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Why does it look like this?
Surface irregularities near the point of H is one flat earth explanation for hull-first disappearance, but it doesn't work as a stand-alone explanation. You need to what and where H is.
H is Rowbotham's "natural law of perspective" explanation for where the earth's flat plane appears to rise to eye level. Geometrically, the convergence to that vanishing point of H occurs for objects or lines of perspective that are the same height above eye level as below. Beyond that, the plane of earth doesn't appear to rise. Instead, objects or lines of perspective above that height equal to eye level converge beyond H, but anything below that line is lost to sight since the earth is not transparent.
Here, I've taken illustrations used to explain this in
Earth Not a Globe and applied the container ship illustration, with eye-level approximately at 50' instead of 100', merely for the ease of illustration. (I couldn't make it work to match the observation if I put the eyeline level at 100')
So, the flat earth theory is Perspective (explanation of H and converging sight lines) and Surface Irregularities at H (waves/swell).
Now, how do we calculate distance to H?