What is an synecdoche metaphor?

Synecdoche is when a specific part of something is used to refer to the whole, e.g. "my wheels" for "my car". It is usually understood as a specific kind of metonymy. A simple sentence that displays synecdoche, metaphor, and metonymy is: "Fifty keels ploughed the deep", where "keels" is the synecdoche, as it names the whole (the ship) after a particular part (of the ship); "ploughed" is the metaphor, as it substitutes the concept of ploughing a field for moving through the ocean; and "the deep" is the metonym, as "depth" is an attribute associated with the ocean.