Prop Slip Calculator

Q: What is the prop slip of a 25-inch pitch prop at 65 mph?

The prop slip of a 25-inch pitch prop at 65 mph is 8.48% , assuming the engine speed is 6000 rpm, and the gear ratio is 2:1. To calculate this result, follow these steps: Calculate the theoretical speed using the formula: theoretical speed = (prop pitch × engine rpm)/(gear ratio × 1056) ...using 1056 as the conversion factor between mph and inches. theoretical speed = (25 × 6000)/(2 × 1056) = 71.02 mph Calculate the prop slip using the formula: slip = 100 × (theoretical speed − actual speed)/theoretical speed slip = 100 × (71.02 − 65)/71.02 ≈ 8.48%

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Krishna Sai is a mechanical engineer with a passion for math and physics. He creates calculators with the aim of helping students understand the mathematical concepts behind the calculation. With an interest in coding, Krishna has recently started learning programming and web development. He enjoys long walks, riding his motorcycle, and loves a good story. Someday, he aspires to write his own fantasy book. See full profile

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This boat prop slip calculator is an excellent tool for calculating the propeller slip of a boat. Knowing the slip of your propeller is crucial when selecting the correct propeller for your boat.

In the following article, we shall learn how to calculate the prop slip for a boat with some examples. But first, we shall look at what prop slip is and whether it affects your propeller's efficiency.

Before we dive in, we recommend you go through our prop pitch calculator to familiarize yourself with some of the terms used here.

What is prop slip?

Theoretically, the propeller's pitch represents the distance a boat should move for one complete rotation of its propeller. This is similar to other threaded fasteners — the thread pitch of a bolt is the distance a nut will move along the bolt for one complete rotation.

🔎 For more on the pitch of threaded fasteners, visit our thread pitch calculator.

In reality, the distance a boat moves per propeller rotation is usually less than its pitch. This is because, unlike a bolt and nut, the propeller is turning in water, a liquid medium. This difference between the actual distance (or actual pitch) and the propeller pitch (or geometrical pitch) is the propeller slip (or prop slip), given as a percentage.

\text{slip} = \frac{\begin{gather*}\rm geometrical\\ \rm pitch\end{gather*} - \begin{gather*}\rm actual\\ \rm pitch\end{gather*}}{\text{geometrical pitch}} \times 100

It is common to find boat propellers with a prop slip of 10%-20%.

Formula for prop slip calculation

The formula for prop slip that we introduced in the previous section is not easy to use in practice since actual pitch is not a reading we can obtain quickly. Instead, we can switch the distance parameters (pitch) for speed parameters, as shown below:

\begin{align*} \text{slip} &= \frac{\begin{gather*}\text{geometrical}\\ \text{pitch}\end{gather*} - \begin{gather*}\rm actual\\ \rm pitch\end{gather*}}{\text{geometrical pitch}} \times 100\\[2em] &= \frac{\frac{\begin{gather*}\scriptsize\text{geometrical}\\\scriptsize \text{pitch}\end{gather*}}{\text{time}} - \frac{\begin{gather*}\scriptsize\text{actual}\\ \scriptsize\text{pitch}\end{gather*}}{\text{time}}}{\frac{\text{geometrical pitch}}{\text{time}}} \times 100\\[2em] &= \frac{\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} - \begin{gather*}\text{actual}\\ \text{speed}\end{gather*}}{\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*}} \times 100 \end{align*}

where:

$\text{time}$ — The time it takes the propeller to complete one rotation;
$\text{theoretical speed}$ — The boat's theoretical speed; and
$\text{actual speed}$ — The boat's actual speed.

We can obtain the boat's actual speed from its speedometer, but how do we calculate its theoretical speed? For that, let's turn to the formula for prop pitch:

\begin{align*} &\begin{gather*}\text{prop}\\ \text{pitch}\end{gather*} = \frac{\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} \times \text{gear ratio} \times C}{\text{Engine rpm}}\\[1.5em] &\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} = \frac{\begin{gather*}\text{prop}\\ \text{pitch}\end{gather*} \times \text{Engine rpm}}{\text{gear ratio} \times C} \end{align*}

where:

$\text{prop pitch}$ — Propeller pitch;
$\text{gear ratio}$ — Gear ratio (number of revolutions of engine for each propeller revolution);
$C$ — Conversion rate between $\text{prop pitch}$ and $\text{theoretical speed}$ ;
$\text{Engine rpm}$ — Engine speed in revolutions per minute; and
$\text{slip}$ — Propeller slip, a ratio value between 0-1.

Boat prop slip calculation example

Let's learn how to calculate a prop slip using an example.

Q: Take a boat with a 6000 rpm rated engine. If the actual top speed is 60 mph and the gear ratio is 2:1, calculate the prop slip if the pitch of the propeller is 24 inches.

A: First, we start by calculating the theoretical top speed of the boat using the formula:

\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} = \frac{\begin{gather*}\text{prop}\\ \text{pitch}\end{gather*} \times \text{Engine rpm}}{\text{gear ratio} \times C}

Since the boat speed is in miles per hour and the prop pitch is in inches, we use the conversion factor $C = 1056$ .

\begin{align*} \begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} &= \frac{\begin{gather*}\text{prop}\\ \text{pitch}\end{gather*} \times \text{Engine rpm}}{\text{gear ratio} \times 1056}\\[1.5em] &= \frac{24 \times 6000}{2 \times 1056}\\[1.5em] \begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} &=68.18 \text{ mph} \end{align*}

Next, we can calculate the prop slip from speed:

\begin{align*} \text{slip} &= \frac{\begin{gather*}\text{theoretical}\\ \text{speed}\end{gather*} - \begin{gather*}\text{actual}\\ \text{speed}\end{gather*}}{\text{theoretical speed}} \times 100\\[1.5em] &= \frac{68.18 - 60}{68.18} \times 100\\[1.5em] \text{slip} &= 12\% \end{align*}

How to use this propeller slip calculator

This boat prop slip calculator is simple to use:

Calculate the theoretical boat speed by providing the following inputs:
- Engine/crankcase rpm;
- Gear ratio; and
- Prop pitch. Before entering a value in a different unit, change the unit by clicking on it and choosing any option from the dropdown.
Enter the actual boat speed.
Instantly, our boat prop slip calculator will display the propeller slip.

For instance, by entering an engine speed of 6000 rpm, a 2:1 gear ratio, and a prop pitch of 24 inches, we get the theoretical boat speed of 68.18 mph. Then, by entering the actual boat speed at 60 mph, we can immediately determine that the prop slip is 12%.

You can use this calculator in reverse, too! Provide the prop slip and actual speed to calculate the theoretical boat speed. Take it further by entering the engine speed and gear ratio to calculate the prop pitch from slip.

🔎 Are you concerned whether the propeller's weight is worth the propulsion it can offer? Our thrust to weight ratio calculator will help you assess this.

FAQs

How do I calculate prop slip?

To calculate the boat's propeller slip, follow these steps:

Multiply the prop pitch with the engine speed.
Divide the product by the gear ratio and the conversion factor C to obtain the theoretical boat speed.
Subtract the actual boat speed from the theoretical boat speed.
Divide the difference by the theoretical boat speed.
Multiply the result by 100 to get the prop slip as a percentage.

You can use our boat prop slip calculator to simplify your task.

What is the prop slip of a 25-inch pitch prop at 65 mph?

The prop slip of a 25-inch pitch prop at 65 mph is 8.48%, assuming the engine speed is 6000 rpm, and the gear ratio is 2:1. To calculate this result, follow these steps:

Calculate the theoretical speed using the formula:

theoretical speed = (prop pitch × engine rpm)/(gear ratio × 1056)

...using 1056 as the conversion factor between mph and inches.

theoretical speed = (25 × 6000)/(2 × 1056) = 71.02 mph
Calculate the prop slip using the formula:

slip = 100 × (theoretical speed − actual speed)/theoretical speed

slip = 100 × (71.02 − 65)/71.02 ≈ 8.48%

Does prop slip depend on the boat's speed?

Yes, the prop slip is lower when the boat travels close to its top speed and increases as its speed decreases. At top speed, it is common for propellers to have a slip of 10%-20%.

Can the weather and sea conditions influence prop slip?

Yes, how much the propeller actually slips in practice depends on the weather (the wind) and the sea. Rougher weather and sea could increase the actual prop slip. At the same time, it is also possible that conditions may propel the boat more eagerly, reducing the prop slip.