How is deflection modeled?


Question:

How is deflection modeled and what reference axis it is measured from?
Specifically:

Is KMT based on the original axis (mine is FP & baseline) ?  


Does the deflection affect where or how GMT is measured ?


Also, if LBP is set and deflection is zero at FP & AP, is "deflection overall"
due to vessel extending beyond FP & AP ? 


I tried to run GHS /KM and this caused a notation "deflection eliminated", is
this normal operation? 


Also, how is deflection modeled ?  Is the hull shape changed, or the waterline
changed to a "wave" of some kind. 


My run with deflection caused an increase in VCB ? If the hull were deflected
below the baseline, I would expect a decrease. 


Basically, is the change in GMT above based on deflected hull or some shift ?



Answer:


Your two GHS-command reports show different displacements; therefore, they are
not representative of the same condition.  This explains the difference in
VCB. Here is a command sequence you can use:

  LBP fwdloc, aftloc                                                                                 
  VCG vcg                                                                                            
  DRAFT ...                                                                                          
  SOLVE WEIGHT, LCG, TCG                                                                             
  GHS                                                                                                
  SOLVE                                                                                              
  GHS                                                                                                

The second GHS command will be for the same weight and CG; only the deflection
will not be present because the SOLVE command eliminated it.  (Any command
which changes the waterplane eliminates the deflection.)

The reported deflection value can be based on the overall vessel end locations
or the locations specified by the LPB or LWL command.  In order to have it
report deflection at the FP and AP locations, you must use the form of the LBP
command which gives these locations, not just the LBP length.

In answer your other questions:

KMt is always measured from the baseline and perpendicular to it.  But GHS
does not compute KMt with deflection.  The fundamental metacenter value is GM,
not KM.  (KM must be derived from GM.)  GM is always perpendicular to the
waterplane; i.e. a plane angled according to the present trim and heel.

Deflection is actually modeled by curving the waterplane, not the vessel.
Therefore, the axes remain straight; and a location such as the CG does not
change its coordinate values when deflection is added.  Therefore, GMt is
measured from the same "G" with and without deflection.