A propeller typically serves as the sleeve device for Merchant Marines serving in the Engine or Engineering Department of a vessel. The image has been used over the decades for a variety of jobs associated with machinery or engineering in both the Coast Guard and Navy, including Machinist’s Mate in the Coast Guard (until 1974) and Navy (still in use), but with a single blade down instead of a single blade up.
But the Merchant Marine rank system does not follow that of the Navy or Coast Guard, with different titles for different numbers and sizes of stripes. Civil Mariners serving in the Engine Department of a Military Sealift Command vessel, for example, are ranked lowest to highest as Third, Second, and First Assistant Chief Engineer, with four stripes designating Chief Engineer—but the two stripes are the equivalent of a Lieutenant (junior grade) rank in the Navy or Coast Guard, i.e., one half-inch and one quarter-inch stripe.