"Propeller pockets" or "Tunnels"

Discussion in 'Powerboats' started by 67-LS1, Mar 12, 2004.

  1. Nomad
    Joined: Feb 2002
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    Location: Perry, Florida

    Nomad Mold Trader/Boat Builder

    [​IMG]
    [​IMG]

    Here's a few pictures.... I'll see If I can dig up a few more
     
  2. Willallison
    Joined: Oct 2001
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    Location: Australia

    Willallison Senior Member

    Now that is interesting....
     
  3. nevd
    Joined: Nov 2003
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    nevd Junior Member

    Tunnels

    From my perspective there are 2 types of tunnels used in deep Vee planing hulls.

    First is vented like stepped hulls to stop water contacting the upper roof of the tunnel and this can be expected to increase efficiency (if properly designed to minimise spray and give good trim angles at designed speeds). The increased efficience is because the low lift/drag portion of a conventional deep Vee hull no longer has skin friction drag and the prop sees less disturbed water.

    Second type of tunnel has no venting and this causes the tunnel roof to remain in contact with the water (even though this may have a negative trim angle) to allow high prop heights like most of the Florida based "Flats boats". Because of the additional skin friction drag, efficiency will decrease unless the additional drag is balanced by some other benefit eg better prop shaft angles in the case of inboard drives as discussed in earlier parts of this thread.

    What do other contributors more expert than myself think?

    Regards,

    nevd
     
  4. nevd
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    nevd Junior Member

    Were the tunnels in pictures posted today by Nomad vented?

    Regards,

    nevd
     
  5. Nomad
    Joined: Feb 2002
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    Nomad Mold Trader/Boat Builder

    They are vented with shut off's...... With their design venting really does not make much of a difference but we still do it.......
     
  6. Willallison
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    Willallison Senior Member

    hmm... now I'm getting a little confused. Surely by venting to the tunnel you are introducing aerated water to the prop - not a good thing. Or are you only supplying enough 'air' to reduce the skin friction, but not so much that this disturbed water flow comes into contact with the prop?
     
  7. Nomad
    Joined: Feb 2002
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    Nomad Mold Trader/Boat Builder

    The air is foreward enough and limited enought that it does not affect the prop. Also with the tunnel we give the boats more tip clearence.
     
  8. Willallison
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    Willallison Senior Member

    I'm intrigued by the 'hexagonal' shape. Is there a reason beyond build-simplicity for this? I would have thought it would cause the prop to induce vibration as the tips get closer/further from the bottom surfaces....
     
  9. nevd
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    nevd Junior Member

    Have you measured the pressure (vacuum) inside the tunnels upstream of the propellors with both vent open and closed?

    nevd
     

  10. Bear

    Bear Guest

    Regarding Tunnel Hulls, you guys might want to take a look at our website which shows a type of 19+ ft. "Tunnel" Jet Boat we Drag Race here in the U.S..

    I do my own hull/keel designs/fabrications/modifications and rigging and would be happy to share info with most anyone ... Realize my Focus is Safely Going Fast in the 1/4 mile so such things as thrust, drag, getting up on plane quickly and maintaining a stable platform, etc., are very important to me.

    http://www.BlackBearRacing.com
     
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