You cetainly dont need to go under the lifting bridge to stop off overnight in Weymouth itself. We had a berth on the south side , known as the Cove (NOT the aforementioned pub). A little noisy as still pretty close to Condors and fishing vessels, but at least not cheek by jowl with them & no restrictions on departure. I believe there isnt much in the overall timings/distance rounding Portland Bill westward between Weymouth & Portland harbours.