When the U.S. Merchant Marine fleet was operating directly under the U.S. Maritime Service, it employed a system of insignias and specialty marks very similar to the one used by the U.S. Navy. The three-bladed propeller, today used to indicate an Engineering officer in the Merchant Marine, was used on three types of marks or ranks. For enlisted ratings, it was the insignia for the Motor Machinist’s Mate rating, and was naturally also used for the Warrant Officer and Chief Warrant Officer Machinist specialty. Lastly, it was also employed as a branch-of-service, or corps device, for personnel in the Engineer branch. Although there were other branch-of-service officers—think of them as today’s staff officers—only Engineer officers were allowed to retain their insignia when serving as Captains.
Today, the U.S. Maritime Service exists only to facilitate training of mariners at the federally run United States Merchant Marine Academy and a half-dozen other state maritime academies. Graduates are licensed in Deck Programs and Engineering Programs, with Engineering graduates going on to obtain their Third Assistant Engineer license in the Merchant Marine.