Except for a few very specific cases, such as variable-pitch props like the ProPulse, the pitch on most common boat props is fixed. You can’t adjust it, not even a little bit. That means that when you’ve discovered your prop’s pitch is off you’ll need to replace it. How will you know? Start by reading Understanding Propeller Pitch, then watch our video on how to find out if you have the right propeller pitch.
If you’re running a brand new boat, the good news is that many dealers are happy to help you swap out props and try different options, until you’re satisfied you’ve got the right one. The same is true of propeller manufacturers, if you’re upgrading or replacing your existing propeller. You may have to pay shipping charges with every prop swap, but usually, that’ll be the only added expense.
Here’s the key factor to remember, when you need to replace your prop with one that’s a different pitch: if your engine is spinning too many RPM, which is called “under-propped,” you need to increase the propeller’s pitch. And if your engine is spinning too few RPM, you’re “over-propped” and need to reduce the pitch. Increasing pitch by one inch will reduce RPM by about 200, and dropping pitch by an inch usually increases engine RPM by the same amount.
But all propellers aren’t available in one-inch pitch increments. In fact, most jump by two inches. Manufacturer recommended RPM ranges are usually wide enough, however, that the pitch-changing window can be met without a problem.
Now let’s say you’ve found the pitch that puts your engine right in the sweet spot. What other effects are you likely to notice? Time to plane can grow with pitch. But fuel economy can, too, since you should be able to attain a given speed with lower RPM. And pitch is only one of several propeller factors that can affect everything from top-end to trim. Propeller material, blade shape and number, rake, cupping—all of these details and more come into play.
Fortunately, most new boats leave the dealership with a propeller that’s close to correct, and pitch is the variable that most commonly needs attention. If you suspect some of these other items need to be addressed, though, it’s time to put on your thinking cap and study up on a number of propeller factors. Here are a few articles that will come in handy:
Outboard Expert: The Right Propeller Part I (establishing a baseline, under-propping, and over-propping)
Outboard Expert: The Right Propeller Part II (alternatives, and the affects they have on performance)
Outboard Expert: The Right Propeller Part III (an on-the-water test session, comparing four different propellers)
High-Performance Boat Propellers: The Art of Trade-Offs
Which is Best: Aluminum or Stainless-steel Boat Propellers
Pitch Perfect: New Performance Propeller Options for Outboards