LCB Def'n - OCR VPP vs Delft

Discussion in 'Hydrodynamics and Aerodynamics' started by DCockey, Aug 4, 2011.

  1. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    (Note - this message is not about the fundamental definition of LCB but about different ways to define its numerical value.)

    What is the definition of LCB used in the residuary (wave) resistance formula in the OCR VPP, eq 70, p 53 of OCR VPP Documentation 2010? http://www.orc.org/rules/ORC VPP Documentation 2010.pdf
    The formula is based on the Delft test series, but is written in a slightly different form than what has been published by Delft.

    - It appears to be non-dimensional, presumably by L. The Delft formulas use a dimensional LCB.

    - Where is the origin? The Delft formulas use LCBfpp defined as distance from the forward perpendicular. The table below the OCR VPP formula has the Maxima and Minima for various terms in the formula and for LCB/(L/VOL^1/3) the range is .85842 to -.08296. Since VOL (volumetric displacement) is always positive that means LCB must go negative. So it probably is defined from the center of the boat, not from the forward perpendicular. Edit - See my message below. Maxima appears to be spurious.

    - Which way is positive? My understanding is usual convention is towards the stern is positive, but Delft uses towards the bow as positive in characterizing their hulls. This is discussed in the second half of a previous thread http://www.boatdesign.net/forums/sailboats/data-vpp-3360-2.html Edit - See my message below.

    - To further confuse matters the formula for added resistance due to wave in the OCR VPP also uses LCB, eq 83, p 62. The valid values of LCB for this equation are listed as 0.50 - 0.56, which would be non-dimensional, fraction of L from the forward perpendicular. Edit - See my message below.

    - Either different definitions of LCB are used in the residuary resistance equation, eq 70, and the added resistance due to waves equation, eq 83, or the permissable values for LCB in equation 70 are incorrect and should not go negative. Edit - See my message below. The Maxima value appears to be spurious.

    My assumption is the OCR VPP formula for residuary resistance uses a non-dimensional form of LCB, measured from the mid-point of the LWL, positive aft, and the added resistance due to waves equation uses LCB measured from the forward perpendicular. Edit - Revised to: uses a non-dimensional form of LCB, measured from the forward perpendicular, the same as the added resistance due to waves equation.

    Can anyone confirm this?
     
    Last edited: Aug 4, 2011
  2. DCockey
    Joined: Oct 2009
    Posts: 5,005
    Likes: 490, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    I've done some thinking about this and concluded that the Maxima value of .85842 for LCB/(L/VOL^1/3) is spurious, and therefore the negative Minima value is suspect.

    L/VOL^1/3 is always considerbly greater than 1.0. So for LCB/(L/VOL^1/2) to be .85 LCB would need to be considerably greater than 1.0 which is impossible (assuming it is a non-dimension fraction).

    So I'm now thinking that LCB in the residuary resistance equation of the OCR VPP is the non-dimensional, fractional distance from the forward perpendicular.
     
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