Re: Folding/feathering props.
While it is true that the difference between a folder/featherer and a fixed prop is most apparent in light air and that hull speed is the limiting speed in heavy air, the real advantage lies not in an increase in speed, but in the increase in windward ability.
In most applications this amounts to 3-5 degrees per tack. This is 6-10 degrees improved angle to windward. A peek at a vector diagram will explain this. The wind in the sails produces a huge force roughly perpendicular to the heading of the boat. This force is opposed by the keel or center/dagger board which must operate at a slight angle through the water moving past it (leeway). When closehauled, which is the condition we're talking about, there is very little sail force left over to move the boat forward. Any drag removed is force freed to move the boat in the direction it is heading. This is why a clean bottom is desirable. A fixed 3 blade is roughly the equivalent of towing a peach basket.
I know gentlefolk never sail to windward, but consider the savings in diesel when you can make 5 knots and 5degrees better to windward in conditions in which you would previously have had to motorsail. Another condition worth thinking about is when deep reefed. In this condition any increase in windward ability results in a better ride.
Of course if you are towing a dinghy all thoughts of sailing higher than a beam reach are out the window.