Turbo 400 trans or other Multi speed transmission with Surface drive.

Discussion in 'Inboards' started by MRover, Apr 27, 2021.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Yes, 12k is unreasonable outside of racing applications. Particularly when you can get a beefed up TH400 for a fifth that.
     
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  2. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    Why not manual trans. Decades ago, from late 40's, papa had a criss craft with a big blue gear lever. I supposed it clashed gears a little switching gears. That boat had 2 steering wheels on the side.
     
  3. MRover
    Joined: May 2020
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    MRover Junior Member

    Big reason is that the 400 in already installed and working very well. I cant wrap my head around how a manual would work well in this application...
     
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  4. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    It wouldn't be as convenient as the automatic shifts from that old valve body, that's for sure.

    What prop are you running to take advantage of the very low gear in first?
     
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  5. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Right. 18x30 SD
     
  6. MRover
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    MRover Junior Member

    I am running a 17x30 for now.
     
  7. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    I'm curious about acceleration in 1st, top speed in 3rd.
     
  8. MRover
    Joined: May 2020
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    MRover Junior Member

    acceleration in 1st is good, not great and comes on plane in about 20 seconds, Once it's on plane it seems to take off. I have only taken advantage of the 1st and 2nd gear thus far and I have not done a top speed run yet. I have far to much work to complete before I am ready for good shake down.
     
  9. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Interesting.

    Top speed in 3rd is going to be very fast. Gerr's Propeller Handbook suggests about 10% slip. I don't see where you stated rpm for your 670hp mill, so I'm guessing around 5000rpm.

    30"×5000×0.9=11250

    That's 128mph

    Actually, at that speed, slip is probably only 5%. 135mph. 217km/h

    Assuming you have the power to get to that speed. If not, a lower pitch prop would really help acceleration.
     
    Last edited: Jul 24, 2021
  10. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    I ran Crouch's formula, just to see. I don't know the all-up weight of your boat, so I guessed 7700#. Adjust accordingly.
    Crouch's formula has been superceded by more accurate formulas in the almost a century since he cameup with it. But not knowing anything beyond 30', 670hp, SP 17x30, the inaccuracies in the formula are a rounding error compared to the other attributes. But using those values I got 56 knots, 65 mph.

    I don't know what the expected slip for a SP prop is at that v. If it's comparable to an immersed prop, you'd do better with a 17x15.
    @sandhammaren05 is the SP prop master on the forum. He will know.
     

  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Oh yes. SP prop. Faster than 65.
     
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