Stern Tunnel?

Discussion in 'Powerboats' started by Easy Rider, Nov 6, 2014.

  1. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    This stern shot of an Eagle 32 reveals what looks like a prop tunnel but I've never seen one like this. Perhaps it's to reduce cavitation or give a bit more prop clearance. Any Eagle guys know?
     

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  2. PAR
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    PAR Yacht Designer/Builder

    This is a common way to decrease draft a little, as the tunnel permits the prop to be considerably closer to the hull bottom. Picture the hull without the cutout and see where the prop would have to live with at least 15% - 20% of the prop diameter above the tip.
     
  3. Easy Rider
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    Easy Rider Senior Member

    Thanks PAR.. I was also thinking the shape of the tunnel probably strengthens the bottom above the swing of the prop. That could allow closer prop clearances, a quieter boat or both to a lesser degree.

    The small size and angular lines caught my interest.
     
  4. Milehog
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    Milehog Clever Quip

    How would this affect prop walk?
     
  5. JSL
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    JSL Senior Member

    I have done several tunnels (propeller pockets) both circular and faceted on single and twin engines and this looks 'different' ~and small. Probably okay??? for a very slow boat but not for any kind of speed or efficiency.
     
  6. gonzo
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    gonzo Senior Member

    Does the shape of the tunnel make any difference when the clearance is that large?
     
  7. PAR
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    PAR Yacht Designer/Builder

    The size of the tunnel (too big or small) can have a big impact on performance, though not on that boat. I'm sure it just offers a few more inches of draft reduction.
     

  8. Pericles
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    Pericles Senior Member

    Twin stern tunnels are also suitable for flat bottomed vessels.

    Here is an image from a previous thread. It's the Ohura that voyaged on the Wanganui river in the early half of the 20th century. The vessel was recovered & put back into service IIRC.

    http://www.boatdesign.net/forums/at...1206441471-atkin-tunnel-revisited-ohura07.jpg

    OTOH, here is another Ohura that popped up when I went looking for the capsized Ohura. Twin tunnels, lovely bottom.

    https://plus.google.com/photos/115373440293817752545/albums/5969500034097744705

    Ain't boating fun?
     
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