Tugs are ships with strong engines used to pull or push other ships or big floating objects. They are commonly seen in ports helping other vessels, but they are now also used offshore to move and position things like oil rigs, pontoons, and installation ships. Tugs are also used for rescue operations and dredging (digging and cleaning seabeds). There are two main types: ones with traditional engines, gears, and propellers, and others with special propellers that can rotate in any direction (called azipods or Voith Schneider drives). The strength of a tug is measured by bollard pull, which is how much weight in tonnes it can pull. Smaller tugs might have a bollard pull of around 30 tonnes, while larger offshore tugs can pull over 250 tonnes or more.