An Easy Guide to Boat Propellers
When it comes to your boat, the three biggest performance differentiators are the shape of the hull, the outboard motor and the propeller.
You can’t really alter your boat’s hull design, so if you want a change you’ll have to buy a new boat. Same goes for the outboard. If you want more engine power, you’ll have to buy a higher HP motor. However, the propeller is something that makes a big difference in your boat’s performance and is easy to replace.
How a Boat Propeller Works
Propellers are devices that convert the rotational motion from the motor into directional thrust.
As the propeller rotates, its blades push against the water, which pushes back against the propeller. This resistance between the prop and water forces the propeller and the boat onward through the water. The size, shape and configuration of the prop determines how much resistance it encounters against the water, and how easily and far the prop moves with each rotation.
Any resistance against the propeller is also transferred back to the motor through the prop shaft and the driveshaft, which affects the RPM and the amount of power the outboard runs at.
- Less resistance in the water = rotates quicker, higher RPM and more top-end power
- More resistance in the water = rotates slower, lower RPM and less top-end power
Boat Propeller Configuration
There are several design elements to a prop, each of which has a big effect on its overall performance characteristics.
A propeller’s size is expressed in a pair of numbers. The first number is the diameter in inches and the second number is the pitch in inches. The diameter is always the first number and the pitch is always the second (ex: 14x19). The prop’s diameter on recreational boats is determined by the manufacturer for best all-round performance, and it’s unlikely you’ll ever have to change it.
- More resistance in the water, but less maximum RPM the engine can achieve
- Reduces top-end speed, but increases the prop’s “bite” and improves acceleration and low-speed traction
- Less resistance in the water, but more maximum RPM the engine can achieve
- Increases top-end speed, but causes the prop to “slip” and leads to poor acceleration and low-speed traction
Prop pitch is the theoretical distance the prop would travel forward after one complete rotation, assuming there’s no resistance or slip.
Pitch has a greater effect on the performance of the motor, and can be altered to suit the way you use your boat (ex: high pitch for good hole shot; low pitch for increased top-end speed). It’s common to have 2-3 different pitch props and switch them out depending on what type of boating trip it’s for.
- More resistance in the water, which reduces the maximum RPM the engine can achieve
- Rule-of-thumb: Increasing prop pitch by 1 inch decreases an outboard’s RPM by approximately 200 RPM
- Less resistance in the water, which increases the maximum RPM the engine can achieve
- Rule-of-thumb: Decreasing prop pitch by 1 inch increases an outboard’s RPM by approximately 200 RPM
The letter after the prop’s size is either R or L (ex:14x19 R, 15x26 L), which indicates the direction the propeller is designed to rotate, as viewed from behind the outboard or sterndrive looking toward the front of the boat.
- R = Prop rotates to the right, or clockwise
- L = Prop rotates to the left, or counterclockwise
NOTE: Sometimes the direction is indicated by RH (right hand) or LH (left hand).
Most engines rotate the propeller clockwise. However, some outboards rotate them counterclockwise, particularly dual-motor setups where one motor runs clockwise and the other counterclockwise to balance the engines.
The number after the prop size and rotation letter (ex: 14x19 R 3, 15x26 L 4) indicates how many blades the propeller has.
Boat propellers can have anywhere from 2-6 blades, but most recreational boat props have either 3 or 4 blades.
- Less resistance in the water
- Higher top-end speeds
- Better fuel economy
- More resistance in the water
- Better bite and hole shot
- More control and a smoother feel
3-blade props are less expensive, and offer a wider range of diameters and pitches. They’re the most common for recreational boats, offering the best all-around performance characteristics and high top-end speeds. 4-blade props are more expensive, but better for rough water. If you primarily use your boat for watersports and aren’t concerned about top-end speed, a 4-blade propeller might be better for you.
The last thing to consider is what material the propeller is made of: aluminum or steel. Most recreational boats come factory-fitted with aluminum props because they’re cheaper to manufacture.
Switching to a more durable stainless steel prop is a good idea if your budget allows it, but aluminum props also have their benefits.
- Less expensive than stainless steel props
- Weigh less than steel propellers, which is better for a lower HP motor
- Relatively brittle, irreparable and more likely to fail
- Can flex at high RPM, which saps the boat’s performance
- More expensive than aluminum props
- Stronger and more durable than aluminum propellers
- Can withstand more damage, and the blades are reparable
- Won’t flex at high RPM, which means better performance
- Ideal for more powerful motors
If your budget is a determining factor, an aluminum prop is a suitable choice. Unless your outboard is 125HP or above, you probably won’t notice much of a difference in performance between aluminum and stainless steel. However, if you run in abrasive, sandy waters and your motor is over 125HP, consider switching to a stainless steel propeller.