I'm sorry I'm calling bullshit. The earth is much larger than that. Six miles along the earth is not going to create a six foot variance. Bullshit. The circumference of the earth is about 24,854.84 mi (40000 km) this would mean that the six miles you are talking about is about .087 degrees. I came to this figure by using simple math techniques, if you really need to know what, I took the 24,854.84, divided it by 360 to discover how many miles one degree was, and then divided six by that number to ascertain what the degree value for six miles would be. On an open horizon, sailors can most certainly see farther than that, so therefore, the first experiment you provided is either extremely ( and I mean EXTEMELY) flawed, or it is blatantly lying to you.