Their size and mass makes large ships very difficult to manoeuvre; the stopping distance of a supertanker is typically measured in miles/kilometers and even a slight error in judgment can cause millions of dollars in damage. For this reason, many years of experience in an operating area are required to qualify as a pilot.[citation needed] For example, the California Board of Pilot Commissioners requires that pilot trainees must have a master's license, two years command experience on tugs or deep draft vessels, and pass a written exam and simulator exercise, followed by a period of up to three years training gaining experience with all types of vessel and docking facilities. Following licensing, pilots are required to engage in extensive continuing educational programs.[8]
By far the most challenging part of any ship's voyage is the passage through the narrow waterways that lead to port and the final docking of the ship. The pilot brings to the ship expertise in handling large vessels in confined waterways and expert local knowledge of the port. In addition to bringing local maritime expertise on board, unlike the vessel's captain the pilot is insulated from the economic pressures (e.g., getting the ship from point A to point B on time, regardless of weather conditions, traffic, or other navigation issues) that can compromise safety. Instead of being part of the ship's crew, pilots are employed locally and therefore act on behalf of the public rather than of the shipowners.