Radio Horizon

Geometric Horizon
h=Height of aerial above sea level
r=Radius of the earth (3440 Nautical miles)
d=Distance to Horizon

Using Pythagoras d²=(r+h)²-r²
hence d=√((r+h)²-r²)

Radio Horizon

To calculate the Radio Horizon because of refraction we achieve this by pretending the Earth has a larger radius which we call the equivalent Earth radius Re which is normally 4/3r in the UK (4587 NM).
There are 6076 feet in a Nautical Mile

dr=√((4587+h/6076)²-4587²)

The radio horizon between the transmitter and receiver is the addition of the horizon of both masts.
horizon
Note the size of the ship makes a considerable difference to the range.
Reception beyond this range is caused by propagation conditions and will be flukey.

Dip

This was checked to see if a ship could actually be below the aerial horizon.
The graph shows the angle down to a ship for different receiving aerial heights. For a ship 1nm from the aerial and the aerial 1000 feet above sea level the angle down would be 10°. From the same aerial the horizon would be under 1°.
dip
geodip
α=Dip angle
To Horizon sin(90-α)=r/(r+h)
To 1 nm tan α=h/d

 This significance is high gain collinear aerials increase horizontal gain at the expense of vertical beam width. The graph shows this could be relevant with a high gain aerial close to and much higher than the ship's aerial ie the ship is "below the radar"

Elevation
For example the Cushcraft Ringo Ranger specification quotes 7dBi gain and a beam width of  7 degrees. The dipole "donut" becomes a "biscuit" !
For more technical info see the references on my aerial page

See also
Beyond the Horizon PropagationTropospheric Ducting Forecast