Three Blade Prop
A couple thoughts here, I believe that cruising speed on a Yanmar 2 GM is closer to 2800 rpm. You are doing your engine no favor operating at a lower speed and you would probably get more speed out of the boat with a higher rpm. If 1500 rpm is the max that the engine will turn then you either have an engine problem or you are over-propped. If you are over proped then the three blade won''t help a bit. A 10 hp diesel engine should push you at close to hull speed in flat water and moderate winds. You would noticable decrease performance with a 3 blade prop.