Prop Slipping: How to Identify, Calculate, & Fix It

Wondering if your prop is slipping and preventing your boat from achieving its full potential on the water?

This quick guide will tell you everything you need to know about what prop slip is, what causes it, how to fix it, and how to easily calculate exactly how much a prop is slipping.

P.S. – If you’re investigating outboard prop slip, also check out our guide to the signs and symptoms that an outboard isn’t at the optimal height.

What is Prop Slip?

Prop slip occurs when a prop fails to achieve its power potential. This is due to resistance from the water that prop is working to propel the boat through. As the prop endures the resistance, it tends to slide back and does not go the actual full distance in that one revolution. The prop takes a shorter path and that shorter distance is called the prop slip.

Many people use the example of the screw to illustrate what is meant by prop slip. The screw head is turned one full revolution. The distance the screw head turns in that one revolution is the actual distance the screw head has to go to make one complete revolution. This one turn is called the pitch.

This example falls short because the screw has no resistance applied to its performance. You just turn the screw and it enters the piece of wood. No problem and no real resistance holding the screw back.

Since a boat propeller turns, it also has an actual distance to go before it makes one complete revolution. That distance or revolution is also called the pitch. That is where the similarity ends. Unlike the screw, the boat prop has real resistance and the pitch is measured slightly differently.

The pitch of a propeller is measured by the actual distance the propeller moves the boat after one revolution. For example, an 20-inch pitch prop should move the boat 20 inches in the water with each revolution. Prop slip will reduce that distance.

Note: It’s important to understand that a slipping prop is not the same thing as a spun prop.

Causes of Prop Slip

Prop slip happens to every propeller and how much slip you experience depends more on the design of the prop than on the drag of the water. It would be easy to always blame this issue solely on the amount of drag the water places on your prop but that would not be the best answer.

This is why many boat owners switch props when they are not satisfied with the performance of their boats. They want less prop slip so that they get better fuel efficiency and enhanced overall performance.

Some props are designed to produce only 10 to 20% slip while others create more and those types of props lower the fuel efficiency. There are other factors involved as well.

For example, the condition of the propeller would contribute to a great prop slip if the propeller was in bad shape.

Other factors include:

1. Speed
2. Weight
3. Weight Distribution
4. Hull Design
5. Condition of the hull
6. Engine mount height
7. Engine angle

How Prop Slip Affects Performance

Prop slip has a direct negative affect on speed and fuel efficiency. This is a delicate topic though, as some prop slip is actually required to move forward. Without prop slip, you have no thrust.

The goal of enhancing your performance and fuel efficiency does not lie in eliminating prop slip altogether. It lies in reducing that slip to the lowest amount possible.

What makes it difficult to do this objective is that there is no constant rate of prop slip. The only constant you have with prop slip is that it is always changing. The faster you go, the lower the prop slip and vice versa.

Then it is impossible to find the perfect prop for every boat hull and design, etc. Everything is done through trial and error. Even the experts cannot find the perfect propeller for your boat or every situation it will encounter.

There are propeller calculators on the internet that will help you make adjustments. Just do not use them expecting to get perfect results for your boat. To help you find the right prop, with the best percentage of prop slip you do need the help of propeller specialists though.

You will also need a lot of data and a few mathematical equations to figure out how large a percentage you have in your prop slip.

How to Know if Prop Slip is Occurring

This is actually the simplest issue of all issues that relate to this subject. If you are not moving anywhere in the water, then you have no prop slip. prop slip happens with all boat propellers no matter their design.

Without it, you would have no thrust and that is not a good thing. Even if you are going at top speed, you will experience some prop slip. It won’t be much as the faster you go the lower the prop slip.

To get an idea of how much prop slip you are getting, take a look at the design of your prop. A well-designed propeller will reach up to 20% or less. A badly designed prop will produce even more prop slip. This means changing your propeller will help you lower the prop slip and improve the performance of your boat’s motor.

You do not need to know or identify that prop slip is taking place. It is. What you need to figure out is how much slippage is taking place and if that percentage can be improved upon.

Formula for Calculating Prop Slip

Figuring out your percentage of prop slip is actually fairly simple. We can use a few simple numbers and measurements to nail down a relatively precise percentage.

Here’s a step-by-step of a simple formula for calculating prop slip:

1. Theoretical Speed = (Revolutions Per Minute ÷ Gear Ratio) × (Pitch ÷ 1056)
2. Prop Efficiency = (Actual Speed ÷ Theoretical Speed)
3. Percentage of Prop Slip = (100% – Prop Efficiency)

It’s that simple!

This formula is an easy DIY and can be a lifesaver when you’re pulling your hair out trying to figure out whether or not you’re dealing with a significant propeller issue.

Final Thoughts

The one thing you need to remember is that you cannot have zero prop slip. If you achieve that low percentage then you won’t be traveling anywhere. The goal is to reduce that percentage to its slowest level while maintaining proper thrust.

This is the optimal point you should strive for.

Check the design of your prop to see if it is well-designed and, if not, change it. Then make sure to distribute the weight aboard your boat so that the prop has less resistance to worry about.