About 5 years ago I ran some tests measuring static thrust of a 6" prop on an electric motor, behind a 4.5" cowling. With just the motor on the mount thrust was 240g, after adding a tachometer beside the motor thrust was 225 g, and adding the 4.5" cowling reduced thrust to 180 g., a loss of 25% compared to no fuselage at all. The cowling area was 56% of the prop disc area.
Alasdair