Tweet It takes about 20 minutes for a fully loaded supertanker to stop when travelling at normal speed. In emergency situations, some can perform a “crash stop” maneuver, which can cut the time down to 14 minutes. The reason supertankers, also known as oil tankers, take so long to stop is the huge amount of momentum they build up as they sail. Because of this huge momentum, most supertankers turn off their engines about 15 miles (25 km) away from their dock. More facts about supertankers: • The average modern supertanker can carry more than two million barrels of oil. That’s enough oil to power all US auto traffic simultaneously for almost four hours. • The turning diameter of the average supertanker is about 1.2 miles (2 km). • Supertankers are among the largest ships in the world — much bigger than aircraft carriers. The longest ship ever built as of 2010 is the Knock Nevis. It is over 1,500 feet (548 m) in length, which is slightly longer than the height of the Empire State building