effects of adding prop pocket

Discussion in 'Boat Design' started by sidetracked, Dec 22, 2015.

  1. sidetracked
    Joined: Dec 2015
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    sidetracked Junior Member

    To follow up on my earlier thread, but with a different approach:

    What are the pros and cons to adding prop pockets from a hull performance perspective?

    A pro would be less shaft angle, but what other consequences will there be?

    The tunnel/pocket adds a little surface area, so does that have any impact?

    There is a lifting strake that is right where the pocket would be, so that would be eliminated - any hull performance issues with that?

    Is there anything else I should be considering?

    thanks,

    Michael
     
  2. philSweet
    Joined: May 2008
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    philSweet Senior Member

    The trouble with a small pocket that doesn't hold the entire prop is that there are sharp changes in the inflow into the prop at the height of the pocket. At speed, and at the cavitation limit of the prop, these perturbations are a killer. You would have to keep the boat attitude perfectly steady and dead on design to eliminate them. There is a good bit of info on inflow pulsation and prop cavitation.

    from http://www.akcaymarin.com/Products/özelklavuzlar/Volvo Penta prevane klavuzu.pdf

    The entire document pertains to lower speed applications and flatter props, but the trends are valid - you can end up needing twice the prop area just to handle the increased tendency for cavitation.

    In contrast, I can't find anything equivalent regarding shaft drop angle, and I'm not convinced that the best 17" prop @ 13 degrees shaft angle on existing hull is going to cavitate at a lower thrust than a 20" prop that is partly in a tunnel at a 10 degree shaft angle. On a perfect day, sure it would, but on most days, I just don't know. I also don't know if anyone manufactures a stock 17" prop that is dead nuts what you would need. The 20" is no problem.
     
  3. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    When you install a pocket and raise the shaft angle you are moving into the Surface Piercing Propulsion World with all that entails.
    Pockets and tunnels are Surface Propulsion devices.
     
  4. sidetracked
    Joined: Dec 2015
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    sidetracked Junior Member

    really appreciate all the input.... after studying the prop further tonight, the cavitation issues seem to be an issue starting at about 35 knots with both the set ups I have been looking at (13 degree shaft angle, 17" prop and 14 degree down angle with 19" prop). Top speed is estimated at 39/40 knots and I will rarely run that % wise, majority of time will be below 35 knots... looks like I will go ahead and leave the shaft angle unchanged and go with 1.2:1 gear ratio and 17" 4 blade prop and use a high blade area Michigan or acme and just try to tweak it as best I can....
     

  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    check out the Saro Trimable steerable tunnel.
     
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