This is a highly informative article regarding the effect of diameter moving to a four blade. In my case the difference is 5/8" smaller so I don't know whether to keep the same 14" pitch or not.
This article hopes to explain how propeller diameter (not pitch) affects performance when searching for the perfect prop.
We bought a Solas Rubex L4 Blade Stainless 15.25" dia. X 20" pitch prop to replace a stock Suzuki 16 X 21 3-blade prop on our 2015 Suzuki DF200AP. This outboard has a 2.5:1 final gear ratio which explains why it can turn such a large prop.
Researching this further we found we needed to go down to a 20 inch pitch because thats what you do when you add a blade - drop the pitch an inch. Ok - easy enough. However the other variable (diameter) is harder to define. There isn't a lot of information out there on how changing diameter affects performance. Changing diameter has a definite impact - just like pitch does. Changing to a 4 blade and dropping pitch from 21P to 20P I expected similar wide open throttle (WOT) RPM between the 2 props. What we actually found was surprising.
The smaller (by .75 inch) diameter resulted in WOT being too high now which set off my engine overspeed alarm every time I tried to achieve full throttle. Compared to previous performance readings this prop now allowed the motor to rev up an additional 200-250 RPM. In other words, our 3 blade 25 mph cruise speed was achieved at 3900 RPM but now we have to run @ 4150 to achieve the same 25 mph cruise speed. Assuming the pitch change was correct (because we added a blade) this extra RPM must be the result of the Solas prop's smaller diameter.
Knowing what we know now we would have ordered the same 4 blade prop but in a higher pitch (22P) to make up for the smaller diameter.
All is not lost in our case as we are fortunate to have a qualified prop shop nearby. We brought the prop to them and explained our predicament. The prop guys first suggestion was to add more "cup" to the trailing edges of the 4 blades. We agreed and had the cupping done and the prop is back on the outboard ready for testing. We're hoping the extra cup brings the RPM's back down where we want them: 6000 RPM @ WOT without the overspeed alarm going off @ 6200 like it did without the cup added. UPDATE: We tested the 15.25 X 20 Solas prop with the added cup and...we're almost there! We hit 6150 with occasional forays into overspeed range but this was a definite improvement! We sent the prop back to the shop for the addition of an inch of pitch added to the 4 blades and returned to the boat with our 15.25 X 21P (with added cup which was equivalent to another inch of pitch) This turned out to be the perfect setup! We hit 6000 RPM with a light load of fuel and 2 persons on board. This is right where we wanted to be! Top speed was a respectable 45 mph and fuel flow was 17 GPH @ WOT. Finally!
In summary: when moving from a 3 blade 16 X 21.5P to a 4 blade prop WE SHOULD HAVE selected the 4 blade 15.25 X 22P in the first place as the .75" reduction in diameter had a dramatic effect on WOT RPM.
At $500+ and up there isn't much room for economical trial-and-error testing when you try new props in this size range. That's why we posted this info - so it might help others avoid the same issue and make prop selection more reliable.
On a positive note: the extra blade did provide more stern lift and a better hole shot as well as better directional control in reverse (yes!) on our single engine boat. These aspects of 4 blade props over 3 blade props proved to be accurate during our testing.
Morale of the story: When you change diameter - add opposite pitch. Smaller diameter? Add an inch of pitch to your selection. Larger diameter? Subtract an inch of pitch. Hopefully this helps boaters select the perfect prop for their boat. The one that comes with your outboard isn't always the perfect match for the boat it's installed on. Dealers/Manufacturers take a guess and many boaters just accept the choice made for them and miss out on the perfect prop.
Final note: All outboards have a rated horsepower at a specific RPM (i.e. 150 HP @ 6000 RPM - check the specs). This means a prop with too much pitch won’t allow full RPM to be developed - yielding less available horsepower. You bought an outboard and want every pony you paid for, right?
Our point is: you have the correct propeller pitch on your outboard when you’re able to hit this 6000 RPM mark when your outboard is at WOT (Wide Open Throttle) and the boat is loaded normally. You really only have to check this once if the speed worries you or your family. Besides, you have to “break-in” your new outboard before this check can be accomplished.