Content will not tilt around the centre

Good day, 
This is breaking my head and then some. I have content that need to rotate around its centre. I want to do this with this expression:

Rotating expression:

amplitude = 1; // Reduced for less tilting
frequency = 0.2; // Slower movement

rotation = amplitude * Math.sin(time * frequency * 2 * Math.PI);
-rotation // Invert the tilting motion to simulate correct movement

Position expression:

amplitude = 60; // Increased for more pronounced bobbing
frequency = 0.2; // Slower movement

y = amplitude * Math.sin(time * frequency * 2 * Math.PI);
value + [0, y]

When use this the rotation seems off centre.  What am I missing?

thanks.

Its part of a position and rotating expression where the movement of a ship  is simulated. The problem however is that the tilting doesnt seem to be happening from the centre, even as the anchor point is in the centre.