Outboard Prop Damage Causes & Prevention Tips

No matter how minor it may be, damage to an outboard engine’s propeller will have a significant impact on the prop’s water flow efficiency and the boat’s overall performance.

Outboard prop damage shows obvious signs such as broken or bent blades, scratches, dents and pitting. However, some signs aren’t so obvious and require a thorough inspection to be noticed. Here are some tips for dealing with outboard propeller damage. 

Signs an outboard propeller is damaged include:

• Noticeable drop-off in speed
• Excessive vibration
• Increased fuel consumption

Inspect the Propeller

You should inspect the outboard’s propeller for dents, scratches, broken or bent blades, and any other visible damage before every trip. Even if the prop looks fine, run your fingers around the propeller’s blades to make sure they’re smooth. If you feel any irregularities, no matter how small, don’t ignore it.  

Any damage to the propeller calls for having the prop removed from the prop shaft and repaired or replaced. Most outboard propellers can be fixed if two-thirds of each blade is intact. Whether you find damage to the prop or not during an inspection, also check it for marine growth such as barnacles and algae, and for tangled weeds, rope or fishing lines. 

If the prop is made of aluminum, inspect the paint for chips and fading, and repaint it if necessary. And if the propeller is made of stainless steel, check the blades for calcium buildup and clean it off. If you’ve noticed a significant decline in performance, you should inspect the prop hub to make sure it’s operating correctly.  

What is a Spun Propeller?

A spun prop refers to a propeller that no longer turns because the prop hub — the solid cylinder at the center of the propeller — is failing or has already failed. 

The prop hub is what keeps the propeller centered on the shaft. It also serves to reduce vibrations, and protect the shaft from shock during propeller strikes. A spun prop can be the result of a prop strike, or from too much torque on the hub. When a prop is spun, the drive doesn’t transfer power to the propeller. 

Signs that a propeller is spun include:

• Failure or delay of a boat reaching plane
• Reduced power and speed
• Poor fuel efficiency and performance
• Excessive vibration

What is Prop Cavitation?

Cavitation happens when excessive propeller speed or loading causes water to vaporize or boil on of the propeller, giving it a bubbling, melting, peeling or rotting appearance.  

Prop cavitation can be caused by incorrect prop matching and pitch; low clearance between the prop and the hull; and damage to the prop or the outboard’s lower unit. Water flow over the blades needs to run smoothly, and cavitation on the propeller will negatively affect the boat’s overall performance.

Signs of propeller cavitation include:

• Pitting on the surface of the blades
• Vibrations when running 
• Reduced prop bite and overall performance

Tips for Preventing Prop Damage 

Never run a boat on an outboard with a damaged propeller for too long, as doing so can cause severe (and costly) harm to the motor. While outboard propellers will gradually wear out, there are several things you can do to prevent prop damage.

Look Out for Shallow Water

Always be alert for shallow water areas to avoid propeller strikes and running aground. Make sure your boat is equipped with a depth finder to help you navigate shallow areas.

Avoid Aquatic Plants

Water vegetation such as seaweed and other types of algae and plants can be hard to spot in murky waters, and can get tangled up in outboard propellers, wreaking havoc on the motor. Always keep an eye out for and steer clear of aquatic vegetation.  

Pay Attention to Dock Lines

Ropes, dock lines, fishing lines and even chains can get tangled up in the propeller. Untangling lines from a propeller is not an easy task, and can cause irreparable damage to the prop and the outboard. Inspect the propeller after every outing to make sure nothing is tangled up in it. 

Clean the Prop After Every Trip 

Both fresh water and salt water can corrode a propeller, so rinsing the prop off with a hose and drying it after every trip can help prevent corrosion. Keeping the propeller clean helps preserve speed and efficiency that can be lost when the prop has growth on it.  Removing the propeller from the shaft, then greasing the shaft, changing out the seals and refitting the prop is also recommended to prevent the prop from seizing onto the shaft.

Inspect Prop Parts Regularly

Besides inspecting the propeller after every ride, it’s also important to inspect the prop hub, prop shaft and prop hardware at least after every 4,000 hours of use. 

And of course after a prop strike, you should inspect the propeller and all of its parts for damage as soon as possible. The last thing you want is to get stranded out on the water because you failed to check the prop after it took a hit. Finally, always carry a spare propeller on board in case of emergencies, as a spare could mean the difference between getting stuck or getting home.