I remembered when I was a kid, I wondered what those round thingy in front of the battleships (especially the WWII battleship Yamato) were. Now I know. It's called the bulbous bow.
Although the detailed physics is not known, bulbous bows are known to give around 12-15% gain in fuel efficiency for the range of speeds they are designed for. The gain is likely to be caused by destructive interferences of the waves in front of the ship and the waves that ride up the bow, giving less drag to the ship.
From the Wikipedia concerning its development:
It is unclear when bulbous bows were conclusively first examined, but research into the effects of bulbous bows was first published in the 1950s. Engineers began experimenting with bulbous bows after discovering that ships fitted with a ram bow were exhibiting substantially lower drag characteristics than predicted, and eventually found that they could reduce drag by about 5%. Experimentation and refinement slowly improved the geometry of bulbous bows, but they were not widely exploited until research conducted at the University of British Columbia in the 1980s increased their performance to a practical level. Bulbous bows were also developed and used by the Japanese before World War II. One example of this was on the battleship Yamato. This research does not seem to have been connected with western research in any way.
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