A prop is just like a wing - it works by increasing pressure behind the blades
so when the difference is great the plane accelerates when it is low it cruises .
a big cowl -up close --screws up pressure differences --
the air can't move AFT between the blades and the cowl - It stirs around in a circle and flows outward to join the flow around the cowl. That is inefficient.
If you don't mind a simple test (no wind tunnel needed )
set up a table fan - then move a disc (a plate will do) -up close behind the fan -into the air flow.-
what happens to the fan speed ?
does it speed up or slow down?
why?