Prop School Part 3: Prop Terms

Mar 20, 2025

By Matt Vander Loop

Prop School: Understanding Prop Terms

Welcome to another installment of Prop School, our series dedicated to the inner workings of boat propellers. In today’s post, we’ll break down some of the key terms used in the prop industry—from rotation and blade count to diameter, pitch, and more. Whether you’re a seasoned boater or just starting to learn about marine propulsion, understanding these terms will help you choose the right propeller for your vessel.

1. Rotation (“Hand”)

Every propeller spins either right-hand (RH) or left-hand (LH). Most single-outboard and sterndrive propellers are right-hand rotating. When watching a boat in action, a right-hand propeller will spin clockwise as it pushes the boat forward, while a left-hand propeller turns counter-clockwise.

Another handy way to determine the rotation is by examining the blade’s slant:

Right-Hand Propeller: Blades slant from lower left to upper right.
Left-Hand Propeller: Blades slant from lower right to upper left.

This simple visual check can be particularly useful when you’re assessing a propeller’s setup on the water.

2. Number of Blades

While single-blade propellers might offer peak efficiency, they also come with high levels of vibration that can be uncomfortable or even damaging over time. That’s why most designs feature three blades—a balanced compromise between efficiency and smoother operation.

Two vs. Three Blades: The efficiency gap is minor, but the reduction in vibration with three blades is noticeable.
Four-Blade Propellers: Increasingly popular, these designs offer further vibration suppression, improved acceleration (due to more blade area in the water), and enhanced rake effectiveness. Higher rake lifts the bow, reducing hull drag and boosting speed, especially on heavier boats.

3. Diameter

Diameter is the distance across the circle formed by the propeller’s blade tips as they rotate. Think back to your high school geometry class—the concept is similar. In propeller design, the diameter must be perfectly matched with other factors like pitch, rake, and cup. This balance is critical to delivering maximum thrust while maintaining mid-range fuel economy. Many manufacturers, such as Mercury, use proprietary methods to optimize these combinations for various boating applications.

4. Pitch

Pitch describes how far a propeller would theoretically move forward in one complete revolution if it were “screwing” through a solid medium. For instance, a propeller marked as “13 ¾ x 21” has a 13 ¾-inch diameter and a 21-inch pitch—meaning it would advance 21 inches in one full turn under ideal conditions.

There are two primary types of pitch:

Constant (or “True/Flat”) Pitch: The pitch remains uniform from the leading to the trailing edge.
Progressive Pitch: The pitch starts lower at the leading edge and increases toward the trailing edge. This design, often seen on mid- to high-horsepower propellers, improves performance under high-speed or partially surfaced conditions.

5. Rake

Rake refers to the angle of the propeller blade relative to the hub. A blade with its face perpendicular to the hub has 0° rake. As the blade slants toward the aft end, the rake angle increases. Standard propellers typically feature a rake between -5° and 20°, with most outboard engines and stern drives around 15°.

High-performance propellers may use a progressive rake (which can exceed 30° at the blade tip) to:

Enhance performance in cavitating or ventilating conditions (when blades break the water’s surface)
Lift the boat’s bow to reduce hull drag and increase speed

Rake can be flat (straight) or curved (progressive), with each style providing different handling characteristics based on boat weight and operating conditions.

6. Cupping

Cupping is the process where the trailing edge of the blade is slightly curled away from the boat. Originally developed to mimic the benefits of progressive pitch and increased rake, cupping has become a standard feature on modern recreational, high-performance, and racing propellers.

Key points about cupping:

RPM Reduction: Adding cup usually lowers full-throttle engine rpm by 150 to 300 rpm compared to a propeller with no cup.
Customization: Prop repair shops can adjust cupping to fine-tune engine rpm and performance.
Blade Dynamics: The location of the cup relative to the blade’s pitch and rake lines can further alter performance:
- Cupping perpendicular to pitch lines increases effective pitch.
- Cupping perpendicular to rake lines effectively increases the rake.

When done correctly, cupping can also help protect against propeller “blowout” and ensure a responsive feel.

7. Slip

Despite sounding like an undesirable trait, slip is an essential part of propeller dynamics. It refers to the difference between the theoretical travel distance (if the blade were a screw in wood) and the actual distance traveled per revolution. For example, if a 10-inch pitch propeller moves 8½ inches per revolution, it’s operating at 85% efficiency with a slip of 15%.

A small amount of slip (around 4 degrees of angle of attack) is necessary to generate the positive and negative pressures that produce thrust. However, too much blade area or an oversized diameter can reduce efficiency by lowering the effective slip below optimal levels.

8. Setback

Setback is a concept that’s gaining traction in the boating world, particularly for outboard engines. By moving the engine further aft, the boat’s center of gravity shifts, which can lead to improved handling and performance under varying water conditions. In practice, this adjustment can allow for a higher engine height and a more balanced setup, ensuring optimal performance and safety.

Final Thoughts

Understanding these prop terms is key to grasping how propellers function and how various design choices impact overall performance. Whether you’re selecting a new prop for your boat or simply looking to deepen your marine knowledge, a firm grasp on rotation, blade count, diameter, pitch, rake, cupping, slip, and setback will set you on the right course.

Stay tuned for the next post in our Prop School series, where we dive even deeper into the engineering and design choices that make modern propellers tick.

Happy boating!