Ganging Outboards - Does it work?

Discussion in 'Propulsion' started by CatBuilder, Jan 14, 2012.

  1. CatBuilder

    CatBuilder Previous Member

    I can do that. :D

    [​IMG]

    See that chart above? That's a typical DC motor torque curve (the straight line). See the curved line? That's the power put out by this motor in Watts. It could just as well be in Horsepower. Same thing. They both go as a function of the RPM, which increases to the right of the graph. The electric motor, at lower RPMs, has huge torque and horsepower compared to a diesel. This power is 100% at the prop because there is no transmission or any other type of thing between the motor shaft and the prop shaft. They are both one.

    Now, let's take a look at the torque curve for a Yanmar:

    [​IMG]

    Power at the propeller is shown at the bottom line in the graph. See how that power output (at the propeller) is next to nothing until you really get high in RPMs?

    That's the difference. Nobody runs their diesel at 3600+RPMs all day. You run it a little lower and it loses its rated HP very *very* quickly, as you see right on the graph above. Just looking at this Yanmar graph, if I'm running this Yanmar 40HP rated diesel engine at 2800RPMs, I'm getting only 15HP at the propeller. Run an electric motor at the same speed and I'm getting much more out of it because it's able to produce higher horsepower than a diesel at reduced RPMs. (the curved line on the first graph)

    The math doesn't lie. The engines just work completely differently from each other.

    This is why you don't need a 40HP electric engine to go in place of a 40HP diesel engine. The diesel engines are way WAY over rated from their true running output, which is almost never going to be at 3600+RPMs.

    Sure, in a pinch, you can get the 40HP from the diesel engine by pushing it to more than 3600RPMs, but when do you do that? I don't think I ever have in 20+ years on the water.

    I think what a lot of people don't realize is that the power of a diesel engine drops off so quickly as you get below the ridiculous RPMs the engine is rated at. It's not that the electric motor is so much better than the diesel. It's that the diesel is rated in a way that makes it sound like it produces much more power than it really does (at the prop).

    If someone made a quality electric outboard, I'd be buying it because I already have a generator in my design for heating and air conditioning. It could pull double duty to power the electric outboards, which do not need to have as high a HP as diesels would for my boat. As far as outboards go, I have no idea. They don't publish the power/torque curves you see above. I do know they are rated at 5000RPM though. Yikes!
     
  2. Brian@BNE
    Joined: Jan 2010
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    Brian@BNE Senior Member

    Crap!

    You are not reading the graph correctly. The 15 HP you refer to is the power the propeller absorbs (at that diameter, pitch and rpm). The lines above on the same graph show the power the engine can produce at 2800 rpm, if you load it up. Its about 35 HP.
     
  3. CatBuilder

    CatBuilder Previous Member

    Crap! ha ha ha

    Maybe I am missing something.

    I'm not clear on that graph I chose then. Can you explain that lower line? The power the propeller absorbs?

    Where is it absorbing it from? The water or the prop shaft? I guess you are saying from the water, since you are mentioning diameter, pitch and rpm.

    So that lower line is what the prop itself puts out, rather than what the prop shaft is putting out? Damn. Crap. :)

    I'd like to see that line on the Torqueedo.
     
  4. Brian@BNE
    Joined: Jan 2010
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    Brian@BNE Senior Member

    CB
    Here's an article that probably covers it better than I can. At times I'm full of crap too!
    http://continuouswave.com/whaler/reference/propellerPowerCurve.html

    Actually propeller demand would be a better description.

    This 'electric horses' are stronger than 'ICE horses' is marketing garbage. What they may mean is that most folks will run an ICE at, say, 75% WOT when cruising. At those normal rpms the ICE will produce a lot less power. So the electric marketers are talking cruise-speed power needs, and sizing their motors exactly to that. But the problem is, if you size an electric motor for cruise speed you have no reserve power for any adverse circumstances that might arise. With an ICE you can go to WOT for a bit if and when required.

    Edit: put the article into a pdf and uploaded
     

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    Last edited: Jan 14, 2012
  5. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Dont be too dissapointed, those things are noisy, definatley not a "stealth" outboard, I suppose one of the advantages is you dont need to check the tell-tale or tacho to know its running though- I had to lift the engine hatch on my Beach Marine cat to check that my 9.9 Yamaha 4stroke was on.
     
  6. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Factors which Torqeedo does not state when saying their 4.0 motor has comparable propulsive power to a 9.9 HP gasoline outboard:
    - What size and type of boat?
    - What speed throught the water?
    - What 9.9 HP gasoline outboard?
    - What propeller on the gasoline outboard?

    I assume they have done some actual testing but I have not seen anything about what that testing was.
     
  7. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Thanks cat but you needn't have gone to all the trouble..I kinda get that stuff...what I meant was for live side by video proof from a manufacturer.

    Anyways I guess you need to figure out the actual hp,torque,prop size etc for electric motors and see if they match the prop hp and torque.

    Of course,when an engine is geared down..the hp at the prop stays the same but the torque multiplies by the gear ratio.

    My 50' power boat is propped and geared to have the best high speed cruise mpg at the torque peak..which is the best consumption at speed.Thats what I've been told,and thats what it seems to be.
    Due to hull shape,props/rpm it gets better mpg at 25 knots than it does at 14.

    So -looking at your charts- a 23 hp@ 2200 shaft rpm putting out 100nm ( 75 lbft) going through a (2:1???) ratio would give you 150 lb ft and 23 hp.

    And like Brian says,there is an extra 13 hp on reserve.

    Two of these at 12 hp and 75 lb ft would be the match for one engine...in a power boat situation I guess...and I'm afraid what they'd cost.Industrial would be better.

    http://www.ikanostech.com/index.php/electric-motor-drives/st-74-electric-motor
     
  8. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Power vs rotational speed curves for engines and motors show the maximum power possible at various speeds.. With the throttle partially closed or speed control turned down the power will be less; somewhere under the curve. Engines and electric motors have devices, throttles, fuel control, voltage/current control, to reduce the power at any speed to less than maximum. To drive a boat at a steady speed slower than the maximum speed possible the power has to be reduced which is done by varying the throttle or speed control.

    The power needed to turn the propeller vs boat speed curve is the same for a given boat with given propeller irregardless of what's providing the power. This is sometimes refered to as the "power absorbed". It doesn't matter whether the propeller is powered by an electric motor, a gasoline engine, a Diesel engine, a gas turbine, a steam engine, animals on a treadmill or anything else, and how much power the engine/motor/etc can produce in excess of what's needed. Change the propeller and the power needed will change as the propeller efficiency changes.

    If a diesel engine is furnished with reduction/reverse gear then the rated power should be at the reduction/reverse gear output. Losses from the reduction/reverse gear output to the propeller with a straight shaft shouldn't be more than 1% or so, and would be due to drag from the shaft bearings and seals. Even with some sort of angle drive I strongly doubt the losses would be more than 10%, probably less. Reduction/reverse gear losses should also be well under 10%.
     
  9. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    There was also the Coventry Climax(?) which I think, like the Ruggerini, was hugely heavy, agricultural and air cooled so very noisy.

    I think the plan with all of them was to sell to the third world fishermen.

    Which was a good idea.

    Instead the fishermen all bought high revving, low thrust outboards. I'm always amazed at how many totally ineffectual outboards are being used as tugboat engines.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  10. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If ganging a gaggle of outboards, I'd say the ability to easily tilt one or more clear of the water and run with just the remainder, would be essential. There could be an angle to increase fuel economy by just using as many as necessary to make the desired headway, which would vary according to conditions.
     
  11. masalai
    Joined: Oct 2007
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    masalai masalai

    An email, containing a pdf brochure, from the Australian supplier of the Yanmar diesel outboards is attached... offers a nice range of propellers and electric tilt which is nice and clearance from the bottom of the transom of 18 inches, on a top of transom to cavitation plate up to 27 inches "ultra long" leg extension... (D27AX-EP-UL) at 104kg (229 pounds) At a pinch the 11.5 inch diameter propeller could fit but "warranty" ? I am comparing what I have comfortably running on my 20hp engines, and pushing my boat could possibly get away with 11.5 x 13 and still do 6+ knots on one and 12 knots? on both? with reasonable efficiency... These are lighter than my 20hp options of 144kg each - BUGGER... I could have avoided mini-keels :eek: but then I would loose the sail option but as an 'ageing old fart' - so what....
     

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  12. DCockey
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    DCockey Senior Member

  13. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Found some pricing information for the Evinrude multi-fuel 55 HP outboard. It's a little over $20,000 or about three times the price of a conventional outboard of the same power.
     
  14. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Aren't Yanmar diesel outboards out of production ?
     

  15. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Yes they are, but NOS ones are around..plus rebuilt ones. Price..I have no idea.
     
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