Beam sea

A beam sea is a sea or swell train running at roughly 90 degrees to the heading, striking the ship on the beam. It is the condition most likely to drive heavy rolling, because the wave encounter period can approach the ship’s natural roll period and excite synchronous or resonant roll. A master in a beam sea alters course or speed to shift the seas to the bow or quarter, the standard avoidance for resonant and parametric rolling. IMO MSC.1/Circ.1228 gives the guidance, and the polar diagram of roll response against encounter period is the planning tool. Beam seas also degrade course-keeping and can ship water on a low-freeboard deck.