Diameter
Propeller diameter is pretty easy to understand – it’s simply the diameter of the circle scribed by the blade tips as the prop rotates. Imagine attaching a pencil to the tip of one blade and then turning the prop over a sheet of paper. That would trace a circle representing the propeller’s circumference; the diameter of that circle is the length of a straight line traversing the widest point of that circle.
Propeller engineers determine the prop diameter based on the RPM at which the prop will be turning and the amount of power delivered to the prop. Diameter usually increases for propellers used on slower and heavier boats, and decreases for propellers intended for faster boats. A prop with more diameter has more total blade area, which allows it to handle more power and create more thrust to move a heavy boat. More blade area gives the engine more “traction,” like a set of aggressive truck tires. But just like those tires, that added prop diameter also creates more drag, which is compounded as boat speed increases. A MerCruiser Bravo Two sterndrive, which is designed for the heaviest pleasure boats, can use a propeller up to 20 inches in diameter, compared to a max of 16 inches for a Bravo One sterndrive and current outboards, which are designed for faster speeds on lighter boats.
Pitch
Propeller pitch is usually described as the theoretical distance in inches a prop would move in one revolution through a soft solid. Think about turning a screw into wood, or a corkscrew into a wine bottle cork. Our 14 x 19 prop will thus move 19 inches forward with each revolution. A 14 x 21 prop would move 21 inches with each revolution, and so is doing more work – and moving faster – than the 19-pitch prop at the same RPM. Some Mercury propeller models are offered in one-inch pitch increments, while others come in two-inch increments.
Changing propeller pitch is rather like changing gears up or down on a bicycle – in low gear you can accelerate quickly from a standing start, but soon your legs will be spinning and you won’t go very fast. In high gear you’ll need to mash the pedals to start moving but you’ll eventually reach a faster top speed. All else being equal, adding propeller pitch will decrease wide-open throttle (WOT) engine speed, while subtracting pitch will increase WOT RPM. If prop pitch is too low, engine RPM at wide-open throttle may be too high (above the engine’s recommended WOT RPM range) and, while boat acceleration may be strong, top speed will suffer. If the prop pitch is too high, acceleration will suffer and the engine will “lug” or not be able to reach the bottom of the recommended WOT RPM range. Either condition – WOT RPM that is too high or too low – can damage the engine and drive components.
Within a propeller model line the diameter may change slightly through the pitch range, as Mercury engineers have determined the ideal diameter to work with that pitch, the overall design of the propeller, and the anticipated application of the propeller. This is why different propeller models may have different diameters, even though the pitch is the same – for example, a Mercury Spitfire® X7 is 13 x 17 while a Mercury Trophy® Plus is 13.75 x 17. There are dozens of variables that go into the design of each propeller, but most boat owners can find a great Mercury propeller option simply by focusing on the prop model and the best pitch. The Mercury Prop Selector Tool is a great resource that will help you narrow down prop options by answering five basic questions about your boat and engine, and how you use them.
NOTE: Operating your outboard or sterndrive engine outside of the recommended RPM range can cause severe damage to your engine and may void the warranty. Always test the boat on the water after you install a new propeller.