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Prop Pitch Calculator

Table of contents

Boat propeller diameter and pitchPropeller slipHow to calculate prop pitchUsing this boat prop pitch calculator

Our prop pitch calculator is a quick-and-easy tool to help you determine the pitch of a boat propeller. You can also use a boat's prop pitch to calculate its speed.

In the following article, we shall learn more about propeller pitch and its significance. We shall also learn how to calculate prop pitch using parameters like engine rpm, gear ratio, etc.

Boat propeller diameter and pitch

Picking the right propeller for your boat is crucial for the boat's performance. The right propeller will keep your engine running at its rated rpm range at full throttle.

A boat propeller is specified by its diameter and pitch. The propeller's diameter is the diameter of the circle formed by the propeller blades. In other words, it is the diameter of the circle that inscribes the propeller blades. The larger the prop diameter, the more water it can displace. Typically, they are used when pulling heavy loads or when maneuverability is desired.

A propeller's pitch is the theoretical distance it will move through a solid medium for one revolution. Imagine your propeller in a solid medium such that it can move through the medium, much like it would through water. For example, a 13"×16" propeller has a propeller diameter of 13 inches and a propeller pitch of 16 inches. So, in theory, the propeller would move 16 inches for each revolution.

🔎 In a way, this is similar to the thread pitch of a threaded fastener like a screw or a bolt. To learn more about the pitch of a fastener, head to our thread pitch calculator.

The higher the pitch, the more distance the propeller would travel for each revolution, and hence, increase the top-end speed of the boat. Conversely, the lower the pitch, the lower the distance covered per revolution and the lower the top-end speed.

But we must remember that the higher the propeller's diameter, the smaller the prop pitch becomes, and vice versa. So, a propeller with a higher diameter would have a smaller prop pitch, which gives higher low-end acceleration and load-carrying capacity but a lower top speed. On the other hand, a smaller propeller diameter will provide a higher prop pitch, which allows for high top-end speed but slower acceleration.

Propeller slip

In practice, the propeller travels a shorter distance per revolution than its prop pitch when moving through water. This reduction is due to "slippage" when the propeller moves through water instead of a solid. Propeller slip is usually around 10%-20% at top speed and is higher at lower speeds.

How to calculate prop pitch

The formula to calculate boat prop pitch is given by:

proppitch=velocity×gear ratio×CEngine rpm×(1slip)\footnotesize \begin{split}\rm prop\\\rm pitch\end{split} = \frac{\text{velocity} \times \text{gear ratio} \times C}{\text{Engine rpm} \times (1-\text{slip})}

where:

  • prop pitch\text{prop pitch} — Propeller pitch;

  • velocity\text{velocity} — Boat speed;

  • gear ratio\text{gear ratio} — Gear ratio (number of revolutions of engine for each propeller revolution);

  • CC — Conversion rate between prop pitch\text{prop pitch} and velocity\text{velocity};

  • Engine rpm\text{Engine rpm} — Engine speed in revolutions per minute; and

  • slip\text{slip} — Propeller slip, a ratio value between 0-1.

🔎 If you need help calculating the gear ratio between the engine crankcase and propeller shaft, use our gear ratio calculator!

The prop pitch is usually calculated in inches, while boat speed is measured in miles per hour, kilometers per hour, or knots. Depending on the unit used for boat speed, the conversion rate CC is a constant to calculate prop pitch in inches.

Boat speed units

CC value

miles per hour

1056

kilometers per hour

656

knots

1215.2

Using this boat prop pitch calculator

Our prop pitch calculator is easy to use:

  1. Enter the boat speed in your desired unit (select the unit first).

  2. Input the gear ratio between the engine crankshaft and the propeller shaft.

  3. Provide the engine/crankshaft rpm.

  4. Enter the propeller slip value as a percentage or decimal.

  5. Our prop pitch calculator will instantly calculate the boat prop pitch using the provided data.

Want to calculate boat speed from prop pitch? Take advantage of the multi-way calculation power of this tool — enter the remaining four variable values, and our tool will calculate speed from prop pitch in the blink of an eye!

🔎 There is more than one way to calculate a boat's speed. Intrigued? Visit our boat speed calculator to learn more.

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