Tugs or tugboats are used to manoeuvre vessels which are unable to move on their own due to being in a narrow canal or a crowded harbour. Tugs achieve this by pushing or pulling the vessel, either through direct contact or with the help of a tow line. Tugs are also used to move disabled boats, log rafts or oil platforms. Tugs are powerful and strongly built with most modern large tugboats averaging a bollard pull of around 60tonnes. Some tugs have been built and designed to serve not only as a conventional tugboat but also as icebreakers and salvage boats. Most tugboats are also fitted with firefighting monitors, so they can help assist with firefighting when necessary. Tugs are extremely manoeuvrable and new innovative propulsion systems are being developed all the time. ASD (Azimuth Stern Drive) and VSP (Voith Schneider Propeller) both provide the vessel with extra manoeuvrability and while also increasing electrical efficiency and lowering maintenance costs.