To determine the worth of a folding prop, would it be valid to experiment by dragging another prop behind the boat? It would have to be on a fixed strut, so it wouldn''t spin, and roughly the same size and pitch. Watching the speedo drop when you did this would give you an idea of the effect of a nonfolding prop on boatspeed. Going to a folding prop would likely increase the speed of the boat by about the same amount as it was observed decreased by the drag of the second one, no?
This wouldn''t work well for a boat whose prop was in an aperture, but for one that''s just stuck out there in front of the rudder, it might work better than guessing. It''s certainly a good excuse to go out sailing.