将下面英文翻译成汉语: In 1968 the first ship over 300,000 tons was built. With their bulk and enormous capacity, these ships are designed and built to be profitmakers, enormously long, wide, and deep, carrying thousands of tons of crude oil pervoyage a the least cost. Few of these elephantine tankers have more than one propeller shaft or rudder. Their navigation bridges are nearly one quarter of a mile from their bows. Their top service speed is so low that a voyage from an Ar oil port to a European destination normally takes two months.