# How to derive the moment of a floating body about a rotation axis on itself?

Hi,

I'm trying to simulate the motion of a floating body in waves. At present, I have only found the calculation method about the fixed rotation axis, but I want to derive the moment of the floating body about its own rotation axis. How can it be achieved?

-momentaxis:x1:y1:z1:x2:y2:z2 Axis for moment calculation

Best regards!

## Comments

One option is to use coupling with Chrono... perhaps you can activate collisions even if there is no and you are not interested on that...ç

but in this way motions and rotations are also computed by Chrono and some output files from Chrono will include mx, my, mz....

You can check if that is what you need.... since those moment values are computed using x,y,z reference axis.... and not your own rotation axis...

In any case, let us ask @imartinezestevez and @jmdalonso

Regards

I am interested in this too, Chrono out of the box has some nice but also some subpar features. For examples if you lock y-translation in Chrono, the force in y will ALWAYS be zero, even if you hit the object with a big wave in y-direction. This of course makes "sense" since ForceWave = ForceObject, but it would still be nice to know the magnitude of it, without having to do fluid forces post-processing.

Similarily for the moment axis calculations it would be nice if one was able to define more than one axis at a time, else one has to keep re-running the forces post-processing. Chrono only gives moments around COG in global coordinate system and can be hard to use.

Kind regards

Thanks for your answers.

Is there a post-processing command that can extract the moment of the horizontal or vertical fluid force on a rotation axis separately? Do I need to start Chrono?

Using ComputeForces with the MomentAxis option seems viable. You just have to post-process and extract the axis of your floating body for each time step after the simulation has run.

Kind regards