Joke on me. I thought the "thimble-skein" were two parts of the axle and wheel of the wagon. It turn out both words mean the same thing, a metal tube on end of axle to strenthen it and also minimize friction.
The original text said each wagon had a tar bucket, and frequently wheel had to be taken off the axle so lubricant could be applied. I wonder what the source of the tar was. I should think salvaged cooking fat could have been used.

Skein
(Skein) n. [OE. skeyne, OF. escaigne, F. écagne, probably of Celtic origin; cf. Ir. sgainne, Gael. sgeinnidh thread, small
twine; or perhaps the English word is immediately from Celtic.]

1. A quantity of yarn, thread, or the like, put up together, after it is taken from the reel, — usually tied in a sort of knot.

A skein of cotton yarn is formed by eighty turns of the thread round a fifty-four inch reel.

2. (Wagon Making) A metallic strengthening band or thimble on the wooden arm of an axle. Knight.

Dear WW: I know of several sites for nautical terms. I'll PM them to you.