Choosing the Right Propeller for Your Boat
Selecting the right propeller is one of the most important and influential factors that affects the performance of your boat. The right propeller ensures the motor operates within its recommended RPMs. It also reduces wear, increases longevity, improves fuel economy, and optimizes performance.
When it comes to buying a propeller, the options are seemingly endless. With that in mind, here's a guide to choosing the right propeller for your boat or outboard motor.
Establishing Performance Goals
Ask yourself what it is about your boat that you're trying to correct or improve. Think about how you normally use your boat and whether you would like its performance characteristics enhanced.
Typical questions include:
- Are my boat's hole-shot and acceleration too slow?
- Is my outboard failing to reach top speed?
- Does my outboard over-rev at Wide-Open Throttle (WOT)?
- Does my outboard fail to reach optimum RPM at WOT?
- Do I want better performance and fuel economy from my outboard?
- Is my current propeller damaged, ventilating or cavitating?
Once you've established what you want from your boat's performance, choose propellers that will deliver those results.
Size and configuration options to consider for propellers include:
Aluminum or Steel Propellers
Aluminum propellers are the most common prop type, and typically come standard on new outboards and boats. They're cheaper than stainless steel, and are perfectly adequate for most boating needs. Aluminum props can be repaired, but the metal becomes weaker over time.
Stainless steel propellers are more expensive than aluminum props, but are also more durable. Their thinner and stiffer blades perform better than those found on aluminum props. Stainless steel propellers can also be repaired without weakening them.
Number of Propeller Blades
Propellers typically have 3 blades or 4 blades. Three-blade propellers are most common on recreational boats, offering the best all-around performance characteristics and higher top-end speeds. With fewer blades in the water, the three-blade prop encounters less resistance. Less blades means increased fuel efficiency, and they don't affect the motor's RPMs as much as their four-blade counterparts.
Four-blade propellers provide better traction and bite due to more propeller surfaces acting against the water. They improve hole-shot acceleration and low-speed pulling power, so they're ideal for ski and wake boats. Four-blade props also enhance handling and control in tight turns; can reduce ventilation; and are better than three-blade props at running in rough water. However, the additional resistance and drag from the extra blade may cost the outboard 100 RPM or more.
The rake of the propeller is the angle that the blades slant backwards in relation to the prop's line of travel. The higher the rake angle, the more the blades are slanted back, and the more the boat's bow lifts out of the water. This improves time to plane and increases top-end speed. However, a rake that's too high causes additional strain on the outboard. The rake is usually set by the propeller manufacturer and is not a major concern.
Cupping is the addition of a curved lip to the trailing edge of propeller blades. A cupped prop gets better traction in the water, which reduces prop slip and ventilation, and improves hole-shot and acceleration. Cupped props enable the outboard to be trimmed so the propeller is closer to the water's surface. A cupped propeller reduces the RPM of an outboard by about 200.
Most outboards rotate the propeller shaft and propeller clockwise. However, some outboards rotate them counter-clockwise, especially dual outboard setups in which one motor runs clockwise and the other runs counter-clockwise to balance them. Double-check which direction your outboard prop shaft rotates, and make sure the propeller you get is designed to rotate accordingly.
Propellers have two numbers stamped on them: diameter x pitch. The diameter is the total distance from blade tip to blade tip as the propeller rotates. A 14½ x 17 propeller has a diameter of 14½ inches and a pitch of 17 inches. A larger diameter propeller encounters more resistance and drag in the water, while a smaller diameter prop suffers less resistance.
Fitting a larger diameter prop reduces the RPM of a motor and with it, the top-end speed. In contrast, the smaller diameter prop increases RPM and top-end speed. However, smaller props are more prone to slip and aren't as good for a boat's hole-shot or acceleration. Because of the many different prop pitch sizes available, it usually isn't necessary to change the diameter of your prop from what's recommended by your engine's manufacturer.
Choosing the right pitch is the most important factor when buying a new propeller. Pitch is the distance the prop would travel forward in one complete rotation, assuming it encounters no resistance or slip. All propellers encounter slip, but the better a prop is matched, the less slip it'll incur.
A properly matched prop/outboard/boat combination should encounter 10% prop slip or less.
- Lower-pitch props encounter less resistance and drag in the water, allowing faster rotation, and giving the motor excellent hole-shot and acceleration, as well as low-speed pulling power. However, because each revolution of the prop generates less forward travel, the ultimate top speed will be lower at WOT.
- Higher pitch props encounter more resistance in the water, so hole-shot, acceleration and low-end traction are reduced. However, the added resistance reduces the motor's RPMs and with it, top speed at WOT.
On average, every inch of pitch increase or decrease alters an outboard's RPM by about 200. Increasing prop pitch lowers RPM, while decreasing prop pitch raises it.
After narrowing down the list of props you want for your boat, test each one to determine which is best. These water tests are to determine that the prop pitch enables your outboard to run in it's manufacturer-specified WOT-RPM range while improving your boat's performance. For example, if you feel your boat's hole-shot is too slow, you may want to try out a couple of lower-pitch props that increase your boat's acceleration.
Step 1. Make sure your boat is at its typical operating weight.
Step 2. Find an open stretch of water and run your engine up to WOT.
Step 3. Trim out the motor and note the outboard's RPM.
PRO TIP: To find the optimal trim, position the motor up until its RPM begins to increase without further increase in speed. Then, trim the motor back down slightly. If the motor is running above the recommended WOT RPM window, the prop pitch is too low. However, if the motor fails to reach the recommended WOT-RPM window, the prop pitch is too high.
Step 4. Continue testing propellers until you find one that enables you to run your motor at WOT within the manufacturer's recommended RPM range.
If you want a propeller capable of fulfilling many different roles, get both a lower-pitch and a higher-pitch prop, and change them out as needed. After all, swapping a prop only takes a few minutes, and the change in the prop's size will adjust your boat's performance.