeller which absorbs more power than the tiny amount available at tickover if that is what you are asking. An electric motor with a suitable controller can turn any size propeller, at fractions of a revolution per minute if necessary, provided the power absorbed isn't greater than the power supplied from the motor.
    Forget all that nonsense about max. torque at zero rpm; it's true but of no use - max. torque x zero rpm =zero power and zero efficiency. Also, torque is proportional to current so max. torque = max. current which will burn out a motor which is not turning.
     

  5. Lepke
    Joined: Sep 2015
    Posts: 85
    Likes: 6, Points: 8, Legacy Rep: 10
    Location: Oregon to Alaska

    Lepke Junior Member

    I knew a boat on a western river that was a sailing vessel converted from a 245 ton WWII mine sweeper hull. Same hull as Calypso. The boat had no engines so the owner got 2 surplus 200 hp Detroit Diesels to run the twin shafts and props that had been run by 800 hp diesels. When the big day came to move the boat, he put the DDs in gear and they promptly stalled. The current pushed the hull into the marina docks, destroyed them, turned many boats loose and eventually a commercial tug had to retrieve the vessel and dock parts. Loose boats were spread out for 20 miles. He also had no insurance.
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.