Reducing prop blade area

Switching to a 2 blade (if you can swing a larger diameter) will give the largest blade area reduction , and a big improvement in prop efficiency due to less induced drag..

Skeens Elements of yacht design is available at most libraries , and will give pitch vs diameter and required blade area results easily.

 

Although theoretically more efficient, I would be reluctant to put a 2-blade prop behind that massive deadwood. I could result in a good deal of vibration and blade fatigue as the blades load and unload as they pass through the area behind the deadwood.

 

The deadwood looks fine from the photo , so a 2 blade would be no problem.

We have been running a large 2 blade prop for 3 decades , about 5000 hours with no actual ill effects. Smooth and efficient .

3/4GPH @6 1/2K burn with old Volvo MD 3B and 18,000 lb sail boat.

Hardest part was waiting for Michigan Wheel to create a 2 blade with wider power boat style blades.

First question is how did you measure RPM?

IF its not with a digital tach your readings can easily be 300 RPM off .

Many cruisers will prefer a slight over prop as few operate at max power for anything but an engine check.

The higher loading at low power RPM lowers the engine noise , increases engine life and usually lowers fuel burn a bit.

Just be sure engine operation is 10% less than WOT RPM (I prefer 300RPMdown) and there will be no overloading hassles.

 

Fast Fred,
I wonder if a 19" 2 blade would stack up with the loading of the 3 blade 18"? Or would I need to go w a 20" ... then I'd probably need to reduce pitch and not achieve what I started out to do.
The very wide blades of the MP prop should deal better than other props with the turbulence of the deadwood. Lots of old fishboats had the wide deadwood and seemed not to suffer from it. Sure dosn't look very ideal does it?
RPM from a fairly new electronic tach.