cavation/ventilation plate height/depth

Discussion in 'Boat Design' started by dukester, Jul 28, 2014.

  1. dukester
    Joined: Jul 2014
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    dukester Junior Member

    I built a wood 3 point hydroplane, installed a 2005, johnson, 50hp outboard with a 11-1/8 X 16 alum prop. The first test run the boat would not plane until son in law, 215 lbs laid up on the bow. I built this same boat, same plans about 30 years ago, had a 40 mercury and never had a problem planing or pulling a skier. Should the prop be even with the bottom of the boat, I guess I got lucky on the first boat. the boat weighs about 400 lbs.
     
  2. hoytedow
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    hoytedow Wood Butcher

    The cav plate should be level with the boat bottom.

    Welcome.
     
  3. dukester
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    dukester Junior Member

    Thanks for the quick reply, I thought that was the correct position, when the marina installed the motor on the boat they said they thought where they had it was correct but they had not had a 3 pointer in their shop. When it didn't react properly I thought I would check. Thanks for the info, Hank.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    With a stainless prop you might be able to go a touch higher, and get more speed.
     
  5. IMP-ish
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    IMP-ish powerboater

    Is the 50hp a four stroke? Weight is a big deal on a small hydro.

    You can also try a cleaver prop that doesn't lift the bow as much.
     
  6. messabout
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    messabout Senior Member

    A lot of this depends on the boat itself. Four hundred pounds is pretty heavy unless the three pointer is pretty big. When you add the weight of two people you may be pushing the limits for getting out of the hole with 800 pounds or more.

    What size is the boat, how much planing surface does it have? When you had trouble getting it up was the prop cavitating? You may need to fiddle with the prop thrust angle, like letting the prop thrust aim ever so slightly down, (that will tend to push the bow down so that you can get out of the hole.....maybe) lots of other variables at work here. Does the engine rev freely when you are trying to get the boat up or is it struggling? If the 50 horse engine is a little bit sick it may be making only 20 horse power......and on and on with questions.
     
  7. PAR
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    PAR Yacht Designer/Builder

    It also depends on the the prop itself. You can get a prop with the appropriate rake to help your stern squat.
     
  8. Jimboat
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    Jimboat Senior Member


    Performance of 3pt hydroplanes are very sensitive to weight and power, so the 2 boats could perform very differently if any differences in either of these. Prop changes can help a lot, if engine height is not ideal. Pictures of existing setup would help evaluation.
     
  9. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    I would state this slightly differently.

    After running the boat on plane, note the position of the cav plate so that you can later raise or lower the engine on its mounting plate so that the cavitation plate with skimon the top of the water surface, but is not too high to cavitate the prop if it lifts higher doing normal turn, when changing course, at speed.
     
  10. SamSam
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    SamSam Senior Member

    I wouldn't think 3 point hydroplanes tilted very much in a turn...?
     
  11. dukester
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    dukester Junior Member

    To everyone that replied to my request, raising the motor on the transom did the job, we took the boat out and the bow came down and it runs like a scalded dog. There were three adults in the boat, running at 4,000rpm it flew across the water and still had another 2,500 rpm I could safely use. Thanks for all of the replies, Hank.
     
  12. dukester
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    dukester Junior Member

    Mercury prop

    Can anyone tell me what the pitch and diameter of a Mercury prop number: 48-32178A1 is. Years ago I had this prop but the only numbers I have are those listed above and would like to get the same prop again.
    Thanks, Hank.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Hi Hank, that prop should be a 10 3/4" x 15".
     

  14. dukester
    Joined: Jul 2014
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    Location: Blairsville, Ga.

    dukester Junior Member

    Baeckmo, Thank you so much for the information, now I know what prop to by. Have a great day, Hank.
     
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