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.

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.

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.

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.

The formula to calculate boat prop pitch is given by:

where:

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

• $\text{velocity}$ — Boat speed;

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

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

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

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

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 $C$ is a constant to calculate prop pitch in inches.

To calculate boat speed from prop pitch, follow these steps:

1. Multiply the prop pitch with the engine rpm.

2. Multiply the product by (1 - slip), where slip is the propeller slip, a decimal between 0-1 (equivalent to 0%-100%).

3. Divide the result by the gear ratio.

4. Divide the quotient by the constant C to obtain the boat speed, where C is:

   • 1056, to get the boat speed in mph;

   • 656, to get boat speed in kph; or

   • 1215.2, to get the boat speed in knots.

Alternatively, use our boat prop pitch calculator to calculate speed from prop pitch without hassle.

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!