A Quick Follow-up Regarding Calder's Calculations
Bottom line: Calder went with a three-bladed fixed 18-inch prop with a 12-inch pitch. He claimed it provided 7 knots at 3,000 rpm and topped out at 8 knots (He must have really been running that Yanmar hard to do that).
I have all Nigel's books and have read them through, but they're not handy at the moment. The information you're citing regards efficiency of the prop under power, but I'm pretty sure he assessed the drag issue as well. If you have it handy, can you remind us / summarize his feelings concerning drag?
Regardless of the efficiency issues, for me the (near) lack of prop walk in reverse is nice as I single hand a lot and it's one less thing to have to manage. I guess as with everything else sailing, it comes down to one's requirements, priorities, and preferences.