Understanding Propeller: Prop Diameter & Pitch
Looking for help with selecting the right prop for your boat?
Trying to figure out the best option for the prop diameter and pitch?
Changing prop diameter & pitch is a good and easy way to improve your boat efficiency, with better top speed or hole shot.
However, you may see mixed opinions through your research and calculation on the “prop diameter or pitch”.
If you are one of those who is struggling to understand prop specs, you are in the right place!
In this simple guide, you will learn what prop diameter and pitch are, how they work, how to measure, their effect on boat performance, and the best ratio for your boat (with steps to calculate).
Hopefully, you will find the best prop size for your boat by the end of your reading.
Table of contents:
Understanding Propeller
Outboard propeller (or “prop”) plays a big role in the boat’s performance.
The basic measurements for a prop are DIAMETER and PITCH.
For example, if a propeller is marked as 14 x 19, it has a diameter of 14″ and a pitch of 19″.
You can easily get the numbers off the stamp on the prop.
However, what is the difference between propeller diameter and pitch?
What do they mean for boat performance?
Let’s say you are running a 15 x 15 prop, what advantage would a smaller diameter prop have?
Continue reading to find out quick answers.
Note: Too large or too small prop pitch/diameter will both cause problems. If not properly sized, the propeller can damage the engine. So make sure you get the right setup for your boat.
What Is Prop Diameter & Its Effect
Diameter is the distance across the circle the blade tips make as the prop rotates.
Here is a simple diagram that shows the diameter of a prop:
What will a bigger diameter prop do? How does it affect the boat’s performance?
A larger prop diameter means more surface area, which allows the prop to handle more power and gives the prop more thrust and also more drag.
And this means two things:
- More thrust allows a big heavy boat to get on a plane more easily with less power
- More drag means you won’t be able to turn your boat as fast.
That’s why high speed boats tend to use smaller props, and slower (or heavier) planing boats use bigger props.
What Is Prop Pitch & Its Effect
Pitch is the theoretical distance the propeller would move the boat in one revolution. It’s about blade angle.
Here is a simple diagram that shows the pitch of a prop:
The greater the pitch, the greater the distance your boat will advance each revolution.
Compared with diameter, prop pitch has a greater effect on the boat’s performance.
That’s why boaters tend to change the pitch to suit their way of boating.
Essentially, adding (or subtracting) prop pitch by 1 inch will decrease (or increase) its WOT RPM by approximately 200 RPM.
So a higher pitch gives you greater top-speeds, but at the expense of your acceleration or hole-shot.
By contrast, a lower pitch lets you reach max RPMs easier, and therefore, a better holeshot and acceleration but lower top-speeds.
Determining Prop Diameter and Pitch
How to determine prop size and pitch? What’s the best pitch to diameter ratio of a propeller?
Generally, the outboard motor manufacturer has preordained the WOT RPM, and the prop diameter & pitch are designed to allow it to reach that RPM setting.
For example, ePropulsion Navy 6.0 Evo comes with a 12.6″ × 10.8″ composite propeller to bring out the best performance of your boat.
Not sure whether the prop pitch is too much, too little, or just right?
You can do a performance test with your existing prop to find out. Here are the procedures:
- Step 1: Look up the max RPMs in your engine manual
- Step 2: Run the engine up to full throttle in forward
- Step 3: Check if the engine reaches its full RPMs (or at least within 100-200).
If it doesn’t reach the standard, your prop has too much pitch, meaning it’s taking too big a bite of water. However, if it reaches max RMPs easily, it’s highly possible you have too little pitch and want to increase the number.
Want to switch props to get more top speed or a better hole shot?
Here is a simple guideline for your prop selection:
- For high speed, a propeller usually has a small diameter and maximum pitch.
- For better holeshot or acceleration, a prop usually has a low pitch.
- For more power or thrust, a propeller usually has a large diameter.
Tip: A good way to determine the prop diameter and pitch is to find out what others are running in a similar setup as you intend to run, and go from there based on what you want from your boat.
After all, boat performance is not all about the prop diameter and pitch, it’s also about the motor mounting location, weight, etc.
If possible, always chat with a professional to get the best prop pitch and diameter for your specific applications.
Click to get in touch with our experts in props to get professional suggestions. Or simply leave us a comment below and we will get back to you soon.
FAQs About Prop Diameter & Pitch
1. Is a 19 or 21-inch prop faster?
In theory, a larger diameter at the same pitch and RPM will have the same speed, but greater thrust. So a 21-inch prop won’t go faster than a 19-inch. However, the increased pitch will increase the top speeds.
2. Does prop diameter affect speed?
As mentioned above, the prop diameter doesn’t affect speed but thrust.
3. Does the diameter of a prop affect RPM?
Generally speaking, prop diameter reduction will increase your max RPMs since it decreases resistance against the water. So if you add diameter, you will need more power to get it up to the RPM.
4. Prop pitch vs RPM. How does propeller pitch affect RPM?
As a rule of thumb, increasing the pitch will decrease engine RPMs and vice versa. Every difference in pitch number equates to approximately 200 RPMs.
5. Is high pitch or low pitch prop better?
It depends on your specific needs. Basically, if you want to get up on plane faster, have a fast hole shot for water skiing, or run a heavy boat more easily, you need to go for a low pitch prop.
6. What happens if prop pitch is too high?
When the pitch gets too large, it will be hard to reach the recommended WOT RPM, acceleration will suffer and your holeshot will become worse. It will also potentially lug the engine when you have more people on board.