Synchronous Rolling

Synchronous rolling is roll resonance in beam seas, occurring when the wave encounter period matches the ship’s natural roll period so that each passing wave applies a transverse moment in phase with the roll motion. Unlike parametric rolling, it is driven by a direct external force rather than by a change in the ship’s own stability, and it builds once per roll cycle rather than twice.

The distinction matters operationally and forensically. The joint investigation into the MSC Zoe container loss of 1 and 2 January 2019 expressly excluded parametric rolling because the ship was in a beam sea, and attributed the losses instead to high stability producing resonant roll at design-limit transverse accelerations in shallow water. A ship with a very high metacentric height has a short natural roll period, which brings it into resonance with the shorter wave periods common in shelf seas.

The full article will cover the resonance condition and its dependence on metacentric height and beam, the roll amplification factor and the role of damping, the difference from parametric rolling in mechanism and in sea direction, the treatment of the phenomenon in MSC.1/Circ.1228, the reason the 2008 Intact Stability Code sets only a minimum metacentric height and no maximum, and the escape actions available to a master.