Are electric horses really bigger?

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

  1. DennisRB
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    DennisRB Senior Member

    Can someone please explain to me why I am constantly seeing people trying to power their boats with electric motors that have much less power compared to the lowest power fossil fuel powered engines recommended?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Probably because the battery bank needed to feed them would be too big.
     
  3. DennisRB
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    DennisRB Senior Member

    Would there be much of a downside to running a bigger electric motor, then just only using it at lower power though? Seems the inability to motor into a strong wind even for a short time may be dangerous? Talking sailing AUX here.

    Most times I see the justification that the smaller power electric motor will be just as good as the fossil fuel version due to "torque" or "blade area" or something. That's what I have the main issue with. I cant see how this could be the case.
     
  4. DennisRB
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    DennisRB Senior Member

    Here is what a vendor has to say.

    http://www.electricyacht.com/motor-sizing/

    "Some suppliers of electric propulsion have claimed an electric propulsion motor is “equivalent” to a diesel engine of 2-3 times higher horsepower rating. On the surface, these claims seem confusing and untrue. In fact, it takes a certain amount of horsepower applied to the propeller shaft to propel a boat at a given speed, regardless of the source of the power. However, there are several factors that lend legitimacy to these claims"
     
  5. DennisRB
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    DennisRB Senior Member

    This sounds sus to me.

    "Oceanvolt electric motors have tremendously more torque than any diesel. A 10 kW Oceanvolt motor easily outperforms and is more power ful than a 30 hp diesel"

    Is powerful spelled wrong by mistake?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I notice you're from Brisbane, Dennis, you probably aren't old enough to remember those old BCC electric trolley buses back in the day, they sailed up hills like they were not there, while the diesel vehicles struggled. Presumably greater torque at low revs, maybe a boat making little forward progress gets the benefit of having more torque.
     
  7. DennisRB
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    DennisRB Senior Member

    Yeah I agree that could be the case. But does that mean a 10KW motor is more power ful than a 22.2KW motor? It may well be more power ful, but it certainly isn't more powerful.

    Does it really easily outperform and is more powerful than a 30 hp diesel? While at the same time "Typically the boat max speed is somewhat lower (0,5 –1.0 knot) than with comparable diesel,"?

    I think there are some great uses for these electric systems. The product is good. They have some good advantages, but please, why such a BS sales pitch?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd regard it with suspicion at least, and you have too much in batteries weighing a cat down, to feed 10KW for any length of time
     
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  9. DennisRB
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    DennisRB Senior Member

    I would actually love a system like this on one hull of a cat, with a regular diesel on the other side with enough reserve power to diver a constant small but usable amount power to the electric on the other hull via a pancake generator. The battery bank would be about 10kwh or so and also be the house bank. Over all this would be lighter and cheaper than 2 diesels. And far lighter and cheaper than 2 diesels and a diesel gen set.

    The electric motor would be used mainly for maneuverability, or motoring short distances on one motor. It would have as much power as the smallest diesel would have. Long distances would be motored with the oversize diesel on the other side, and possibly with the electric motor going in the other hull fed by the pancake motor if it proves more efficient. Which I expect it might when prop blade area is important like into strong headwinds.

    One day. :)
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think there are at least two or three things at play here.
    1. A diesel engine is most efficient running at 50% rated power.
    2. People that go electric are more likely reduced top speed for economy.
    3. Bucking a head wind is more a matter of torque, not power.
     
  11. DennisRB
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    DennisRB Senior Member

    Thanks for the reply.

    1: Do you have a source for this? I thought it was higher, but nonetheless. This is around where most people may "cruise", however the full power is there when required.

    2: That may well be true, but its not a choice. Unless they reduce speed they will have flat batteries quickly or a heavy and expensive setup. When operating in a strong headwind there is no option to reduce power since the electric motor is under powered to begin with. Also reducing top speed for economy works well on any engine.

    3: As long as there is sufficient engine power/torque available above the prop power curve, the diesel should work well. Now if the electric had the same power as the diesel, and was geared to reach the same top speed, the electric would have more power available above the prop power curve and so be the winner. But if the electric motor has half the power?
     
  12. Stumble
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    Stumble Senior Member

    The fact that electric motors have more torque is only important if for some reason you refuse to install a transmission on the diesel motor. Talking about one HP versus torque is definition ally stupid.

    The relavent formula is
    HP = Torque * RPM / 5252

    the fact that a diesel turns faster than the electric just means you install a transmission to step down the rpm from the engine shaft, to a reasonable prop shaft rpm. But a 20hp output motor will always have more power than a 10hp output motor.


    The only truth to this nonsense is that small diesels have a lot of parasitic systems attached, as a percentage of rated hp. The alternator, water pumps, fuel pumps, and the transmission efficiency all rob the diesel of some amount of power. How much really depends on the installation but on a 20hp diesel it wouldn't suprize me in 5hp is siphoned off to ancillary systems.

    The electric drive can get away without all this, and a larger percentage of the power it generates goes directly to the prop.


    But be careful with electric propulsion motors, many of them are not rated at their output, but the input. Basically they 'forgot' the engine isn't 100% efficiency in converting electricity to propulsion.
     
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  13. DennisRB
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    DennisRB Senior Member

    Exactly. I would also refute some of the losses of a diesel such as an alternator. A diesel engine works without one so its not required, and it can be turned off anyway. Diesel engines can also be fitted with a dive compressor, watermaker etc. Should we take that off too?

    I cant even see how steady state lower RPM high load performance would be that much better. If the diesel has sufficient power above the prop power curve it will be able to reach its rated RPM and therefore max power at any speed. The extra power above the curve will certainly spin it up faster though (with the same total power that is)

    In reality with prop sizing to suit an intended speed through the water, this may mean max HP can not be reached on a bollard pull test with zero speed. How would the electric fare here? One with equal power may win in this test, but one with half the power? I seriously doubt it. Seems a 30hp diesel will still reach about 25hp on a bollard pull test depending on the prop? What might an electric 30hp reach, 26hp if it has some extra HP above the curve? What might a 15 hp electric reach? Over 25hp like the diesel did? Impossible.

    BTW the vendor with the suspicious claims uses a sail drive. It must have a crown wheel and pinion, well one less than a diesel but its still there.
     
  14. gonzo
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    gonzo Senior Member

    The power curves are different between engines. Electric engines have also a variety of winding systems which can be optimized for a power/torque curve. However, power is the same regardless of source. Bollard pull is a measurement used for tugs where they have a slow turning propeller and large blade area. The same test applied to a small, fast turning propeller, will give different results. The reason is the the small propeller will start cavitating sooner.
     

  15. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I don't think the full advantage of electric motors is realized without controllable pitch propellors. This is because electric motors can operate over a larger range of output, from essentially 0% to 100% full power. This has useful applications such as berthing, coming to an anchor or mooring in various wind and wave conditions, and when combined with sail. Electric motors are also more easily adapted to smaller motors, less than 2 horsepower, whereas diesel doesn't really show it's strength until above say 30 hp. But certainly the case for electric motors gets weaker above say 15 horsepower, unless combined with controllable pitch propellors, and a desirability to do so. The other application for electric drives is when the electric power needs of a boat are comparable to the motive power needs, whereby you may already have a diesel generator in place, and so add an electric drive motor rather than an additional diesel engine.
     
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