effect of close clearance tunnels on gear ratio and wheel selection

Discussion in 'Boat Design' started by Dan V, Nov 5, 2011.

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  1. Dan V
    Joined: Nov 2011
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    Location: Beaufort NC

    Dan V Junior Member

    I´m looking for advice concerning the effect of close clearance tunnels on gear ratio and wheel selection. I was very dissatified with the performance of a boat I designed with a tunnel. I found research of the late Donald Blount and redesigned the tunnel as per his reccomendations. The boat is 32´ and about 14,000 lbs. Cummins 450C w/ 1.48 gear. Upon relaunch w/ a 22 x 26 four blade (medium cup), the top speed was 32 knots but top rpm was only 2350. Calculations show a slip of only 5%, so the tunnel w/ its .375" tip clearance was doing its job perfectly, but that restriction had to be hard on the engine. So thinking hydrodynamic friction, I started whittling away at pitch, then diameter and back to pitch again. I´m finally down to 20-1/2" x 22" and just am getting 2600rpm at wot. Performance sucks: top speed is 24k, 20k cruise at 2200 and I´m burning 15% more fuel than before. I think I have to go back to filling the tunnel w/ propeller to get the system to work as designed and will need to change gear ratios to protect the engine. How do I determine if I need a 1.79:1 gear, 2:1 or something different? Or is there something else I have overlooked?

    Thanks,
    Dan Varner
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    First: State rated engine power @ rpm, then main dimensions of tunnel, including propeller position.
     
  3. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    I had a meeting with DLB a few months ago. He was on time for the meeting, so I wouldn't have characterized him as late.....

    We all gotta go sometime, but I think your description of him as deceased is premature....
     

  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

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