Boston’s newest and most powerful harbor tug has been christened, crewed up and has started serving customers of Constellation Maritime Co. The tug Leo is a 78-foot, 5,000 horsepower Dolphin-Class vessel built at Foss Rainier Shipyard in Oregon. The brand new ASD-powered tug arrived in Boston on March 7 along with the Orion, a 3,000 horsepower, 95 foot Voith-powered tractor formerly in service for Foss Maritime on the Columbia River. Together, the two high-powered tugs give Constellation, a Foss subsidiary, a one-two punch that amounts to a significant competitive edge in handling ship assists, harbor work, and future tanker escorts.