Never Seen This Before

Discussion in 'Hydrodynamics and Aerodynamics' started by Bornyesterday, Nov 27, 2020.

  1. Bornyesterday
    Joined: Aug 2020
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    Location: Southern california

    Bornyesterday Junior Member

    There has been addition to the rudder and the keel, you can see the weld line from the original and addition. Also a type of prop wash ducting above the props with smaller rudders added.

    I am interested in this series of boats which 4 were built in the early 70’s by Benetti. None of the other 3 ships were modified with these changes.

    Does anyone have any ideas of why these modifications were added?
    DSC_0015.jpg
    Thanks in advance for your comments.
     
  2. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    Could be to avoid ventilation of the propeller, also provide some pitch damping. Some Minorcan boats with pointed aft have patforms above the propellers
    at waterline level, similar effect together with swimming platform use !
     
  3. brendan gardam
    Joined: Feb 2020
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    Location: east gippsland australia

    brendan gardam Senior Member

    Agreed the half round plate is an anti ventilation plate. The small rudder gives improved steerage at low speed because the main rudder is not in the prop stream.
     
    Alexanov likes this.
  4. Alexanov
    Joined: Feb 2003
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    Location: Norway Sola

    Alexanov Senior Member

    Agree, this is excellent explanation. Not sure if this solution can be effective. It is really new for me also.
     
  5. Bornyesterday
    Joined: Aug 2020
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    Location: Southern california

    Bornyesterday Junior Member

    patzefran and brendan gardam thank you for your reply.
    Having both of you agreeing in reassuring.

    My questions stem from me being familiar with the sister ships.
    This one is different and the other 3 are the same below without the ventilation plates, small rudders, addition of the enlarged rudder, and addition of the keel, that this one has!

    Thank you for giving reason to parts of my curiosity. There are still lingering questions:
    1. I can see by the weld lines on where they increased the trailing edge and bottom of the rudder.
    2. Because I know the sister ships so well I can see the weld line of adding to the keel that lines up with what they added to the the bottom of the rudder.
    3. What kind of drag does all this create?

    101 common sense tells me there was problem with low speed maneuvering, what about the addition of the keel?
    Your input would really help, Thx in advance
     
  6. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    Increasing the keel area improves lateral directional stability, and also protect the rudder !
     
  7. Bornyesterday
    Joined: Aug 2020
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    Location: Southern california

    Bornyesterday Junior Member

    Thanks Alexanov and all that have replied,

    Hanging all that extra stuff from the hull below the water line; addition of keel, addition to main rudder, ventilation plates, and two small rudders with apparatuses to make them move with main rudder ......... how much do you think it effects the hydrodynamics and fuel economy?...
     
  8. Alexanov
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    Location: Norway Sola

    Alexanov Senior Member

    Run CFD with and without thees appendages. Difference in resistance give you quite good estimation. Looks like owner of the ship try to fix some design problems. Resistance will be higher.
     
  9. Alexanov
    Joined: Feb 2003
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    Alexanov Senior Member

    Here is some example of wrong existing ship rebuilding. (Wrong bulbous bow shape). In some conditions resistance uncreased dramatically. Owners use it after rebuilding 10 years. You can imagine how much extra they paid. You boat much smaller, but extra resistance can be significant, if ventilation plates is not inline with water flow.
     

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  10. Bornyesterday
    Joined: Aug 2020
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    Location: Southern california

    Bornyesterday Junior Member

    Alexanov,

    Your reply validates my suspensions.
    Not being close to a naval architect and even born yesterday it visually looked like something was wrong and the fix could be a bad brandade making things worse.
    Maybe Benetti is getting more credit than they deserve or the owner had to much money and time on his hands because the other three sister ships have not been modified.

    I am not familiar with CFD but I imagine it has to do with a drag coefficient.
    Do you think this could slow her down more than a knot? I realize fuel consumption would be higher ... its just how much.

    Alexanov thank you for sharing your knowledge with me I hope it comes back to you 10 fold.
     
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