Locks functioned as steps in the Susquehanna and Tidewater Canal, raising or lowering boats over changes in elevation. Twenty-nine lift locks raised boats a total of 231 feet between Havre de Grace, Maryland, and Wrightsville, Pennsylvania, and Lock No. 15 raised and lowered boats almost 9 feet. Its double lock chamber let tandem boats, two boats tied together and pulled by one team, pass through. Horses or mules pulled boats along the canal at a state speed of 4 miles per hour to prevent wakes from damaging the canal edges, while the driver walked the towpath and the steersman aboard handled the tiller and sounded a conch shell horn when approaching the lock. The team pulled the boat into the chamber, the lock tender closed the gate behind it, and then opened wickets in the downstream gate so water flowed out and the boat dropped as the water level subsided. Afterward, the downstream gate opened and the boat continued south, while boats heading up the canal went through the reverse process. Lowering a boat took about five minutes, raising one took a little longer, and a boat could be locked through every eight minutes. The work kept lock tenders busy; in 1855 alone, more than 5,700 boats passed south through the Susquehanna and Tidewater Canal.