reverse kick with partial tunnel

Discussion in 'Boat Design' started by eyschulman, Jul 10, 2012.

  1. eyschulman
    Joined: Jul 2011
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    eyschulman Senior Member

    My boat is a 38ft Pearson true north with a single motor with the upper third of the 5 blade prop in a tunnel. When I try to kick the stern in reverse the reaction seems dampened is this due to the tunnel? What is the physical explanation if this is so?
     
  2. Village_Idiot
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    Village_Idiot Senior Member

    Many tunneled boats have cupped propellers to deal with aerated water going to the prop. If your prop has this cupping, it could be a significant contributor to lack of reverse thrust.
     
  3. midnitmike
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    midnitmike Senior Member

    What do you mean exactly when you say "kick the stern in reverse"?

    MM
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    My boat has twin tunnels with normal 3 blade props at the tunnel exits. They gobble up a lot of air when reversing with too high rpm, so I put them in reverse very gently, then wait until the boat responds.....
     
  5. Milehog
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    Milehog Clever Quip

    By 'kick' do you mean prop walk?
     
  6. eyschulman
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    eyschulman Senior Member

    Yes I mean lateral prop walk.
     
  7. midnitmike
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    midnitmike Senior Member

  8. Milehog
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    Milehog Clever Quip

    To the layman (me) it would seem that the pocket would enhance the kick.
    How much of an influence does the propeller blade count have in this matter?
     
  9. eyschulman
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    eyschulman Senior Member

    I am begining to think the smaller amount of port yaw(from what I expected) I am getting in reverse may be indirectly related to the tunnel in that the tunnel allows for a shallow shaft angle. Less angle less yaw.
     

  10. midnitmike
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    midnitmike Senior Member

    There are other guys on this forum far more qualified then myself to discuss scientific nature of this phenomenon. I will say however that in my experience the amount of prop walk that one sees in slow speed manuvering is proportional to the pitch of the prop. If your boat where fitted with the correct wheel using three, four, and then five blades you would have the greatest effect with the three blade configuration and the least with the five bladed due to the decrease in pitch.

    Although shaft angle may have some measureable effect the size of the wheel (diameter) and it's pitch are going to be the major factors in determining prop walk.

    MM
     
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