* The most common is that ships were thought of as floating castles, and the word referred to the defensive "circles" or maru that protected the castle.
* That the suffix -maru is often applied to words representing something that is beloved, and sailors applied this suffix to their ships.
* That the term maru is used in divination and represents perfection or completeness, or the ship as a small world of its own.
* A legend of Hakudo Maru, a celestial being that came to earth and taught humans how to build ships. It is said that the name maru is attached to a ship to secure celestial protection for it as it travels.
* For the past few centuries, only non-warships bore the maru ending. It was intended to be used as a good hope naming convention that would allow the ship to leave port, travel the world, and return safely to home port: hence the complete circle arriving back to its origin unhurt.
a circle; round; spherical