As you said plenty of ways too prove the world is spherical, but some others have there own thoughts.. I don't know what they exactly mean when they say the earth is flat, but I did see lots mentioned about a "ice Wall".. Yes an ice wall at the face of the earth apparently...
But of course a ship that has set sail out too sea does not just disappear, but slowly drops away in till you only see the mast.. People observed this is and was only more proof that the earth being not flat.
And the science, gravity, physics etc I just can't see working on a flat earth, pretty sure there is experiments you could possibly even do at home too see why the planets are formed round (Just I guess from what I remember)
Cheers
Cody