I don't get what you don't understand. The sinking ship effect is where ships, as they go beyond the horizon, appear to sink starting with lower most portion and as they get further away so too then goes the top as it appears to sink below the horizon. They don't just suddenly disappear. This is an expected occurrence on a round earth. Mirages don't do this. They get fuzzy and parts of them or all of them will suddenly vanish. These are two distinctly different observations.