Are electric horses really bigger?

Discussion in 'Electric Propulsion' started by DennisRB, Apr 9, 2016.

  1. Stumble
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    Stumble Senior Member

    The only advantage that the torque of an electric motor may buy is that you can turn a bigger prop. Since most vessels are turning the largest prop that will fit in reality there is almost no gain to be had from lots and lots of extra torque. Fundamentally props can only turn so much torque into motive power for a given size, and Diesel engines are pressed right to that threshold. Absent designing a 30' sailboat that can swing a 30" prop it just doesn't matter.

    Electric motors do have some advantages, but frankly Hp=Hp and the prop doesn't care if it comes from a diesel or electric, or steam motor. Once it starts turning the prop shaft it's all the same.
     
  2. gonzo
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    gonzo Senior Member

    Not really. An electric motor producing 1HP at 1RPM has the same torque as a gas or diesel engine producing 1HP at 1RPM. There are plenty of low torque, high RPM electric motors. For example, the one in your computer's fan.
     
  3. powerabout
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    powerabout Senior Member

    sure but its still down to what standard have you used, plenty for gas and diesel engines
    When do you see a hp/torque curve for electric engines?
     
  4. gonzo
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    gonzo Senior Member

    The hp/torque curves for electric engines are usually grouped by winding type. Each has a different characteristic which is typical for each.
     
  5. Chuck Losness
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    Chuck Losness Senior Member

    Mainecat (sp??) did actual testing of electric motors in the prototype of their power catamaran. I couldn't find their test results on their website so I don't know if the test results are still available. What Mainecat found was that horsepower is horsepower whether it be from electric motors or diesel engines. I forget which electric motor they used but the supplier had claimed that their electric motors were the equivalent of much larger diesel engines and would push the prototype to speeds over 20 knots. Actually testing did not bear this out. The electric motors pushed the prototype at around 10 knots which was the same speed that Mainecat sailing cats achieved with diesel engines of the same horsepower as the electric motors. Mainecat then installed 160 hp diesels and the prototype easily ran at 20+ knots. I would suggest that you contact Mainecat and ask for a copy of their test results. No need to reinvent the wheel here.
     
  6. DennisRB
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    DennisRB Senior Member

    I cant believe MC needed to spend so much money on testing to show them that a ton of feathers is no lighter than a ton of bricks. Nonetheless, I would love to see those results.
     
  7. WestVanHan
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    WestVanHan Not a Senior Member

    There was a company about ten years ago,making claims such as "our 6hp electric motor can replace a 20 hp diesel" etc etc but like mentioned by others you can get torque from a diesel with a transmission,and the electric gives no reserve hp.

    The main idea was that if you have a cat and don't mind dragging around an extra couple tons of batteries,you could recharge while sailing.

    The company went public,and IIRC they got into some sort of trouble about their claims, changed names a couple times and then delisted.
     
  8. gonzo
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    gonzo Senior Member

    There are no batteries that have the energy density of fossil fuels.
     
  9. Chuck Losness
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    Chuck Losness Senior Member

    I am going from memory here because I never saved a copy of Mainecat's test results but as I recall there was a diesel generator that charged a 144 volt battery bank which provided the power to the 144 volt electric motors. Don't remember the horsepower of the electric motors. Somewhere in the 30 hp range I think.
     
  10. daiquiri
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    daiquiri Engineering and Design

    This thing about diesel vs. electric HP has been beaten to death so many times here. Yet, it doesn't want to die. :)
    See here, for example: http://www.boatdesign.net/forums/boat-design/tko-electric-solar-concept-43940-4.html#post568467
    I am copying that post here for a future reference:
    -------------------------------------------------------------------------

    So where does that claim (about 1 electric HP equaling 4 diesel HPs) comes from? It comes from considerations on torque only. Yes, the 12 HP ST74 electric motor will indeed give a torque equal to a 48 HP diesel (like a BUKH model DV48 ME: http://www.bukhdiesel.com.au/spec_docs/BUKH DV 48 ME.pdf).
    The ST74 gives a constant torque of 74 lb ft (around 10 kgm) over the whole RPM range, whilst a BUKH DV48 gives a nearly-constant torque between 8 and 10 kgm over its operating range.

    So... where is the catch? It is in the RPM range. The ST74 works between 0 and 900 RPMs, while a DV48 works between 1200 and 3600 RPMs - 4 times higher revs. Hence, a same prop connected to the output shaft of, respectively, an electric and a diesel engine will work in extremely different conditions in two cases.

    Now, we know that:
    1) Power is directly proportional to the Torque times RPMs
    2) Torque is directly proportional to Power divided by RPMs.​

    So if you attach a, say, 4:1 reduction gear to the output shaft of a diesel engine, then both engines will deliver their power to the prop shaft at the same RPMs. However, by reducing the diesels RPMs by a factor of 4, the torque will be multiplied by the same factor, because the output power remains constant 48 HP and the mathematical relationship n.2 is valid.

    The above situations are well visible in these comparative graphs: View attachment 12 HP Electric vs. 48 HP Diesel.pdf
    The first one shows the RPM-power and RPM-torque curves for the ST74 and the DV48 engines. Essentially, the diesel carries on where the operational range of the electric motor stops.
    The second graph shows what happens when a 4:1 reductor is attached to the diesels output shaft. Here the 48 HP diesel clearly shows his muscles compared to the 12 HP electric motor, and is superior over the electric both in power and in torque.

    So the claim "12 HP electric motor is equal to a 48 HP diesel" is true if only torque is considered and there is no reduction gear at the diesel's output shaft.

    For curiosity, let's see it from another point of view. What happens if we compare a 12 HP electric motor (ST74) to a 12 HP diesel engine, like this one: http://www.dek.co.nz/diesel-engines?
    These two graphs show that situation: View attachment 12 HP Electric vs. 12 HP Diesel.pdf
    The first one shows again the different RPM range for the motors, and a clearly inferior torque value of a diesel (at high RPMs).
    The second one shows the same diesel with a 4:1 reductor. Hey, is that a kind of a miracle, or what? The two curves now look almost identical, except for the low-RPM (below 400) zone, where diesel cannot work.

    So, clearly, a 12 HP electric gives the same power and torque as a 12 HP diesel, when both are reduced to same RPMs.

    It is a physical fact which can be deduced directly from the above mathematical relationships between Power, Torque and RPM. However, I thought that it might be more comprehensible if expressed via nice colored graphs like the ones we just saw.
     
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  11. DCockey
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    DCockey Senior Member

    Daiquiri, Excellent post!
     
  12. Caroute Motor
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    Caroute Motor Junior Member

    electric motor‘s efficiency is higher than gas motor. The elctric motor's power is out directly, but gas motor need the gear box. But for bigger boat, it's better to choose the gas motor.
     
  13. gonzo
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    gonzo Senior Member

    Gas motors don't necessarily need a gear box. High performance applications have a direct shaft. Can you explain what you consider when you claim efficiency differences? You need to take into account the losses in the production of electricity too. Even larger losses if you use energy storage. Further, only low RPM electric motors would have a direct drive. High RPM motors would need a reduction box too.
     
  14. serow
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    serow Junior Member

    The DC electric motor is about as good a self regulating torque converter as you can get but the real advantages are the regulation aspect. No sensible person chooses an ill matched drive and motor and relies on the wonders of the motor to get them out of a hole. This is akin to putting a large reduced shank bit into a home Black and Decker instead of a workshop drill and wondering why there is this funny smell.
    The idea of using a diesel motor/generator to charge batteries to power an electric drive only makes sense under specific circumstances. One is if average use allows a smaller power generating capacity than peak use requirement, as in an electric car or similar vehicle. I suppose a short range ferry could fit into this category; high power needed to start and stop, less power when under way, no power while loading etc.
    I suppose a subset of this is where battery capacity is about right, but you cannot be too sure, so a small genny helps out. Another is that it allows the generation plant to be remote from the drive, although you do not need a battery bank for that.
    Green issues are not a valid reason of themselves. They rarely reduce emissions so much as displace them; the UK for example has reduced emissions by manufacturing zilch and getting stuff from China, the biggest belcher forth of pollution going.
     
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  15. DennisRB
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    DennisRB Senior Member

    Do you know what else works as well as a torque converter? Well imagine a set of correctly matched vanes operating in a fluid medium being powered at the correct speed and torque. This fantastic device would allow the engine to reach substantial rpm and power with zero boat speed. Much like a hi stall torque converter in a drag car!

    This amazing device already exists. Its called boat propeller ffs and renders all this "electric torque" totally redundant for powering a boat prop.
     
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