Limiting KG analysis error with Additional Windage Area

Question

Hi,
For my model, in case of doing Limiting KG analysis, I get VCG, maximum KG value negatives which seem very weird. It happens when I specify additional area in criteria window like below:

However, If I select total area [model area+additional area] and its centroid, KG value looks fine. Could you please tell me why is this happening? Where is the problem please?

Moreover, I presume additional area option should be used in case of large angle stability analysis since the model windage area changes due to different loadcase and relevant drafts. However, I am confused in case of limiting KG analysis in intact condition as it doesn't care about the loadcase; should I use total area or additional windage area?

James Clarkson · Answer

Hi Ku Ru, 
 Thanks for letting me know you got it working. The only difference I can think is if you are calculating a new windage area at each angle of heel, then the two areas could actually be quite different (Edit | Preferences | Calculate Inclined Windage Profile) 
 The KG Analysis iterates the Equilibrium and Large Angle stability analyses to find the right KG value. You can create a loadcase (actually a loadgroup without tanks is best) with a single weight item and you should be able to run the LAS and Equi analyses to iterate it manually. 
 The Windage Area isn't treated any differently between LKG and LAS or Equi. So any result you can get in the LKG, you should be able to repeat in the LAS and Equi analyses. 
 LKG analysis does assume that a higher VCG is worse stability. However, this isn't always true. Rules limited by deck edge immersion or DF angles are largely independent of the VCG. Other rules such as the angle of Maximum GZ being greater than a value can become worse as the VCG gets higher. It is possible that there are multiple solutions and that different starting points will oterate to different values. 
 If you do work out what happened then please let me know. 
 
 Regards, 
 James