Backstays are generally adjusted by block and tackle, hydraulic adjusters, or lines leading to winches. Smaller electrically driven winches were installed beside each mast. Winches and capstans are machines designed to pull ropes. Motorised winches, electronic navigation and sonar systems are usually installed. Large ships generally tighten their mooring lines using heavy machinery called "mooring winches" or "capstans". The vessel can employ one of two winches for towing operations. The Chinese used winches for large mounted crossbows. These and other launch methods require assistance and facilities such as airfields, tugs, and winches. Some types, originally used as sail winches for model yachting, can rotate continuously. Bullbars are sometimes used as a platform for mounting spotlights and vehicle recovery winches. Later mechanical winches and slipways were introduced. Locomotives and cars were lifted with the assistance of horses and steam winches. The lines can be hauled in manually or by small winches. These are known as "self-tailing" winches. Other drawing mechanisms such as winches and oxen were also used.