Calculating frictional drag

Discussion in 'Boat Design' started by Cpalm, Oct 5, 2011.

  1. Cpalm
    Joined: Mar 2011
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: USA

    Cpalm Junior Member

    What are the common drag coefficients used for calculating drag from wetted surface?
     
  2. philSweet
    Joined: May 2008
    Posts: 2,466
    Likes: 270, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    What are you trying to do? Post a picture of the surface and explain how it moves and describe what it is moving through. You might want to shift to the aero and hydro subforum here and scan the threads. There are quite a few pointers to relevant material there.
     
    1 person likes this.
  3. Cpalm
    Joined: Mar 2011
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: USA

    Cpalm Junior Member

    Basically I am trying to estimate the frictional drag from the wetted surface on my boat, moving forward through water.

    Using this drag formula:

    D = 1/2*fluid density*velocity^2*Area*drag coefficient.

    I assume there is a drag coefficient for water against common hull surfaces (bottom paint, gelcoat, etc.)

    Is this the correct approach?
     
  4. DCockey
    Joined: Oct 2009
    Posts: 4,997
    Likes: 488, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Clean bottom paint vs clean gelcoat will have a negligable difference in drag. Fouled bottoms are a different story.

    Area for ship and boat frictional/viscous drag is usually defined as the wetted area.

    Most common drag coefficient used for boats and ships is ITTC 1957 which is based on Reynolds number:
    Cf = 0.075/((log(Re)-2)^2)
    It's around 12% higher than the Cf for an equivalent flat, rectangular plate. The difference is sometimes claimed to roughly account for additional viscous drag due to 3D effects. There are different thoughts on what length to base the Reynolds number on. Simplest is static LWL. 70% of static LWL is frequently used for sail boats.

    Frictional/viscous drag generally dominates up to a Froude number of 0.3 or so. Wave making drag becomes dominant at higher Froude numbers.
     
  5. philSweet
    Joined: May 2008
    Posts: 2,466
    Likes: 270, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    We're making progress. Is it a motor boat or a sail boat. What does it weigh, how long is it, and what speed is it going. Is it on plane, semi planing, or non planing. Photos would really help because it isn't nearly a simple as you're hoping it to be. If you are trying to figure out how much power you need just say so. That is a lot easier.
     

  6. Ad Hoc
    Joined: Oct 2008
    Posts: 7,047
    Likes: 975, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    That depends upon the L/B ratio and L/D ratio.

    Very long slender hull forms have minimal residuary compared to frictional resistance. Here it is the opposite...

    As always, horses for courses.
     
Loading...
Similar Threads
  1. Raptor88
    Replies:
    24
    Views:
    619
  2. Jo Ho
    Replies:
    7
    Views:
    421
  3. Peter Binns
    Replies:
    40
    Views:
    2,051
  4. thenavalarch
    Replies:
    0
    Views:
    509
  5. SBarlow
    Replies:
    10
    Views:
    1,337
  6. thenavalarch
    Replies:
    0
    Views:
    776
  7. thenavalarch
    Replies:
    0
    Views:
    2,049
  8. Annode
    Replies:
    11
    Views:
    6,998
  9. Pammie
    Replies:
    107
    Views:
    7,901
  10. Jmooredesigns
    Replies:
    13
    Views:
    1,682
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.