The matter Manuel is they do not spot the ships.....
they spot the top of the masts and follow some times the convoys looking the top of the masts.
It is well modelled in SH1.
Look at this......
http://radarproblems.com/calculators/horizon.htm
From a conning tower of 4m, and a theoretical clear day.....
You can spot the 30m masts tops of a cargo ship from near 26.7Km.
And a 65m mast tops of a Warship from 35.9Km.
Considering a small ship as a destroyer, its bridge and its radar antenas may be at 15m over the sea level, looking it from a coning tower of 4m it will be visible at 21km.
To start up to see the ships under 17km ...about 16.5 as in your mod...... is not bad.
And the horizon (zero height) will be at 4.23 nautical miles about 7.8km, look bellow.
http://www.boatsafe.com/tools/horizon.htm
About the smoke columns, it is correct, in your mod, smoke columns appears first than the ships, smoke is visible a little bit under 18km and ships start up to take body a little bit under 17km.
Un abrazo, Red.