Prop Pitch Calculator
Table of contents
Boat propeller diameter and pitchPropeller slipHow to calculate prop pitchHow do you calculate boat speed with prop pitch?Using this boat prop pitch calculatorFAQsOur prop pitch calculator is a quickandeasy 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 topend speed of the boat. Conversely, the lower the pitch, the lower the distance covered per revolution and the lower the topend 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 lowend acceleration and loadcarrying capacity but a lower top speed. On the other hand, a smaller propeller diameter will provide a higher prop pitch, which allows for high topend 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:
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 01.
🔎 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:

Multiply the
prop pitch
with theengine rpm
. 
Multiply the product by
(1  slip)
, whereslip
is the propeller slip, a decimal between 01 (equivalent to 0%100%). 
Divide the result by the
gear ratio
. 
Divide the quotient by the constant
C
to obtain theboat speed
, whereC
is:
1056
, to get theboat speed
in mph; 
656
, to getboat speed
in kph; or 
1215.2
, to get theboat 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:

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

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

Provide the engine/crankshaft rpm.

Enter the propeller slip value as a percentage or decimal.

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 multiway 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.
What is the boat speed of a 16inch pitch propeller?
The top speed of a boat with a 16inch 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:

Multiply the
prop pitch
with theengine rpm
:16 in × 5500 rpm = 88000 in/min

Multiply the product with
(1 − slip)
, whereslip = 10% = 0.1
:88000 in/min × (1 − 0.1) = 79200 in/min

Divide the result by the
gear ratio
:79200/2 = 39600 in/min

Finally, divide by
1056
to obtain theboat 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 24inch prop pitch. To calculate prop pitch from boat speed, follow these steps:

Multiply the
boat speed
withgear ratio
and1056
:50 mph × 2 × 1056 = 105600

Divide the product by the
engine rpm
:105600/5500 = 19.2

Divide this result by
(1 − slip)
, whereslip = 20% = 0.20
:19.2/(1 − 0.2) = 24 inch
To verify this result, head to our boat prop pitch calculator.
Is a 19inch prop faster than a 21inch prop?
A 21inch propeller will have a higher top speed than a 19inch prop, but the 19inch propeller will have faster acceleration and hole shot (racing start) than the 21inch prop. Think about your use case and pick the right prop for you. To learn more, visit our prop pitch calculator.