I'm not sure about its meaning, but I sure do feel seasick! Also, the term, 'pendulum eight'? I searched You Tube and came up with a simulation where 4 pendulums are released just nano seconds apart, after swinging synchronous for a short time, they then create a chaotic behaviour of their own. Not sure if you meant this term as a simile?
The writer brought me into a place where I felt next to this sailor working and making mistakes in a precarious ocean. I especially loved the lines "The prow dives, rises and dives again / as swells lift the craft and drop away" (gripping!) and "in the darkening sky making idiocy / ever more apparent in the transit" (this line stands out at me). Great read.