It all depends how high you are
The distance to the horizon is incredibly dependent upon how high you are, and how high the object is (or how tall). So both the 11 miles and 26-27 mile answers can be correct.
Hence the spotting tops on battleships etc.
10-11 miles is a reasonable estimate for two small ships to mutually see each other. Battleships and the like would have a reasonable chance of seeing each other in the 20-25 miles range. But of course although the ships are that distance apart, the horizon is a little closer, it's as if you were looking over a hill.
That's one reason why you can see France from Kent (and vice versa) - as long as you are up on the cliffs. Trying to look from beach to beach is probably not possible (Straits of Dover are about 25 miles wide, from what I recall)
Speedbird, if I could work out how to draw a picture that I could post it would be a much simpler explanation. Doesn't seem to be a useful picture facility here (which is odd, some other vBulletin sites do have that feature, I guess it's disabled to conserve bandwidth)