Oversized prop compromise

Discussion in 'Props' started by DogCavalry, Sep 24, 2020.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    That is consistent with propellor theory, and Gerr's propellor bible. Props work by momentum transfer. Momentum is mass times velocity change. But the energy to do it is kinetic energy. Mass times velocity change squared. If you can only use half as much reaction mass (water) you have to accelerate it twice as much. At 4 times the energy. Mass isn't as much of an issue going fast, since so much water is coming into the prop any way. But when moving slow, or starting, a bigger prop is always better.
     
  2. DogCavalry
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    DogCavalry Senior Member

    Seandepagnier, no video link seen.
     
  3. seandepagnier
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    seandepagnier Junior Member

    As you say, a bigger prop is always better, this is true as "moving slow" meaning < 35 knots boat speed. Most boat's have terrible propeller efficiency, then combined with the dismal efficiency of the engine overall efficiency from fuel ends up only 3% or less: a terrible waste.

    Future technology could be 80% efficient total efficiency but the world will be ravaged before the technology is realized.

     
  4. DogCavalry
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    DogCavalry Senior Member

    Sailboat propellors are always a compromise, and that's just unavoidable, unless they are on an outboard. Powerboat efficiency is more amenable to change and improvement.
     
    Last edited: Oct 16, 2020
  5. gonzo
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    gonzo Senior Member

    What video?
     
  6. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    You should be seeing it in post #33 above.
     
  7. brendan gardam
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    brendan gardam Senior Member

    I have had 4 home built boats with 3 speed transmissions hooked up to marinised car engines. Prop sized to suit second gear, they worked great. Reverse was slow because of the large reduction. 1 of them had a 3 speed auto which actually dropped into 3rd on its own when you got enough speed up. A friend of mine built the boats, smallest 22ft, longest 32 ft. I especially liked the auto because I could pull up on a fishing spot and put it in park, easier than anchoring.:)
     
    DogCavalry and BlueBell like this.
  8. DogCavalry
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    DogCavalry Senior Member

    Haha! Okay, that gave me a pretty good laugh. Parking.
     
  9. DogCavalry
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    DogCavalry Senior Member

    Okay, so P for parking is a joke, Brendan, but is the rest of it true?
     
  10. brendan gardam
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    brendan gardam Senior Member

    Yes. Car transmissions do work well. But you do have to bump the revs up to get a decent reverse.
     
  11. brendan gardam
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    brendan gardam Senior Member

    There was a company here that used to sell professionally marinised 2 speed powerglide autos for ski boats. But they may have been locked in 1 gear.
     
  12. DogCavalry
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    DogCavalry Senior Member

    Can't imagine why they'd do that.
     
  13. brendan gardam
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    brendan gardam Senior Member

    I don't know if they did, I just don't remember them talking about using the 2 speeds
     
  14. missinginaction
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    missinginaction Senior Member

    There could be others but the only 2 speed that I can find with any real data posted is from Coan Racing. It retails for $12,000.00. That buys about about 4,000 US gallons of decent marine grade (non-ethanol) fuel at today's prices. Considering the work involved and the costs (and I have no idea about the reliability) I'd have a hard time justifying the cost.
     

  15. DogCavalry
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    DogCavalry Senior Member

    Considering I can pick up a TH400 that would outperform the hell out of that for 1200, I agree that that horrific rip off would not be justified.
     
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