non-physical heave response

[janerikhy]

What?

When simulating CSAD_DP_6DOF in different sea states, it seems to obtain a way too large heave response compared to the wave amplitude. The vessel will slowly but steadily increase its response - resembling that of resonance.

This is not what we observed in the experiments. Therefore, it should not be what we observe in the simulator either.

There might be multiple sources of error here, but it is assumed that the large response is due to the hydrodynamic coefficients selected for the model parameters.

[janerikhy]

The EOM of CSAD 6 DOF model seems to be implemented incorrectly. Currently, the EOM is defined as $$\dot{\nu} = M^{-1} (\tau - D\nu_r - G J(\eta)^{-1} \eta).$$

This formulation gives the large oscillations observed as mentioned in the issue description. It is in the last term where the stiffness matrix is multiplied with $J(\eta)^{-1}\eta$ the error is observed. When changing this term to $G\eta$, the simulation gives a much more reasonable response!