I had a "disagreement" with a sailor who claimed he could see the curvature of the earth from the bridge of the ship he sailed on. The bridge was about 40 ft above sea level. By curvature he meant he could see the curve of the earth by scanning from left to right (north to south - east to west).
I couldn't see how this was possible given that from any point on the earth's sea surface you would be in the "middle" of the earth and therefore would see a "flat line". (NOT adocating a flat earth! :)
If this is so, how far above the earth would one need to be to see the curve of the earth?
Photos from space clearly show the curve of the earth. I understand that these photos are taken more than 300km in space. My question is"How high above the surface of the earh would you need to be to discern the curve"?

I just talked to my boss who is an "old salt" who served 10 years in the US Navy and I have also heard my husband who retired after 20 years in the US Navy say he has seen what you describe.

With an unobstructed view of the earth the curvature is more easily detected. My boss described it as "feeling it" more so than "viewing it." He says you can watch ships go over the curve - they don't just drop off or instantly disappear. That is a distance of approximately 7 miles. The higher up you are the better the view becomes so a person on the control tower of a carrier would see more than a person on the deck of a destroyer.

I think you are getting horizon confused with curvature. Sailors see the horizon.