You can increase pitch, but the blades will cavitate when you exceed certain limits. The main defense is diameter (power absorption being proportional to D to the fifth power), secondarily blade area. A single blade is most efficient of all, but has structural problems, so two blade is preferred. However if you do not have enough clearance for a properly sized two blade, you go to a too-small diameter 3 blade, which having more blade area forestalls cavitation, if not enough diameter for the 3 blade then on to the 4 blade. This being a 75 hp motor on a saildrive (which limits prop diameter by the length of the leg) is not ideal from this standpoint. Most prop sizing is done to get the minimum number of blades and diameter that will absorb the power without cavitation. In this case the manufacturers say that 20 or 21" would be best, except I only have 12" radial clearance to the hull, so a tip clearance of 15% rule-of-thumb means a 18.5" prop. So we cheated on that and went with a 19" four blade. Like to use about a 24" two blade.

So I still don't know what is wrong, doesn't sound like a tip clearance problem which is usually more of a noise issue, can't find anyone with a four blade on a saildrive (other than the Volvo folder). Autostream (Martec) says they would cheat further on the tip clearance and get to 20" which is what I will try. 20" almost makes up for the 4 blade (thrust and load are proportional to D^4 so 20/19^4 is 1.22) and more than makes up in D^5 power. All this of course is just theory. I think less easy to understand factors are ruling here, so its a crap shoot. But going with three blades that are further away from the leg changes several things and gives me a chance at eliminating the problem?

Its just guesswork. I with it was cheaper guesswork!