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# Prop Pitch Calculator

Boat propeller diameter and pitchPropeller slipHow to calculate prop pitchHow do you calculate boat speed with prop pitch?Using this boat prop pitch calculatorFAQs

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.

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:

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

where:

• $\text{prop pitch}$ — 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.

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

Boat speed units

$C$ value

miles per hour

1056

kilometers per hour

656

knots

1215.2

## How do you calculate boat speed with prop pitch?

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.

## 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!

FAQs

### What is the boat speed of a 16-inch pitch propeller?

The top speed of a boat with a 16-inch propeller is 37.5 mph with 10% prop slip at 5500 engine rpm and a 2:1 gear ratio. To arrive at this answer, follow these steps:

1. Multiply the prop pitch with the engine rpm:

16 in × 5500 rpm = 88000 in/min

2. Multiply the product with (1 − slip), where slip = 10% = 0.1:

88000 in/min × (1 − 0.1) = 79200 in/min

3. Divide the result by the gear ratio:

79200/2 = 39600 in/min

4. Finally, divide by 1056 to obtain the boat speed in mph:

39600/1056 = 37.5 mph

### What is the prop pitch required for a top speed of 50 mph?

For a boat's top speed of 50 mph at 5500 engine rpm with a 2:1 gear ratio and a 20% prop slip, you need a 24-inch prop pitch. To calculate prop pitch from boat speed, follow these steps:

1. Multiply the boat speed with gear ratio and 1056:

50 mph × 2 × 1056 = 105600

2. Divide the product by the engine rpm:

105600/5500 = 19.2

3. Divide this result by (1 − slip), where slip = 20% = 0.20:

19.2/(1 − 0.2) = 24 inch

To verify this result, head to our boat prop pitch calculator.

### Is a 19-inch prop faster than a 21-inch prop?

A 21-inch propeller will have a higher top speed than a 19-inch prop, but the 19-inch propeller will have faster acceleration and hole shot (racing start) than the 21-inch prop. Think about your use case and pick the right prop for you. To learn more, visit our prop pitch calculator.