calculating torque with buyouant force and hardly definiable arm

Hej Guy,

Here I am again with a question on torque basically.

Please see the image below.

I have a half tube underwater. The tube have air inside. Now I would like to calculate the torque for this system. As you see the tube will move to a balanced position, but I don't how much torque it will have on the axis of rotation..

My question is it legal to approximate the torque as you see in the 3rd image that I divide the tube volume and use those volume segments with the distance to the center.

I am having this problem because basically inside nothing can act as an arm, but the tube have its volume and this volume have distance from the center of rotation and well the volume creating the buoyant force...

Or is it just a whole volume as a force upwards and the toque is basically the Torque =buoyant force x center-cylinder-radius ? This seems to be not valid for me.

Anybody can help me on this?
Thanks in advance!