Propellor selection electric drive

Discussion in 'Hybrid' started by Marc78, May 14, 2014.

  1. Marc78
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    Marc78 Junior Member

    I have read quite a few posts on power differences on electric driven versus diesel driven. I conclude that you can go for a lower shaft power on electric compared to diesel. Most likely because there are no auxiliary components to drive like pumps and alternators etc. at least, I like to think so. Please correct me if I'm wrong!

    The biggest win I think in using electric drive is to take advantage of the RPM range combined with torque, you can make it go at 1 rpm and deliver power to the prop.

    Now my big question is, what propellor to select? What gives the best efficiency? Low rpm, big diameter, high pitch (low slip)? If needed combined with reduction gear, to facilitate even larger diameter/pitch?

    Or would the best way to go be high rpm?

    For clarity, I am talking about a displacement boat.
     
  2. jehardiman
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    jehardiman Senior Member

    As far as the prop is concerned, SHP makes no difference; i.e. a specific prop turning at a given RPM with a given speed of advance will absorb a specific amount of torque. It is up to the prime mover to supply this torque, and this torque increases with increasing RPM. This is where ICEs excell, as they generally put out more torque as you increase their RPM and follow the prop curve. Conversely, electric motors, unless you get real creative with multi-pole frequency chopping or M-G sets, generally lose torque as you increase RPM giving only a single operating point.

    As far as efficiency goes, always swing the biggest, lowest RPM prop you can.
     
  3. daiquiri
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    daiquiri Engineering and Design

    I am not sure what posts have made you arrive to that conclusion. Perhaps on some other forums? On this one, it has been stressed out so many times that 1 HP is 1 HP regardless of the type of motor.
     
  4. Yobarnacle
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    Yobarnacle Senior Member

    Yes, it's been stressed again and again, that 1hp is 1hp.
    Most of the engineers/architechs here are diehard fans of internal combustion engines.
    I ain't an engineer. but I have more than 40 years commercial sailing. I've sailed on vessels powered by steam, electric, and diesel, and combinations of those technologies.
    They are comparing apples and oranges.
    It takes SEVERAL thousand HP diesel to drive a vessel the same speed as a 1000 HP steam plant can. Same with electric. I've LIVED it. Seen it. Know it.

    HP doesn't move ships. Thrust does. To get thrust from any wheel, (paddle or screw), needs TORQUE. Both steam and electric can provide STATIC torque. No internal combustion engine can.

    So, experts who are experts in ONE technology, sometimes make erroneous claims regarding other technologies. I'm no expert. But I know what I've experienced first hand.

    To answer your question, a BIG, slow turning prop, driven by steam or electric, will give maximum thrust for the HP.

    The compound steam engine image is animated.

    Gearing isn't the answer. Trucks have lots of gears. Trains and boats don't. For max torque over greatest range of RPM, steam or electric.

    Ask yourself, Why are portable drills electric instead of gas engine powered? Torque!
     

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  5. gonzo
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    gonzo Senior Member

    You are confusing propeller efficiency with applied power
     
  6. Yobarnacle
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    Yobarnacle Senior Member

    I've worked several tugs that were re-engineered while I was captain.
    First arrangement, they had two EMD diesel engine GENERATORS rated at around 2000 HP each, driving two 500 HP electric motors geared in common on the reduction gear, total 1000 Hp, turning a single large propeller.

    The office decided to keep the same diesels, but hook them up to TWO smaller props, one to each diesel, through clutches and reduction gears.

    Results, boat more maneuverable, BUT slower top speed, LESS bollard pull (thrust), and worse fuel efficiency.

    Three different tugs, different companies.

    All were ex-military diesel electric tugs.

    After the first conversion, with the other two tugs, I suggested adding small wing propellers, electric powered, for maneuverability. but keep the original arrangement.

    The engineers at the office laughed.

    Ha,ha, ha. The joke's THEM!

    Been there, done that, seen with my own two eyes.

    I have also served on a steamer converted to diesel. Took 4000 HP diesel to do what the one thousand HP steam plant did, and worse economy of fuel. But gained by smaller engineroom crew and wheelhouse control.
     
  7. daiquiri
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    daiquiri Engineering and Design

    1 HP is always 1 HP, Yobarnacle. Regardless of what you might have experienced. 1000 HP diesel gives the same power as a 1000 HP steam engine - namely, 1000 HP. That is all that the boat knows and all that the prop feels coming from the shaft.
    The matter of the importance of proper gearing in order to obtain the right RPM at the prop has also been discussed and beaten to death, in this as well as in other forums. Of course that a prop optimized to run at XXXX RPMs will not deliver at YYYY RPMs. Hence - gearing.
    These are the facts. Your experience with diesels and steam engines, if correctly narrated, tells only one thing - the substitution of engines was not done in a correct way. That's all. But 1HP remains 1HP, regardless of all.
    Cheers
     
  8. Yobarnacle
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    Yobarnacle Senior Member

    daiquiri.
    I respect you and your opinions, and often find we agree on various topics.

    There are GOOD reasons why train locomotives are diesel electric, why trucks have umpteen gears, and why even modern ULCCs and VLCCs are single screw.
    And why atomic powered submarines are reactor/steam/electric drive and single screw.

    The internal combustion engine ONLY delivers it's rated HP in a very narrow RPM range. IE: Trucks have myriad gears.

    A diesel generator running at a constant optimal RPM is at it's best.

    An electric motor will produce higher torque longer beginning from 0 RPM through 1500 RPM than an ICE.

    That's why locomotives are diesel electric.

    It's been proven over many years as a viable marine propulsion system.

    comparing the torque speed curves of induction motors and ICEs, the ICE does NOTHING below 1000 RPM, it's idle speed.
    The AC induction motor has high torque at zero RPM through about 1500 RPM.
    For lower RPM work, the electric motor corners the market. The ICE isn't working.
    For high speed craft, yes the ICE is appropriate. For displacement vessels, the diesel generator/electric motor hybrid wins the prize.

    Just because the technology has been around awhile doesn't make it obsolete! :)

    I've been around awhile myself, and I'm not obsolete, not YET! :D

    http://www.mpoweruk.com/piston_engines.htm
     

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  9. Marc78
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    Marc78 Junior Member

    1 HP will indeed will always be 1 HP, no magic here or any expected magic. However the engine characteristics are way different, Electric motor @ idle: 0 RPM, Diesel @ idle: 600 - 900 RPM. I'm not trying to reengage any HP threads, I'm trying to figure out if there is any significant to achieve in propellor efficiency which takes advantage of RPM differences.
     
  10. Yobarnacle
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    Yobarnacle Senior Member

    A single large prop is more efficient than multiple props.
    it's important to correctly match prop to selected drive.

    Originally, my boat was single screw and 20 HP diesel and 16 inch 3 blade prop.
    Later production of the same boat increased engine to 36 HP for a bit more speed, and same prop.
    I converted my boat to a single 10 HP diesel generator running two, 220 volt 3 phase electric driven props. Larger 5 bladed props. For slow speed control in maneuvers.
    These props are lifted out of the water when under sail.

    Speed under power isn't important to me. Control and economy are. Speed under sail IS important, but not the MOST important thing. Safety and comfort are more important.

    what are YOUR requirements?
     
  11. Marc78
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    Marc78 Junior Member

    Thanks Yobarnacle,

    For a (potential) customer we are setting up an idea for an electric driven boat, full electric, or hybrid electric. The customer wants to have extra power when needed. Going upriver or running for bad weather. Hybrid electric is our first thought, however it would be a waste to fit a prop designed for a diesel engine and drive it electric, since both have different characteristics. My gut feeling says with electric drive there will be more efficiency with the right prop. But this is based on nothing scientific.

    The boat is +/- 30' and around 3000 lbs
     
  12. Yobarnacle
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    Yobarnacle Senior Member

    My boats are 25 ft LOA. Two Albin 25s. One has the 20 HP diesel and 16 inch 3 blade prop in a shaft log.

    The other has the 10 HP gen set and two 5 bladed electric props and homebuilt outdrives made from seagull parts.
    The props are hydro fans from ancient British seagulls. Originally 11 inch diameter. I've added on to the blades with carbon epoxy so now are 18 inch diameter and enclosed in a carbon/epoxy ring for strength. I am experimenting. Ideal? Not yet.

    Bigger diameter, more blades, and less pitch, give better low speed thrust. Backing power. Stopping power. Jump ahead power.
    Sacrifice is speed.
    How fast is that current?
     
  13. jehardiman
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    jehardiman Senior Member

    Marc, I would suggest you do some research into prime mover selection by not only looking at all the threads here in the boat design forum, but also what manufacturers of motors say. If you do your homework, you will find that electric propulsion is not selected for propulsion efficiency, but for other needs such as arrangements or hotel & service loads. The propeller selection and efficency has nothing to do with the prime mover selection, but the prime mover may be selected by the overall needs of the vessel.

    FWIW, a typical motor efficency curve looks like this...
    [​IMG]

    As you can see torque increases only with slowing down (i.e. loading) the prop (i.e. motor speed), but power out is relatively flat while power in is always increasing (and so is current which leads to other issues...large gage copper cables are not light). Compare this to the diesel curve where power out in increasing with prop speed. Because of this, power in for an electric drive is always greater (~130%) than a similiar performing ICE, steam, or GT prime mover. Not only is power in greater, you also have to pay a double weight penalty...i.e. you have to have the motor and a larger generator/battery set. Where electric drive excells is where propulsion is only a small percentage of total electrical load, such as dredges and cruise ships, or where arrangements prohibit the placing of other types of prime movers, such a SWATHs or small tractor tugs (big tractor tugs can overcome this arrangement issue).

    Notice, I never mentioned the propeller here...that is because it is a totally seperate issue only teniously linked to prime mover selection. If you are going real fast, then you need low weight, which generally precludes electric propulsion due to weight issues. If you are going slow, then issues other than prime mover selection will determine propeller design.
     
  14. Marc78
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    Marc78 Junior Member

    Thanks for the replies! My prime mover selection is not at discusion, there has to be an electric drive. This is by customer choise, local regulations, space issues and/or ambition. We have a CW Hood 32 daysailer fitted with a 2kw Poddrive and a lithium battery stacked out of our way, this provides enough home coming power without the big engine box in the cockpit.

    Tesla has marketed it's car quite well, main mover electric. The issue is not efficient power input, shore power and batteries, solar, generator or whatever. The point is, use an electric motor and make the propellor as efficient as possible.

    If watts/kg and watts/€ or $ on batteries increase, electric power can become more interesting over time.
     

  15. daiquiri
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    daiquiri Engineering and Design

    Yobarnacle,

    I do respect your opinions too. A big number of my posts was sent from a mobile device lately, because I have very little time to write extensively. So some posts may sound harsh or non-respectful of other's opinions, because too short or written quickly. But that is just an apparence, trust me. :)
    Getting back to the diesel vs. electric issue, you will probably remember this old discussion where I have put a practical example, comparing two motors: http://www.boatdesign.net/forums/boat-design/tko-electric-solar-concept-43940-4.html#post568467
    As you can see from that example, a gearbox is the device which brings the diesel and electric RPM to the same level and makes their power and torque outputs comparable. It does introduce a small loss of efficiency to the diesel drivetrain, but it is really small. Please check it out once again, just to refresh your memory, then we can discuss it, if you want.

    I completely agree with Jehardiman when he says:
    That pretty much sums it up.

    Cheers
     
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