Hello Stefan,

About 10 years ago a couple of NDB chasers have been investigating 
this issue by comparing reception of NDB's over a sea path and a 
land path. To exclude sky wave propagation as good as possible, the 
tests were done in summer at the time of the sun's solstice.

Though by no means a scientific research, it was found
that the length of a sea path was up to four times the length of a 
land path!

I live about 8 km from the sea wall here and my furthest daytime 
signal has been the now decommissioned NDB AB, Akraberg, Faroer, 
1268 km away. There is another one on the Faroer, MY, Myggenaes, on 
337 kHz, 1360 km away, which is still active. I could hear it at 
daytime by ear in the past. Unfortunately power seem to have been 
reduced as the carrier on 336.970 kHz is about 4 dB above my noise 
floor at the moment.

73,
Roelof Bakker, pa0rdt