Oracal:

We are all taught to feather a prop. The real trouble starts when they fail to.

I’m not joining the line of people telling others how to fly an aircraft they have only seen in pictures or film clips. My SLG (ASH 31 Mi) has a fixed pitch propeller and when shutting the engine down, if the speed is too high it doesn’t stop windmilling. After reducing the speed and the propeller has stopped windmilling, the sink rate is reduced considerably. Next, when in the correct (vertical) position, the propeller is retracted into the rear fuselage.

When performing a planned shut down, it is no problem maintaining the correct speed and the propeller automatically stops but if you loose power, the gut reaction may be to increase the speed which keeps the propeller windmilling. In that situation, the possibility to feather the propeller would stop it windmilling, right?

A stationary fixed pitch propeller still has the same frontal area which a featehered propeller does not but I speculate that the reduction of frontal area is not a large contribution towards a lower sink rate.

Try starting a stopped piston engine with a fixed pitch prop in the air by diving! 

Exactly, I have seen (in pictures  and film clips) that it requiers several people to manually rotate a large engine by the propeller and that force is extracted from the airflow when windmilling. When the propeller stops, that force is no longer extracted and that reduces the sink rate much more than a reduction of frontal area, right?