Sentence:  During about three and a half months of the year , in the summer , there are three boats that run from the mainland to the Island carrying passengers , food , and cars ; but the rest of the year only one boat is needed , which <WSD>ties</WSD> up at the mainland nights and makes the trip down to Nantucket in the daytime . 
Meaning of the word marked with <WSD> tags:  secure with or as if with ropes