Free Surface Effect

Discussion in 'Stability' started by QUICKNAVAL, Jan 23, 2013.

  1. QUICKNAVAL
    Joined: May 2012
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Philippines

    QUICKNAVAL Junior Member

    Please Help. I Want Just To Clarify On How To Calculate The Free Surface Effect Of Liquid (s.w.) In A Certain Tank That Has Swash Bulkhead?

    Say For Example The The Tank Dimensions, Length= 5m ; Breadth=3m
    I Solve The Free Surface Moment Automatically By By Using (5)*(3^3)*(1/12)*(1.025). Not Considering The Factor Reduce By Swash Bulkhead.

    Note: Swash Bhd Location - Mid. Long'l. Of The Tank


    Your Kind Suggestion And Ideas Is Much Appreciated.:)
     
  2. jehardiman
    Joined: Aug 2004
    Posts: 2,489
    Likes: 202, Points: 63, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Are you looking for the moment shift (ultimate stability) or GM reduction (initial stability)?

    For moment shift, you need to work up a set tank curves for all your stability angles (which will include heeled free surface at large angles of heel).

    For initial GM reduction, the corrected inertia of the tank is (LB^3/12) divided by the specific volume (tonnes/m^3). The GM reduction for an individual tank is then the corrected inertia divided by the vessels volumetric displacement.

    Generally, swash bulkheads are ignored. While they do have some effect on initial statbility at certian roll frequencies, those effects are tank capacity dependent. Swash bulkheads have on effect on ultimate stability. See Principles of Naval Architecture, Chapter 2, section 5, Effect of Free Liquids and Special Cargos.
     
  3. QUICKNAVAL
    Joined: May 2012
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Philippines

    QUICKNAVAL Junior Member

    Thank You So Much. Your Reply Helps A Lot To Answer My Doubt. Actually I Am Making Stability For A Floating Dock. As A Newly Grad. Naval Architect,,,i've Still To Learn More.


    :)
     
  4. jehardiman
    Joined: Aug 2004
    Posts: 2,489
    Likes: 202, Points: 63, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    A floating Dry Dock? Or just a floating pontoon? FWIW, water balasting a floating pontoon that will have a live load (i.e. mooring loads, people or cargo handling) is a bad idea. Use solid ballast for that.
     
  5. QUICKNAVAL
    Joined: May 2012
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Philippines

    QUICKNAVAL Junior Member

    Yes a floating dry dock (100m x 32m) with 4 towers on its corner (fwd and aft). Although it's very tiring to my brain,,,i just consider it a challenge.
     
  6. Convenor 12217
    Joined: Jan 2013
    Posts: 9
    Likes: 1, Points: 0, Legacy Rep: 13
    Location: Southampton, UK

    Convenor 12217 Convenor 12217

    Jehardiman (post #2) is right.
     
  7. TANSL
    Joined: Sep 2011
    Posts: 5,801
    Likes: 202, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Do not worry, a floating dock is like any other boat, with the advantage that their tanks are usually rectangular and it is much easier to define and calibrate. The hydrostatic curves and KN have discontinuity when the dock submerges its deck, but no calculation (to my knowledge) different or more difficult than, for example, a dredger when opens its bottom to discharge.
    Cheer up, it's easy. If you have a program of naval architecture, the calculation is as fast as a traditional boat.
     
  8. jehardiman
    Joined: Aug 2004
    Posts: 2,489
    Likes: 202, Points: 63, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yep, the hardest and most critical thing with a wing wall floating drydock is making sure the Tank Level Indicators (TLIs) work correctly. If they don't you can break the dock...or topple the docked ship...(and so in there hangs a tale...that I can't tell, LoL...) Just make sure they are working correctly and monitor their actual soundings ALL THE TIME!
     

  9. QUICKNAVAL
    Joined: May 2012
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Philippines

    QUICKNAVAL Junior Member

    Thank you so much for your ideas. I'm already done with my calculations.
     
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.