A pilot is a mariner who manoeuvres ships through dangerous or congested waters, such as harbors or river mouths, and completes the berthing / unberthing operation of the ships by controlling the ship's manoeuvrability directly and the tugs and shore linesmen through a radio. Pilots are expert shiphandlers who possess detailed knowledge of local waterways. They are transported by high speed pilot boats or helicopter from shore to an inbound ship and from an outbound ship back ashore. Most ports have compulsory pilotage.