Stiffness matrix and translation

[mancellin]

I'm not actually sure if this is a bug or a feature that I don't understand:

from meshmagick.mmio import load_mesh
from meshmagick.mesh import Mesh
from meshmagick.hydrostatics import Hydrostatics

reference_mesh = Mesh(*load_mesh("SEAREV.hst", 'hst'))

for shift in range(0, 100, 20):
    shifted_mesh = reference_mesh.copy()
    shifted_mesh.translate_x(shift)
    print shift
    print Hydrostatics(shifted_mesh, cog=(shift, 0, 0)).hs_data['stiffness_matrix'][2, 2]
    print Hydrostatics(shifted_mesh, cog=(shift, 0, 0)).hs_data['Iyy']

The inertia matrix is invariant when the body is translated in the x direction. I would have expected the same to be true for the stiffness matrix. Are the rotation dofs defined around the same axis in both cases ?