Sometimes a horse is just a horse

Discussion in 'Electric Propulsion' started by jet14, Oct 23, 2017.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    unless we will be discussing electrical torque. I don't have the time anymore, I have a new controller made with a different IC , which gives me hardship. One of the phases is not performing as it should and my torque is zero. Thanks folks,
  2. Angélique
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    Angélique aka Angel (only by name)

    Wish you Godspeed with the new controller Bert !
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Unfortunately, the only way to optimize a propeller is to size to the maximum power available. If you optimize it for less power, then the power supply (regardless of source) should be reduced.
  4. Jedidiah
    Joined: Mar 2018
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    Location: Indiana

    Jedidiah Junior Member

    This one really took a dig through the weeds. When I first read it, I was trying to get clear on what the OP meant by the following ...

    ... & so I kept reading.

    If he were talking about the thermal efficiency of a motor vs an internal combustion engine ... but no, that doesn't seem to be the case, though that is a factor in resource consumption, or whatever you're fueling your engine with be it electric or liquid fueled.

    Then I thought somebody would mention that modern boats are typically overpowered & under propped because a weak internal combustion engine with a lot of prop will stall when engaged at idle - the ICE needs to be turning it's output shaft with vigor & making power before any demand is placed upon it. Assuring enough low RPM power to prevent that has led big engines with silly little trinkets for props.

    My own understanding is, we've evolved to a point where the common setup is a great deal more power & torque than required to achieve & maintain hull speed with a far smaller prop than would be ideal, simply to prevent that stalling when a prop is engaged (or placed into gear). There's also the part where running a boat at speed on the plane involves a very different power delivery than puckling around at displacement speeds - the two different profiles are ... different.

    This isn't a problem with electric motors that are adequate to the task. They make their peak torque with a stalled shaft - assuming that you're not trying to drive your boat with your wife's sewing machine motor, no over-sizing required to prevent stalling in the course of slow-speed maneuvering. This high torque at no or slow RPM also allows the larger props that were very typical in the late 19th & early 20th century when steam engines ruled, electric vehicles of all kinds were more statistically common than now, & internal combustion was in it's infancy.

    Consider early marine gasoline engines that weighed a ton or more, & made gobs of torque but not a lot of horsepower at low shaft speeds. They tended to run far larger props than we're accustomed to seeing today, & they were remarkably efficient vessels given just how crudely constructed their engines were by modern standards. Blow-by? Who cares. Oil is cheap. When the crankcase oil stinks of gasoline & soot, just change it.

    I'm not looking to throw kerosene back into an argument, as it were, just adding another perspective - one that's supported by the historical development of marine propulsion.

    The assertion that the horsepower not required but typically seen driving any given hull can be divided by 3 has merit, in a displacement mode, but my math is saying that achieving & exceeding hull speed is when a horse is a horse is a horse.
  5. Joakim
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    Joakim Senior Member

    This is not the case. A propeller needs very little power&torque at idle speed. The torque needed to rotate a propeller increases at rpm^2 and power rpm^3 (assuming high slip, e.g. bollard pull). Both torque and power increase at D^5 (again high slip). A typical IC engine has about 1:5 - 1:6 rpm ratio between idle and full power. Thus torque required for full power is about 30 times more than at idle. Not all installations can reach close to full power at high slip conditions, but they can do at least 50% of max rpm (some high speed planing boats might not), which is already 6 times the torque on idle. An IC engine has much more than 1/6 of maximum torque at idle.

    Choosing a propeller is all about maximum and cruising speed. You don't need to worry about idle except for having too high idle boat speed. The same is equally true for IC and electric. However electric engines do not have the same amount of energy to waste, thus they are usually optimized to much lower speed. The optimum propeller diameter comes from the shaft rpm, power and speed. E.g. all outboard and stern drive propellers are very close to that optimum when they are used what they are designed for. They are not designed for displacement speeds for which they have too fast shaft rpm with too small propeller diameter. That includes high thrust models, which are more marketed towards displacement boats.

    Also almost all sailboat propellers are at that optimum diameter, but only for their shaft rpm. Say a sailboat with 20-30 hp engine, 15-16" propeller and 1200-1400 maximum shaft rpm. It has about 60% propeller efficiency at both cruising speed (higher efficiency) and maximum speed (lower). You could have maybe 80% efficiency, but then you would need to have very low shaft rpm (less than 300 rpm probably) and high diameter (30" or even more). That would give you "only" 30% more propulsive power at the same shaft power, but you would loose part of the shaft power due to much higher gear ration needed (also for the electric motor). Also the drag and weight of the propeller installation would be much higher.

    Yes electric engines have high torque at small rpm, but as I explained earlier it is not needed when used to rotate a propeller. Power is still torque * rpm, thus in order to output high power an electric motor needs high rpm. Both IC and electric motors can be geared to output the needed shaft rpm.
    kerosene likes this.

  6. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    Joakim said it well and it has been said numerous times in this and similar threads.
    E-horses being worth more is a myth.
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