1 The name given to that portion of the mast of a large sailing vessel where an upper mast overlaps the lower, as a topmast with a lower mast, or a topgallant mast with a topmast. It is a word more often used in the plural than in the singular, as in normal square-rigged sailing vessels each mast will have two doublings.
2 The operation of covering a ship with extra planking or plates when the original skin is weak or worn. In wooden ships the term was only used when the new planking was more than 5 centimetres (2 in.) thick.
3 Doubling a cape or other point of land means sailing a vessel round it so that on completion the land is between the ship and its original position. The verb, to double, can also be used for the last two definitions.