Listing
List, or heel, is the sideways inclination of a vessel from the vertical. It’s measured at midships and usually expressed in degrees using an inclinometer. When this isn’t available, the list can be calculated from the difference between the port and starboard midships drafts. The vessel rotates around its longitudinal axis through the longitudinal centre of flotation, which divides the underwater body into two equal halves.
Hydrostatic data assumes the vessel is upright. Because of this, the ship should have zero list when drafts and tank soundings are taken. In practice, list should not exceed half a degree. Anything larger reduces the accuracy of the entire survey. Reading drafts becomes harder, and determining ballast quantities becomes less reliable. Topside tanks are especially challenging when the ship is leaning.
When a vessel lists, the waterplane area increases and the ship rises slightly. This is why list correction, when used, is always positive. That said, the better option is almost always to remove the list rather than calculate it. Adjusting ballast or shifting weight can usually bring the list below half a degree. When it’s that small, the correction becomes negligible and should be ignored.
If the list is significant and a correction is required, a simple relationship can be used to estimate it based on the midships draft difference and the change in TPC between both sides. In rough conditions, a manometer can help determine list when outboard drafts cannot be seen. By comparing water levels across the deck and scaling for the ship’s beam, the true list can be derived and the outboard draft corrected.
