Port of Portland

Hello, My relatives, Ole C. Malterud and family, left Oslo on April 7, 1882 on the ship named "Rollo". I am trying to figure out what ship they traveled on after arriving in Hull, England. They state on citizenship papers that they arrived at Portland, ME on May 1, 1882 but I cannot locate any ships that went to that port on May 1. Would anyone be able to tell me if sometimes the ships just made random stops along the way or if the database is incomplete? Thank you, Mike