Boat propeller diameter and pitch
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 sections, we'll learn more about propeller pitch and its significance, and how to calculate prop pitch using parameters like engine RPM, gear ratio, and propeller slip.
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 — the circle that inscribes the blades. The larger the prop diameter, the more water it can displace. Larger diameters are typically used when pulling heavy loads or when maneuverability is desired.
A propeller's pitch is the theoretical distance it would move through a solid medium for one revolution. Imagine your propeller moving through a solid medium, much like it would through water. For example, a 13"×16" propeller has a diameter of 13 inches and a 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.
The higher the pitch, the more distance the propeller travels for each revolution, and hence the higher 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 remember: the higher the propeller's diameter, the smaller the prop pitch becomes, and vice versa. A propeller with a higher diameter has a smaller prop pitch, which gives higher low-end acceleration and load-carrying capacity but a lower top speed. A smaller diameter provides a higher prop pitch, allowing 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 as 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 boat's speed is related to the propeller RPM and pitch. First, find the propeller RPM from the engine RPM and the gearbox reduction (gear ratio):
Prop RPM = Engine RPM ÷ Gear ratio
The theoretical speed (with no slip) is the distance the propeller advances per minute, converted to a speed. Accounting for slip, the actual boat speed is:
Speed = Prop RPM × Pitch × (1 − Slip)
Rearranging the formula to solve for pitch:
Pitch = Speed ÷ [ Prop RPM × (1 − Slip) ]
The calculator handles the unit conversions automatically. In the American (Imperial) system, pitch is in inches and speed is in miles per hour (mph). In the Metric system, pitch is in millimetres and speed is in kilometres per hour (km/h).
How do you calculate boat speed with prop pitch?
To find boat speed, switch the calculator to Calculate Boat Speed mode and enter:
- The engine RPM at full throttle.
- The gear ratio of the gearbox (e.g. 2.00 : 1).
- The propeller pitch (in inches or millimetres).
- The estimated propeller slip (typically 10%–20%).
The calculator divides the engine RPM by the gear ratio to get the propeller RPM, multiplies by the pitch to get the theoretical speed, and reduces it by the slip percentage to give the realistic top speed.
Using this boat prop pitch calculator
To find the prop pitch (default Calculate Prop Pitch mode), enter the engine RPM, gear ratio, your boat's measured speed, and the propeller slip. The tool returns the required propeller pitch along with the propeller RPM and the theoretical (no-slip) speed.
Example (Imperial): Engine RPM = 5,000, gear ratio = 2.00 : 1, slip = 15%, prop pitch = 16 in.
- Prop RPM = 5,000 ÷ 2.00 = 2,500 RPM
- Theoretical speed = 2,500 × 16 in ≈ 37.88 mph
- Actual speed = 37.88 × (1 − 0.15) ≈ 32.20 mph
FAQs
What is propeller pitch?
Propeller pitch is the theoretical distance a propeller would travel forward in one full revolution through a solid medium. A 16-inch pitch propeller would, in theory, move 16 inches per revolution.
What is propeller slip?
Propeller slip is the difference between the theoretical distance the propeller should travel (its pitch) and the actual distance it travels through water in one revolution. It is usually 10%–20% at top speed and higher at lower speeds.
Does a higher pitch mean a faster boat?
A higher pitch increases top-end speed because the propeller advances farther per revolution, but it reduces acceleration and load-carrying ability. A lower pitch improves acceleration and pulling power at the cost of top speed.
How does diameter affect pitch?
Generally, the larger the propeller diameter, the smaller its practical pitch, and vice versa. Larger diameters favour low-end thrust and load capacity; smaller diameters favour higher top speed.