Side-to-side. There's a certain design range where it's not too fast and not too slow, and some inspectors check for it. https://en.wikipedia.org/wiki/Metacen

Side-to-side. There's a certain design range where it's not too fast and not too slow, and some inspectors check for it. https://en.wikipedia.org/wiki/Metacentric_height#GM_and_rolling_period https://en.wikipedia.org/wiki/Metacentric_height#GM_and_roll... Also, there's random motion that may look like roll when it's the sea surface that's showing the motion, so there are methods to filter that out and extract the actual rolling period.