Yes, it's "rattlings".
As someone pointed out already, English
"sailorspeak" is closely related to the sailorspeak in the languages of the sundry ethnic/linguistic groups that ring the North Sea.
I think it's a safe assumption, therefore that the "ing" part of the word "ratlines" corresponds exactly to the "ing part of the Scowegian word "vævlinger" and the German/Frisian/Platdeutsch word "Wefflingen". An older German word for ratlines was "Webelienen" ("Weaving lines")
The grammatical function of the "ing" suffix is to transform a verb into a noun rather like gerunds do in modern English
. Thus the Scowegian word "vævling" (a single
"crossline") is the verb "to weave" transformed into a noun. Just exactly analogous to German "Webelienen".
"Rat", in the modern English word "ratlines, is, I believe, a corrupted form of the word "raddle". "A raddle" is a weavers tool used to assist in spacing the "warps" (threads that run fore'n'aft) on a loom, just exactly as ratlines "space" the shrouds on a sailing ship.
So there you have it: Clear and concise. The pronunciation is most definitely "rattlings" :-)
Selv tak :-)!