Δοκάρια που τρικύμιζαν = beams which were shaking.
Τρικυμίζω can refer to something which is moving rhythmically, back and forth in a wave-like or undulating fashion, usually due to strong winds (eg. picture a flag undulating in the wind).
τρικυμίζω (ρήμα)
κινούμαι κάνοντας έντονους...