Design against propeller ventilation on 16' snekke

Discussion in 'Boat Design' started by mattis, Feb 13, 2017.

  1. mattis
    Joined: Feb 2017
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    mattis New Member

    Hello all, this is my first post here...

    I'm designing a 16' double-ender with a Yanmar 1GM10 inboard diesel. It should displace about 500 kg, and I would expect 6 to 7 knots cruising. The design is based on a traditional Norwegian sjekte/snekke, and as such is quite shallow aft.

    The company supplying the propeller have suggested a 12", three blade prop. I am a bit worried about the propeller possibly ventilating, and was wondering if there is a rule of thumb for minimum depth of the propeller. At the moment, the centre of the propeller shaft is ~10" below DWL.

    The image shows the profile of the stern, depth in mm, and the propeller, including a tip clearance of 10% of the diameter.



    Many thanks for any advice!

    Mattis
     

    Attached Files:

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The depth of the propeller depends on many factors. For example, a propeller that is under a wide stern with a flat bottom will tend to ventilate less than your application. The overall design of the hull will also have a large influence. For example, how much it will pitch in head or following seas. That may be the most limiting factor. Since this is based on a traditional design, what is the typical position of the propeller on these boats?
     
  3. mattis
    Joined: Feb 2017
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    mattis New Member

    The propeller on original boats of this type tends to be in much the same position (albeit not quite as low as I have drawn), however these boats typically had much smaller engines (on the order of 2 HP) and smaller propellers. I am just trying to decide whether to give it a bit more draught aft. I see your point re pitching. My main reason not to lower the keel aft is that the boat should also row reasonably well.
     
  4. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    If you don't want to fret too much about the design, you can just add cup to your prop. Baumann Marine in Tx. is one of the best. It will cost you about $60 plus shipping and you won't believe the difference.

    For example, on my micro drafting flats boat, my first prop couldn't even get this flat bottom on plane. Now I can run with the motor jacked so the prop is completely above the surface, fed by a tunnel and I have 0% slip numbers.

    of course first try with a stock prop first.
     

  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    More prop blades might offer some protection.
     
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