Identify inboard boat with prop in tunnel which cannot strike bottom ?

Discussion in 'Boat Design' started by preventec47, Jul 31, 2016.

  1. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    I remember way back running across a boat design that somehow was an inboard
    but did not have a problem in shallow water because the prop did
    not stick down below the bottom of the boat. I am wondering how
    the prop stayed in the water instead of spinning in air ?
    I know there was a tunnel under the boat hull but I am certain the water line
    on the hull was much lower than the top of the tunnel. Does this design
    have a specific name which will allow easier searching ?
     
    Last edited: Aug 7, 2016
  2. Heimfried
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    Heimfried Senior Member

  3. keysdisease
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    keysdisease Senior Member

    Penn Yan, never saw one that ran right.

    :cool:
     

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  4. Ike
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    Ike Senior Member

    The Coast Guard had quite a few 22 foot Penn Yans with tunnel drive. They were ok in forward, but would not back up in a straight line, if at all. However they were pretty rugged and could take a beating. We used them to patrol offshore power boat races.
     
  5. CDK
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    CDK retired engineer

    From experience: the key word is "nightmare"!
     
  6. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Mersey class lifeboat has tunnels, now replaced with Shannon class with jets.

    [​IMG]

    [​IMG]
     
  7. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    The Bluewater coastal cruisers have tunnels,I think the props stick out a few inches.
    The bow is lower in the water than the props,so if the bow clears you know the props will.
    A regular poster here has one.
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Bayliner for many years had prop pockets in boats around 35 to 45 feet.
    But they found that the tunnels impacted lift and dropped that idea.
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    To make a tunnel(s) big enough to keep a prop hidden, you have chopped out a lot of volume. The hull would really need to be designed starting with the tunnel in mind, to be successful.
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Correct
    Additionally there is a reduction in planing surface as well as a reduction in pressure as the velocity of the water speeds up into the tunnel.
    While there are advantages to a tunnel protected propeller, optimization of lift is not one of them
     
  11. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    I think the boat you are looking for is the Atkin Rescue Minor.
     
  12. rnlock
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    rnlock Junior Member

  13. PAR
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    PAR Yacht Designer/Builder

    The Atkins tunnel hulls are very limited in top speed. The best ones can do 25 MPH, but most are slower. Tunnel design isn't an easy thing to do right, but prop protection is pretty easy. Maybe it would be simpler if you describe you concerns and see if more conventional methods can address them.
     
  14. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    Thanks for the reply...
    I see that I do not have a need for tunnel hulls but was simply seeking education
    and had no idea what their purpose was. .... much in the same way it took me
    a while to figure out what a needed log was as relates to inboards.
     

  15. rnlock
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    rnlock Junior Member

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