A reference point from which we start measuring angles would be handy, for a start. I'd suggest that 0° would usefully refer to a mast or funnel that sits perpendicular to the deck and when the ship is at rest on a calm surface. Capsizing 90° to either port or starboard would then see the mast parallel to the water's surface, while 180° would see the mast pointing straight down, i.e. a precise 'turtling'.
But Vixen actually requires maximum confusion, so I'm not expecting her ever to be systematic about these things.
Why would 0° usefully refer to a mast or funnel that sits perpendicular to the deck and when the ship is at rest on a calm surface?
No, in simple terms, it doesn't mean "capsizing 90° to either port or starboard would then see the mast parallel to the water's surface, while 180° would see the mast pointing straight down, i.e. a precise 'turtling'."
Forget about listing for a moment.
Think of a number between say 10 and 260. Draw a circle. Draw a straight line which passes through the middle. Jot down your number at one end of your straight line. If the number you chose was less than 180, add this to 180 and jot down the total at the other end of your line. If your number was more than 180, subtract it from 360 and jot it down.
Now draw a line that bisects the other line at right angles. This will be 90°
away from the start and end of the other two lines, whatever the figure isyou have.
So, 10°, 20°, 30°, etcetera, will always be 10°, 20°, 30° relative to that point, no matter what configuration you have of your circle, whether it is upside down or back to front.
So, the centre of gravity is not necessarily bang in the physical middle of an object. It lies at the average where all of the points of its average weight lies. So we call this point – where there is perfect balance, the centre of gravity.
So imagine you now have a boat floating in water, which is not flat, but has a rounded hull. Depending on how its cargo is loaded, the centre of gravity is not necessarily in the middle. If it floats, it means there is a centre of buoyancy, also not necessarily at the middle of the boat. (Albeit the crew will aim for trim so that it is.) There is gravity which is perpendicular to the boat (straight downwards, regardless of where it is) and buoyancy which pushes upwards, also perpendicular. The metacentre is the imaginary straight line that passes through the point where the centre of gravity line and the centre of buoyancy line meet and this will vary depending on the angle that the boat is listing in. It is not necessarily upright. As the vessel lists to one side (Z) , say to starboard, say by a movement of one unit downwards, the centre of gravity moves likewise as the weight average is now weighted to its right and the centre of buoyancy moves likewise, thus it carries on floating. This is because the rotational force, the righting arm, exerts a momentum pulling the list back into equilibrium. The more the boat lists, say by two units and then three, this righting force weakens. There comes a point where the boat capsizes owing to the angle of the list, and the side of the ship hitting the surface of the water and concomitant surge of water flooding into it, after which point it flips, Z becomes a rotational force in the opposite direction from the centre of gravity. We have negative stability at the point Z reverses with G. Once toppled over, the boat will continue to interplay gravity versus buoyancy, but this time, whilst upside down. Because there is lesser list whilst upside down, due to the hull shape limitation, one could say a boat is more stable in this position than when it is upright.
So, if we call the centre of gravity in its upright position 0°, the angle of list is relative to this centre of gravity not to an abstract right angle or where the mast stands upright.