As I mentioned earlier, I'm personally not much interested in etymology with respect to understanding the chantey. For purely academic interest, however, here's one more scrap to add to the pile. from a short article in American Speech vol 2:12, 1927, "More Hobo Lingo":

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"Bull" as an adjective commonly means "large." In logging operations with steam donkeys the big hook on the end of the main cable is the "bull hook." This may be the explanation of "bullgine" for engine, too.
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