My late grandfather was a stoker in WWII (HMS Whitehall), and my g.grandfather a stoker 1900-1912 on pre-Dreadnought battleships.
Usually, "promotion" from 2nd Class to 1st Class was - at least, in my g.grandfather's time - on conduct. My g.grandfather had a habit of being thrown into the cells for drunkeness, so found it difficult to maintain his 1st Class status.
My understanding accords with what's already been said: my g.grandfather would have spent a lot of his time shoveling coal, but my grandfather did more in the way of engine maintenance, reflecting the change from steam engines to internal combustion engines.