What does it mean when a ship keels?

Answer 1

I believe the complete term is "keel over". This sometimes referred to as a knockdown. If the keel turns straight up out of the water, your boat has "turned turtle". If a large wave turns your boat over end-to-end, you have been "pitch-poled". These conditions are usually cause by rogue winds, rogue waves, or lack of attention to the sails and weather conditions. They do not necessarily mean that the boat will sink.

Another take:

"Keeling" is a malaprop -- it's not really a word. People often say "keeling" when they mean heeling. When a boat heels, it tilts laterally, with the wind.

The term "keeling over" is something else: it refers to a position when the boat shows its keel; the bottom-most part of the boat, which means it's turned turtle or gone upside down. So "keel over" is a legitimate term; keeling is not.