The difference between Crow's nest and Main-truck
When used as nouns, crow's nest means a small open-top shelter, originally a cask, on the top of the foremast, large enough to accommodate a lookout. it was used by whalers to watch for a blow (spout), or in icebound waters to seek a channel, whereas main-truck means on a wooden mast, a circular disc (or sometimes a rectangle) of wood near or at the top of the mast, usually with holes or sheaves to reeve signal halyards.
check bellow for the other definitions of Crow's nest and Main-truck
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Crow's nest as a noun (nautical):
A small open-top shelter, originally a cask, on the top of the foremast, large enough to accommodate a lookout. It was used by whalers to watch for a blow (spout), or in icebound waters to seek a channel.
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Main-truck as a noun (nautical):
On a wooden mast, a circular disc (or sometimes a rectangle) of wood near or at the top of the mast, usually with holes or sheaves to reeve signal halyards; also a temporary or emergency place for a lookout.