Recommend me an angle of attack for outboards on a catamaran

Discussion in 'Multihulls' started by Wally2, Jun 2, 2019.

  1. Wally2
    Joined: May 2018
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    Wally2 New Member

    I'm working on permanently setting the angle of the outboards on a Schionning Waterline1480.

    It's hard to get a measurement but I'm going to take it with a level across the exhaust exit cone on the back of the prop while at rest in the marina.

    Do I want a bit of lift ? ......... 2° ?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Can you explain this a little ?
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Most outboards run at 12 degrees. I built a cat transom recently at 14 degrees. It is easy to trim up, bit trimming down requires wedges, so I get 2 degrees of no need for wedges.

    I also planned for my transom to run the engines a bit deep to avoid cavitation in rough stuff, but they ended up about at spec for the fact I wrapped all the glass around the top (8 layers of 1700). Turns out I forgot glass thickness, so was glad I built it an inch deep. I mighy try to vac a piece of aramid on the top for anti-theft as well.

    The angle is set by the transom angle buildout. That is transom angle to design waterline.

    Not sure why you would mess with the engine. The engine maker has a spec sheet with that info.

    Assuming the vessel is sitting on her lines; you measure the existing transom against a plumb level into a motorwell.

    The distance from the top of the transom to the bottom of the well is a hypotenuse. The distance to a square line off the horizon (waterline) is an opposite angle. The distance up is the adjacent angle.

    The math is simple.

    Example- 7" motorwell angle
    Vertical height 6.847"
    Cosin^-1 (6.847/7) = 12 degrees.

    Or
    Horizontal distance from top of transom to edge of square. 1.455"
    Sin^-1 (1.455/7) = 12 degrees

    Metric is easier here, btw.
     
  4. goodwilltoall
    Joined: Jul 2010
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    goodwilltoall Senior Member

    I would say perpendicular with waterline is best
     
    Last edited: Jun 2, 2019
  5. fallguy
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    fallguy Senior Member

    ?
     
  6. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    More info needed!
    Pictures speak volumes!
    Your question asks any respondent to dig into researching your boat, and begs to know what motor, intended use and conditions, desired speed, boat loading, etc...
     
  7. Wally2
    Joined: May 2018
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    Wally2 New Member

    Where the outboard leg exits the hull through the cetreline it is in water that is (hopefully) following the lines of the hull. Which is curving up at this point.

    Would I be better to:
    A). Conform to the water flow
    B). Parallel to the waterline (at rest or dynamic)
    C). Angle up to reduce squat
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Won't make much difference, but parallel to water surface is probably the best way.
     
  9. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I believe Goodwilltoall may mean so that the prop is vertical, zero angle.
    And for the most part, I would agree.
     

  10. fastsailing
    Joined: Sep 2017
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    Location: Finland

    fastsailing Junior Member

    How many degrees is the difference between A) and B) ?
    If > 10 degrees, orient axis of the prop in the middle of those a closely as practically possible.
    If it's between 5...10 degrees, no more than 5 degrees off either one for the prop axis.
    If' it's less than 5 degrees orient the prop axis anywhere between those as you like.
    Just how powerful outboards do you have, if you really think it makes any measurable difference in squat for a cat over 14 meters in length?
    In most cases it does not make a difference.
     
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