Encounter Frequency and Encounter Period

Encounter frequency is the rate at which a ship moving through a wave system meets successive wave crests, and it differs from the absolute wave frequency by a Doppler shift that depends on ship speed and on heading relative to the waves. It equals the wave frequency only when the ship is beam-on to the sea, is higher in head seas, and is lower in following seas, where it can fall to zero when the ship travels at the wave celerity.

The quantity matters because every resonance a ship can suffer is defined in terms of the encounter frequency rather than the wave frequency. Synchronous rolling occurs when the encounter period matches the natural roll period, and parametric rolling occurs when the righting arm varies at a frequency that is a simple multiple of the roll frequency, both of which the bridge controls only through course and speed.

The full article will cover the derivation of the encounter relation from ship speed and heading, the behaviour in head, beam, quartering and following seas, the singular case at the wave celerity in a following sea, the encounter spectrum in irregular waves, and the practical use of the relation in avoiding resonance.