Geographic Range
This calculation yields a range to a distant object in miles.
Geographic range = √land height + √eye height (units in feet).
If you can see a beacon and know it is 25ft high, and assuming your eye height on the boat is 9ft, then the square root of 25 = 5 and the square root of 9 = 3.
5+3 = 8
The beacon is approximately 8 nmi away.