2 blade or 3 blade propeller
I don''t think you are evaluating the right issue. Cruising has been my primary activity and an extra 1/2 knot makes a big difference if your sailing for 4-5 hours. The main thing is matching your diesel engine to the size and pitch of the prop. When you run full throttle, you don''t want to engine to "over rev" at one extreme (too little bite out of the prop) and you don''t want to kick out black smoke (too much bite). You should be able to size a two blade prop that will fit into these extremes. That''s what I did and my 27 hp diesel pushes my Apache 37 at 6.9 knots at 2900 rpms full throttle with a two blade prop. With a decent breeze, the boat goes its hull speed of 7.5 knots.
Remember, an extra 1/2 knot in 8 hours of sailing gets you 4 miles further.