If you say the axis what I defined as a body fixed axis, the body is changing continuously, so my horizontal is changing. Take a submarine it can dive here like that, so will you call this to be heave or the vertical to be heave right.
The axis are fixed to the body then
That is ok
Even then, no, ok fine, axis fixed to the body there is a ship here, it is the submarine is diving like that; axis is fixed to the body this x as this z. You are calling the motion along z axis is to be heave will you call now this is my vertical remember, this is my water water surface will you call this to be heave.
But inside the angle
Yeah, but will you call this heave when you say the ship is heaving.
I will select the reference.