The Coast Guard and the Steamship Authority are investigating what caused a ferry carrying 78 passengers to lose power shortly after leaving Martha’s Vineyard last night.The ferry’s main engines lost power about 15 minutes into a trip to Woods Hole after leaving Martha’s Vineyard about 8:30 p.m. last night, according to Steamship Authority General Manager Robert Davis.Two Coast Guard vessels were dispatched to the scene and three tugboats brought the ferry back to Vineyard Haven about 2 a.m.