We had this discussion at our handicapping meetings in Nova Scotia. Under our system 1 point ASPN is roughly 6 sec/mile. Previously we did not differentiate ratings between two identical boats one with a folding prop and the other with a fixed two bladed prop. The main reasoning was that anything less than 6 sec/mile would be a partial point under our system and we did not wish to go there.
Using IMS numbers and a modelling program one of our members who is a marine engineer ran various scenarios against a C&C 30 and a C&C 40 (cannot remember length other than greater than 40) under various wind strengths and directions. This was done with an exposed shaft with no propellor, an exposed shaft with folding or feathering propellor, the same with 2 bladed fixed and then with three bladed fixed.
Under lighter wind conditions the difference between 2 bladed fixed and a folding or feathering prop was never less than 5 sec/mile in this experiment and as high as 7 sec/mile. As the wind speed was increased to over 15 knots (true) the difference lessened.
As a result of this process it was decided that 1 point ASPN (6 secs/mile) credit be given for fixed two bladed prop over a folding or feathering prop on the same boat.
There were many sidebar discussions but this was the basic outcome. Please be warned that this is all from memory and if specifics are desired I would have to look them up.