Choosing the right propeller

The propeller is an essential part for the operation of any boat with a motor. It is the propeller that transmits torque and engine power to provide propulsion. There are many models with various specifications adapted for different engines, boats and uses. The choice of a propeller is important and can radically change a boat’s performance and energy consumption. In this guide, we will discuss the criteria to consider when choosing a propeller.

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  • How to choose the best propeller?

    SOLAS boat propeller

    Certain precise criteria make it possible to select a propeller adapted to your needs, ideally you should test a propeller before making a purchase if possible as only real tests in navigation will determine the best solution. A propeller must be changed if it is damaged due to contact with the bottom of the water, rocks or even sand. You can also choose to replace an old propeller with a more recent model, in order to take advantage of technological advances. The objective in this case is to benefit from better engine efficiency and to reduce consumption.

    When choosing a propeller, you need to consider a number of key criteria:

    • The pitch
    • The diameter
    • The number of blades
    • Materials
    • The intended use
    • The engine
  • Which pitch is best?

    Side-Power propeller

    The pitch of the propeller is the distance traveled by the propeller to make a complete turn on itself. It is expressed in inches. The greater the pitch (we also say “longer”), the greater the quantity of water pushed at each rotation of the propeller. A smaller (“shorter”) pitch allows a more direct acceleration, which is useful for certain activites, such as getting a waterskier up out of the water for example. On the other hand, consumption will be higher so the overall yield will be less efficient. The ideal pitch must allow the maximum engine speed recommended by the engine manufacturer to be reached.

    It’s important to note that the pitch will affect the engine rpms. An increase in pitch will cause a decrease in engine rpms, and vice versa.

    There is always some slip between the propeller and the water (generally 10 to 15%) so the actual distance traveled is somewhat less than the theoretical value. The role of pitch is speed, similar to gear selection on a car’s transmission.

  • What diameter should you choose?

    The size of the propeller is determined by two numbers: the diameter and the pitch—the number of the diameter is listed first. This is the diameter of an imaginary circle passing through the end of all the blades. It can be calculated by multiplying the distance from the center of the hub to the tip of any blade by two.

    The larger the diameter, the more water the propeller will push, which will create more power. But a large diameter propeller also puts more strain on the engine. The choice of diameter depends on the space available under the anti-cavitation plate.

  • How many blades should you choose for your propeller?

    Mercury Outboards propeller

    The fewer blades a propeller has, the more efficient it is. Unfortunately, fewer blades also means more vibration. Three-bladed propellers are the most common for pleasure boating because they offer the best compromise between smooth performance, speed and consumption. Propellers with 4 or 5 blades allow better acceleration and significantly reduce vibrations. On the other hand, it is to the detriment of maximum speed and fuel consumption.

    Each propeller blade has two surfaces that displace water to move the boat. As the propeller rotates, the blade back creates a low pressure that helps pull the boat forward. The blade face creates high pressure as it rotates. This pressure forces a stream of water away from the propeller. As the water is pushed to the rear, an equal force pushes the boat forward. Today, many propeller blades have a cup incorporated at the trailing edge which allows for a better bite on the water; it reduces ventilation and slipping, while aiding in a quicker acceleration.

    Here are some basic propeller terms:

    • Leading Edge: The edge of the blade closest to the boat.
    • Trailing Edge: The edge of the blade farthest from the boat.
    • Blade Tip: The point on the blade farthest from the hub. It separates the leading edge from the trailing edge.
    • Blade Root: The area where the blade attaches to the hub.
    • Blade Back: The side of the blade closest to the boat (low pressure side).
    • Blade Face: The side of the blade opposite to the boat (high pressure side).
    • Rake: The angle the blade tip tilts away from the gearcase, either flat or progressive (angle increases with distance from the hub). Most propellers have between 0º and 20º rake.
    • Cup: The small curved lip edge of some propellers. It reduces ventilation and slipping, especially when operating in disturbed or aerated water.
  • What is the difference between the materials available?

    Stainless steel and aluminum are the most common materials used to make propellers. More rarely, bronze or alloy propellers are found. Stainless steel is 5 times stronger than aluminum, which is beneficial for the longevity of the propeller blades. But stainless steel is also 2 to 3 times more expensive and its strength means that the damage avoided on the propeller can have repercussions on other parts of the engine. A cheaper aluminum propeller will act as a sort of lightning rod in the event of an impact. Aluminum is particularly suitable for lower power engines (up to approximately 100 horsepower) while for above 150 horsepower, stainless steel propellers are systematically recommended.

  • What’s the intended use?

    Each use requires different capacities. The practice of water skiing or other towed water sports requires a boat capable of clear acceleration and a high maximum speed. On the contrary, for fishing or joy-riding, it is the power available at low speed that will be important.

  • What about the engine?

    The propeller must match the specifications of the engine manufacturer. Each engine is designed to operate optimally at a certain speed. The engine owner’s manual provides this information. This is the power delivered at a given speed.

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