Sailing ships were the cutting-edge technology of the 18th century, and the shipbuilding boom spawned many phrases we still use today.

Loose cannon: Today, an unpredictable person. Then, a literal unsecured big iron tube that was going to hit something and cause a lot of pain. 

Batten down the hatches: Today, get ready for trouble. Then, literally get an iron bar to cover the door of a ship for terrible weather.

All hands on deck: Today, there's a problem and we need everyone here now. Then, it was the same, except on an actual ship's deck.

Show one's true colors: Today, a dishonest person who reveals who they really are. Then, a ship flying a fake flag to confuse enemies; pretty much the same thing.

Learn the ropes: Today, learn the rules and skills of a job. Then, literally learn how to tie and use ropes.