How to Measure a Boat Propeller
The propeller is the part of the boat that is most in contact with the water. As a result, it is the one part of the boat that is most likely to degrade quickly due to the mechanics of the engine or the salinity of the water.
Before considering any change, it is important to know how to measure the propeller of your boat.
With this in mind, the National Boating Safety School’s boating specialists explain how to measure a boat’s propeller.
Why measure a boat propeller?
The propeller is one of the many parts of a boat. It is designed to propel the boat forward or in reverse. There are times when you may need to change your boat’s propeller if it is no longer working or becomes damaged.
It is important to know the characteristics of your propeller as well as your boat’s electric motor. According to these characteristics, you will have to determine the measurements of your propeller if you plan to replace it or ask a dealer for a part.
What are the measurements of a boat propeller
Before we understand how a propeller is measured, it’s best to review the essential measurements you need to know to define its size.
Diameter of the propeller
The diameter of a propeller informs how the boat’s engine is used. The larger the diameter, the more strain the engine has in starting the blades.
Pitch of the propeller
The pitch of a propeller is the distance traveled by the blades during one complete revolution or spin cycle. This measurement is also an essential point since it informs the engine’s round per minute (RPM). This information indicates the engine speed and the top speed of the engine.
- A short pitch makes your starts quick, which is useful for sporty driving or water skiing
- A long pitch makes starting and accelerating more difficult because the propeller can’t reach its maximum speed as quickly
If you choose a propeller that doesn’t match the power of your engine, you risk damaging your boat. It’s therefore important to know how to properly measure your boat propeller.
What do you use to measure a boat propeller?
A simple way to calculate a propeller’s measurement is to find this information on the equipment itself. Whether you have an inboard motorboat or not, this data is found either on the inside or outside of the hub of your propeller.
It is presented in the following format: 15 X 27.
The first number (15) is the diameter of your propeller and the second (27) is the pitch.
The notation may change depending on the type of engine, the type of propeller and the number of blades.
How to measure your propeller
The calculation of a boat propeller includes the measurement of its diameter and pitch. This information is usually expressed in inches unless stated otherwise. Over time, your propeller’s manufacturing labels may weather. In this case, it is possible to manually measure your propeller.
To illustrate the calculation method, we provide an example below. Note that, whatever the result obtained in all the steps, you must always round up and avoid decimals. This will make it easier to choose your propeller when you need to replace it.
To determine the propeller’s diameter:
- Measure the distance from the hub (center part) to the tip of one of your blades
- Multiply this distance by 2 to get the total diameter
For example, if the distance from the hub to the tip is 12.5 inches, then the total diameter will be 25 inches.
To determine the propeller’s pitch:
- Determine the angle between the hub and the propeller blade using a protractor. To do this, draw a line perpendicular to your hub. Place the base of your protractor on a blade. Then measure the angle between your line and the blade
- Write down the angle – we’ll use 20 degrees as an example. Then use the TAN function on your calculator:
| Angle | Tangent |
| 20° | 0,36397 |
- Then multiply the tangent by 6.2831 (this multiplier is a fixed number and cannot be changed):
| Tangent | Multiplier | Result |
| 0,36397 | 6,2831 | 2,28685 |
- Then multiply the result by the radius of your propeller. The diameter is 25 inches, so your radius will be 12.5 inches. This will give you the pitch of your propeller:
| Step 5 results | Radius (half the diameter) of propeller | Pitch of propeller |
| 2,28685 | 12,5 | 28,58 |
In this example, the propeller has a diameter of 25 and a pitch of 29. The information on your propeller would then be 25 X 29 because you need to round up to avoid decimals.
Given these steps, you may as well determine these measurements yourself or use an online boat propellor calculator that help you determine the pitch of your propeller.
Boat safely with National Boating Safety School
Determining the measurements of your propeller before having it replaced is a process that requires some comfort with numbers. Knowing how to determine them allows you to understand the operation and power of your engine, which is an important step in obtaining your Canadian boat license.
To perfect your knowledge, you can register for our online boating exam to become more comfortable with the process.