The Ultimate Guide to Understanding Propeller Pitch

You’ve used a boat payment calculator to secure the ship of your dreams, tackled the open waters, and enjoyed every minute of it. Now, the time has come to buy a new propeller. If this is your first time purchasing a new motor for your boat, then you need an understanding of propeller pitch to move forward. Consider this your ultimate guide.

The Pitch

A quick search for used outboard motors will yield dozens of options, but you need to know both the pitch and diameter to find the right model for your personal watercraft. While both elements are essential to performance, the pitch comes first when shopping. Thankfully, understanding propeller pitch is simple.

The pitch of a propeller is the number of inches it moves forward in a single revolution. So, a 20-pitch propeller moves 20-inches each time the blades make a full turn back to their starting positions. These ratings are determined by the propeller’s ability to move through a soft solid, often compared to the rotations of a screw through wood.

For many boaters, it helps to think of pitch like a car’s axle ratio. A lower ratio equals more power off the bat when the car is sitting still. The same is true of a propeller’s pitch. The lower the number, the easier time you have getting the boat into gear.

That power comes with a tradeoff, however. Lower pitches cause your boat to reach its maximum number of RPMs at a lower speed. Pick a higher pitch, and you can satisfy your inner speed demon but at a slower rate of acceleration.

Pro Tip: Never outfit your boat with a higher pitch propeller if the engine boasts lower horsepower. This causes the internal engine parts to wear down at an incredibly fast rate, leading to damage and constant repairs.

The Diameter

If you remember from your high school geometry class, the diameter is the distance across a circle. Your propeller’s blades create a circle as they spin, and their diameter is the distance created by the blades’ tips. Part of understanding propeller pitch is to understand why this aspect matters.

That distance in the circle is directly related to RPMs. Generally, slower boats rely on a larger diameter while faster boats require smaller ones. This distance is never constant, though. Increasing power causes the blades to increase the size of their circle, while other variables (like the condition of the water) work to minimize that distance.

Choosing the Right Propeller

Finding the best propeller for your ship is a vital part of enjoying your investment. With current power boat prices, you don’t want to wear down your engine with the wrong diameter or pitch. So, how can you tell what works best for your boat and your needs?

The simple answer is to purchase the exact same style motor as the one that came with your boat. This ensures you’re using the recommended diameter and pitch, as well as helping you enjoy the same experience you’ve always had with your boat.

If you’re looking for a different experience that won’t destroy your engine’s components, then the trick is to buy a propeller with decent acceleration and a preferable top speed. Any qualified dealer or marina can help you find what you’re looking for, but you’ll save money searching online if you know about propeller pitch.

When shopping online, there are several vital pieces of information you need to know. First, you need to know what type of engine you’re running and what its horsepower rating is. Next, what is your boat’s weight and hull style?

It also helps to know what type of propeller you’re currently running. That includes the number of blades, the diameter, and the pitch. You can find these pieces of information stamped on the propeller, though the placing of that stamp changes from one manufacturer to the next.

Finally, you need to know your engine’s RPM when it is at a wide-open throttle. That statistic changes but take down a reading when you’re running a usual load and have your engine trimmed for the best speed.

Performance Goals

One of the most important things to consider when buying a new propeller, after all of the above information, is what your performance goals are. Namely, you need to decide between low-end torque and speed. Unfortunately, there’s no such thing as both.

If you own a powerboat, like a pontoon, then you’re probably better off with higher top speeds. That means a smaller diameter and a higher pitch. If you regularly cart around a heavier load or fishing gear, then you might want to choose the lower-end torque to get your boat moving from a dead stop.

Those aren’t set in stone, of course, and you should ultimately choose a propeller that provides the experience you’re looking for when out on the water. Just remember to stay in line with your engine’s RPMs.

Another point to consider is that most boats can utilize a faster top speed than their current engine. That’s enticing to a lot of boat owners, but it can be a lot like switching from a regular sedan to a sporty roadster. Make sure you can handle the increase in speed if that’s the option you choose. Higher speeds usually mean you need better boat insurance as well.

The final aspect to consider, of course, is price. While it might be tempting to choose the highest end propeller on the market, always stay within your budget. No one wants to be “boat poor” from strapping their finances too thin on an engine. There are plenty of fantastic used options on the market as well as affordable new propellers to choose from.

Putting It All Together

Understanding propeller pitch isn’t difficult, but it is essential to choosing the perfect engine for your boat and your experience on the water. Pitch and diameter entirely change the way your boat handles. So, choose the combination that makes the most sense for the usual activities you enjoy when boating.