Oblique sea

An oblique sea is a wave system whose heading relative to the ship lies between head (180 degrees) and following (0 degrees), so bow-quartering and beam and stern-quartering seas are all oblique. The wave encounter frequency depends on heading mu through omega_e = omega minus (omega squared V over g) times cos mu, so beam seas (90 degrees) give the encounter frequency equal to the wave frequency. Oblique seas couple the symmetric motions (heave, pitch) with the antisymmetric ones (sway, roll, yaw), drive roll near resonance, and raise the risk of parametric roll and broaching in stern-quartering conditions. Seakeeping and load assessments sweep heading in steps, often 30 degrees, around the full circle.