The Question:

At what length does a boat become a ship?

The Answer:

The determining factors that distinguish boats from ships has evolved over time. The term "ship" formerly referred to a sailing vessel having three or more masts. In modern times, however, a ship is usually a vessel of more than 500 tons of displacement.

Displacement tonnage is a way to measure a ship's capacity. The number, expressed in tons, is the weight of a vessel as it floats, or the weight of the water it displaces. By a common definition, a boat is small enough to be carried by a ship. And all submarines are called boats, regardless of their size.

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—The Fact Monster