Ships lengthening

Discussion in 'Hydrodynamics and Aerodynamics' started by Alexanov, Jun 6, 2020.

  1. Alexanov
    Joined: Feb 2003
    Posts: 176
    Likes: 20, Points: 28, Legacy Rep: 10
    Location: Norway Sola

    Alexanov Senior Member

    How to predict speed of lengthen vessel if original vessel 1o9 m speed was 12 knots and we add 3om in parallel midbody? Engine and propulsion same as for original ship.
     
  2. Alik
    Joined: Jul 2003
    Posts: 2,833
    Likes: 192, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I would do it this way:
    - calculate resistance by existing method, say Holtrop,
    - 'anchor' these results to original ship performance - say, by coefficients and correction factors
    - calculate the new ship using Holtrop method and correction factors from old ship
     
    Alexanov likes this.
  3. Alexanov
    Joined: Feb 2003
    Posts: 176
    Likes: 20, Points: 28, Legacy Rep: 10
    Location: Norway Sola

    Alexanov Senior Member

    I think about something like that... thanks.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,453
    Likes: 530, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    What method/procedure did you use to calculate the resistance of the original 109m length ship?
     
  5. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    If you use the Holtrop-Mennen method you must recalculate the following values:
    • Waterline length
    • Length between Perp.
    • Moulded Draft at Lpp / 2
    • Displacement
    • Longitudinal C.of B. from AP
    • Center of bulb area above keel line
    • Waterplane area coefficient
    • Prismatic coefficient
    • Transom submerged area
    • Hull wetted surface
    • Bilge keels area
    In other words, there is no way to extrapolate the results from the initial boat to the new boat. You must perform the calculation as if it were a new boat.
     
    Last edited: Jun 7, 2020
  6. Alexanov
    Joined: Feb 2003
    Posts: 176
    Likes: 20, Points: 28, Legacy Rep: 10
    Location: Norway Sola

    Alexanov Senior Member

    CFD calculations.
     
  7. Ad Hoc
    Joined: Oct 2008
    Posts: 6,453
    Likes: 530, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Then why don't you simply rerun the CFD?
    With your new data points.
     
    Alexanov likes this.
  8. Alexanov
    Joined: Feb 2003
    Posts: 176
    Likes: 20, Points: 28, Legacy Rep: 10
    Location: Norway Sola

    Alexanov Senior Member

    Finally we decided to calculate CFD for short vessel at required speed and long one with the same power to get new speed.
    Probably it will be most correct method.
     
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 6,453
    Likes: 530, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    That depends if you validated your first set of CFD results.
     
  10. Alik
    Joined: Jul 2003
    Posts: 2,833
    Likes: 192, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I would not trust any CFD only. Systematic series or regression methods, then CFD. Usually, if there is no tan test data, we do resistance curves by 2-4 methods (regression of systematic series), then CFD to finalize, say, stern shape.
     

  11. Alexanov
    Joined: Feb 2003
    Posts: 176
    Likes: 20, Points: 28, Legacy Rep: 10
    Location: Norway Sola

    Alexanov Senior Member

    We compared resistance value from CFD and several other sources. It’s quite close. Usually resistance prediction for ships with length/beam > 7 works very well in many different methods.
     
    Alik likes this.
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.