Increase Length, decrease skin friction?

Discussion in 'Hydrodynamics and Aerodynamics' started by designing, Mar 27, 2017.

  1. designing
    Joined: Mar 2017
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Brazil

    designing New Member

    Dears,

    I was studying about resistance in this material found online:

    https://www.usna.edu/NAOE/_files/documents/Courses/en400/TEXT%20-%202011/12-Chapter%207%20Text%202011.pdf

    and there is an affirmation saying that "Note from the above equations that for a given speed, as ship length increases the skin friction (tangential) component of viscous resistance decreases."

    The "above equations" are the Reynolds number formula and the friction resistance formula. Taking for example the ITTC 1957 formula for Cf, the reynolds number's log is used, which for me seems to be almost insensitive to length variations of the hull, in the order of a couple of meters. Plus, if we think about the dimensional skin friction resistance

    Rf=Cf*0.5*rho*v^2*wetted surface area

    if we increase the length, the wetted surface area increases and so does the resistance. So coming back to the affirmation, what does it mean when relating increase of L to decrease of Rf?
     
  2. Mermaid Co
    Joined: Oct 2012
    Posts: 50
    Likes: 4, Points: 8, Legacy Rep: 10
    Location: Singapore

    Mermaid Co Junior Member

  3. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    As length increases, the Reynolds number also increases.
    As the Reynolds number increases, the coefficient of friction decreases.

    The result is that, for a given area, resistance will be less for a longer surface.

    However, if making the surface longer increases the area, the increased area can* equal or outweigh the reduction in coefficient of friction. In that case, the frictional resistance will increase, but it will not increase in direct proportion to the increase in area.

    *Often does.
     
  4. AusShipwright
    Joined: Aug 2016
    Posts: 39
    Likes: 5, Points: 8, Legacy Rep: 10
    Location: Australia

    AusShipwright Junior Member

    It is probably also important to understand the main resistance component for different ranges of froude number(low, medium, high).

    For instance, increasing length for high froude numbers increases the frictional resistance, but overall resistance decreases due to a reduction in wave-making resistance. (How much depends on hull form)
     
  5. jehardiman
    Joined: Aug 2004
    Posts: 2,609
    Likes: 259, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Actually what it said was the "component" of Rv (viscous resistance) caused by Cf decreases. It did not say the absolute Rv decreases. The entire quote is
    What this says is as the ship becomes longer while keeping the same beam and draft, then the proportion of Rv caused by Cf decreases. It says nothing about absolute Rv.
     

  6. SyuzVR
    Joined: Apr 2017
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Arizona

    SyuzVR New Member

    Thanks a lot for your informative posts. They helped me too.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.