She may not be the prettiest boat, nor the fastest. But when Claire (formerly known as Anna) glides into the Winthrop port several times each morning, hers is a distinctive chug. For those who use it, this Winthrop-to-Boston commuter ferry is a welcome alternative to more traditional transit options: taking a bus and the T or driving into Boston through the trafficky corridor north of the city. Indeed, the 25-minute trip is as close as you’ll get to traveling as the crow flies. (Some other coastal commuters have their own such boats.) As of this writing, Claire, operated by Boston Harbor Cruises, sails from late May through mid-October. The town of Winthrop, whose subsidy keeps the fares low, has been mulling expanding the service to year-round. The rumble of Claire’s three diesel engines is so unusual, says Winthrop resident Eva MacLeod, a 53-year-old business analyst for a financial services firm, that she listens for it from her house as she’s getting ready for work. “I can hear her enter the channel coming back, and I have just enough time to throw clothes on and go flying out the door,” MacLeod says. Claire navigates through the moored sailboats and fishing trawlers, pulling into the dock as commuters descend the ramp to meet her. After everyone’s aboard, she gives three blasts of the horn and then she’s off, headed for the glistening city beyond, just visible past the runways of Logan Airport.