Ganging Outboards - Does it work?

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

  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    IO drive unit on the transom of each hull, driven by an electric motor inside the hull?

    I was looking at listings of electric motors yesterday for a different application and saw reference to motors designed to work in exposed, wet enviorments. Sealed housings with a lip seal or similar on the shaft.
     
  2. CatBuilder

    CatBuilder Previous Member

    Gunboats are semi-custom. They leave propulsion up to the owner. The one in the video has diesel electric inboards, through normal shafts that are angled to the boat's overall centerline from each hull. I've seen others with sail drives mounted at 45 deg. I've seen yet others with standard saildrives.
     
  3. CatBuilder

    CatBuilder Previous Member

    Could you provide a link to the water tight motor?
    Did you look at the YouTube video at 0:31? How could an i/o work in that condition without aerating the prop?
     
  4. CatBuilder

    CatBuilder Previous Member

    I don't mind some drag when motoring. It's when sailing that I need to eliminate the drag.
     
  5. whitepointer23

    whitepointer23 Previous Member

    could be, i remember the people using them did not like them, unreliable plus a lot of noise and vibration. something i did see a while back which looked quite good was a long hydraulic outboard on a piling barge, i guess they already have a huge oil supply to run the hyd pile driver, i would not like to pay for a hyd setup in a cat though.
     
  6. CatBuilder

    CatBuilder Previous Member

    AH! Here is the answer to the different HP ratings between electric and diesel.

    The answer is, "Electric motors are rated at continuous output. Diesel/gas is rated at peak output." This means you can exceed the rating of an electric engine for brief periods, just like you can use peak diesel or gasoline power at peak periods.

    http://evalbum.com/motor

    From the link:

    "Electric cars are driven by large electric motors usually rated between 3.5 and 28 horsepower. For those accustomed to gas engines, this may not seem like much power, but the rating systems used for gas engines and electric motors are so different that the numbering system is almost meaningless. Gas engines are rated at their peak hp, electric motors are rated at their continuous hp. The peak hp of an electric motor is usually 8 to 10 times its continuous rating."


    This explains why you can use a lower rated HP or KW electric motor than you can an internal combustion one. Aside from all the parasitic losses associated with ICEs, you can get more reserve HP from an electric than listed.

    I knew I had the math right from my initial comparison of inboard electrics. I just forgot the steps I took to find that a 15-20HP electric motor was the same power as a 30HP Yanmar.

    Now... does anyone know where to get the lightest weight electric motor in the 20HP range? They keep weighing in at 300-400lbs! That won't work.
     
  7. michael pierzga
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    michael pierzga Senior Member

    With a PTO pump on the front of your generator you are ALWAYS dragging on the generator.
    With electric you can choose how you load the generator and how to use its energy. I would investigate diesel electric.

    Another advantage of diesel electric is stored energy. When you do " A COSTA" it might be significant
     
  8. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    It probably would aerate the prop. But with an electric motor which is speed controlled that shouldn't be a problem for the motor because it won't overspeed.

    What is you "SOR" for the mechanical propulsion? What kind of seas do you want to be able to motor into at what speed?
     
  9. CatBuilder

    CatBuilder Previous Member

    The boat should be able to motor into seas just like you see in that video at 0:31 without slip. The problem would be a loss of forward momentum with slip, not so much an over revving problem.

    The forward speed would be whatever the motors are capable of, with 8 knots to be a good speed.

    This is a sailing machine with auxiliary propulsion to motor places at 6-8 knots or so.
     
  10. michael pierzga
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    michael pierzga Senior Member

    What is the problem with normal shafts ?
     
  11. CatBuilder

    CatBuilder Previous Member

    Too much drag. See SOR, "Props must lift out of water when not in use."

    It might also say, "No junk under the boat"
     
  12. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    All propellers slip when producing thrust. I assume you mean not aerate.

    How deep does the propellers need to be to remain submerged in that sort of seas? Looks to me be be 4 or 5 feet if mounted at the stern. Moving the props under the hulls reduces the required depth but makes it much more difficult to retract the props efficiently.

    From your other posts it appears that cost is a significant factor.

    A SOR needs to be balanced and feasible, and may require some compromises/tradeoffs from the initial wish list. If there doesn't appear to be a feasible design which meets all of the SOR then it may be necessary to modify the SOR. Developing a final SOR is usually an iterative process if the envelope is being pushed.
     
  13. CatBuilder

    CatBuilder Previous Member

    And yes, this is pushing the envelope. You have reasoned out a fact I knew from experience - props go foward to motor in a rough chop. Excellet. You figured that out very quickly.

    The sor doesn't mention cost at all, because it's not a huge factor. Weight and performance are the factors.
     
  14. DCockey
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    DCockey Senior Member

    Can you mount the drive system from the bridge deck considerably forward of the stern?

    What are the available mechanical design and engineering capabilities? Does the mechanical portion of the system need to be completely or mostly "off-the-shelf" parts? What are the available mechanical fabrication capabilities?
     

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

    I was on a Gunboat in Florida...all I can say is wow.

    The HP thing has been gone over before..cars need 5-10 hp to go down the road,but need the 200hp for acceleration and going up hills.

    Generally aircooled 20 hp DC motors used in EVs should be about 50 pounds but can go way up depending on type.
     
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