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Propeller maintenance

Your boat’s propeller connects the engine to the water, providing all of the thrust required to propel and lift the boat. Keeping the propeller in good condition is vital to maintaining peak performance and efficiency.

Regular inspection

Propellers are prone to damage ranging from a few nicks in the blade edges to major damage from a severe hard-bottom strike. The propeller hub can also be damaged in a bottom strike, or simply from age and use. Get into the habit of inspecting your propeller regularly during the boating season.


Step 1

With the gear position in neutral, you will be able to turn the propeller. For safety, always remove the key from the ignition and disconnect the engine cut-off switch before inspection.

Step 2

Spin the propeller to inspect for nicks in the leading edge of each blade or a bent blade, which will appear out of alignment with the other blades.

Step 3

Striking a soft bottom can often slightly bend propeller blades without causing other obvious damage. If you notice vibration or diminished performance after running through sand, mud or clay, your Mercury® authorised dealer can inspect the prop or send it to a propeller repair shop to see if the blades are true.

Step 4

Carefully inspect the leading edge of the prop blades and check for burrs, which can occur if the prop is run through sand. You can remove these burrs yourself with a file.

Propeller repairs

A propeller with bent blades will not function properly and is also likely to be unbalanced, which can cause vibrations at high speed that may damage shaft bearings, seals and other gearcase components. If blades are bent or otherwise damaged, the propeller should either be replaced or repaired.


A good repair shop can restore a propeller with very significant damage, often by welding new material onto the affected blades and then grinding and shaping the blades back to their original geometry. Because of the higher replacement cost, repairing a stainless steel propeller often makes economic sense. It may be more cost-effective to simply replace an inexpensive aluminium prop.

Mercury authorised dealers

Propeller repairs require special skills, tools and experience, so most Mercury authorised dealers have a relationship with a specialty shop and send propellers out for servicing. If you strike bottom hard enough to damage the prop, have your dealer inspect the propeller shaft. If the shaft was also bent on impact, it will need to be repaired or replaced.

Carry a spare

Heading out on the water without a spare propeller is as risky as leaving on a road trip without a spare tire. In the event of damage, changing to a spare propeller and hub kit kept on board can get you back to port, just as the spare tire in your trunk can get you home.


For general lake and inshore boating, your spare propeller only needs to be “good enough” to get you back to the dock. Carry an affordable aluminium propeller as a spare for your more valuable stainless steel propeller or purchase a less-expensive used or repaired unit to keep on board as a spare. If your boating takes you far offshore, it is more critical that the spare propeller is rated for the power of your engine and is of a size similar to your main propeller, so that the boat is able to get on plane and cruise back to port at an efficient speed.

Changing a boat propeller

Practice changing the propeller in the marina or with the boat on its trailer so you know the drill. As you remove the propeller, pay close attention to how washers and spacers are installed so that you can put them back in the same order and direction when you install the spare.


In calm conditions and in safe waters the simplest way to change a prop is to back the boat up near shore, so you can wade out and change it while standing. Changing a prop at sea can be tricky if the water is very cold or rough; if that is the case, it may be safer to hail assistance or a tow.

Mercury Flo-Troq propeller hub kits

If you are using a late-model Mercury propeller with a Flo-Torq® hub system, also keep a matching Flo-Torq replacement hub kit on the boat. The Flo-Torq hub is designed to be sacrificial. In the event of a hard strike to the propeller, the hub gives way to protect the prop shaft and gearcase from damage. If that happens, the propeller might still be functional, and you can get back underway by removing the prop and replacing the hub components.

Your propeller changing kit

In addition to the spare prop, keep on board:


  • a propeller wrench or a breaker bar handle with a deep-well socket that fits the propeller nut. The Mercury floating propeller wrench is made of plastic and will float, which is a handy feature if you are forced to change the propeller in deep water.
  • A spare hub kit just in case you drop a small part into the water. Spare kits are available from a Mercury authorised dealer.
  • A prop block or small piece of wood placed between the propeller blade and the anti-ventilation plate of the lower unit will keep the propeller from turning as you loosen and tighten the nut.
Propeller wrench
Hub kit
Prop block

Fishing line menace

How to check your prop shaft seal

When fishing line is discarded or lost in the water, it often floats. When a boat passes by, the line can become tangled in the propeller. Eventually the line can work its way into the gap between the propeller hub and the gearcase and become wrapped around the prop shaft. With time this line collects at the point where the prop shaft exits the gearcase, and the thrust of the propeller presses the line into the prop shaft seal. This can be very bad news – if the seal fails, water can enter the gearcase and cause expensive damage. Remove the propeller several times per season to check the prop shaft for any accumulation of fishing line.

How to check for fishing line

Check your owner’s manual for specific instructions on removing the propeller.

Some engines have a thrust washer that fits between the propeller and the prop shaft seal – be certain this washer is removed.

A tangle of line may be obvious as soon as you pull off the propeller. A smaller piece of line can get pressed into the grease that accumulates around the area where the shaft passes through the seal. Use a fingernail or a sharp pick to find line embedded in the grease.

If no line is present, reinstall the propeller after coating the prop shaft with a quality marine grease such as Mercury 2-4-C marine grease.

If you discover fishing line

  • If you pull fishing line out of the seal, or if the line appears to be melted, it is good practice to check the gearcase lubricant for water.
  • Tilt the outboard or drive down.
  • Position a catch pan under the gearcase.
  • Remove the drain screw from the bottom of the gearcase (see your owner’s manual for its location). If there is water in the gearcase it will often run right out. If you do find water, replace the lube and have the drive inspected by a professional.
  • Your Mercury authorised dealer can also inspect your gearcase, change the lubricant and pressure-check the gearcase to test the seals.

Next topics

Maintenance: What's in it for you?

Boating is an activity that fuels pride of ownership, and for many owners a strong do-it-yourself attitude. Knowing you have covered the basics of maintenance gives you confidence that your boat is in tip-top shape every time you leave the dock.

Fuel basics

Using the correct fuel in your Mercury Marine® engine and properly maintaining your fuel system can help ensure many seasons of reliable service. For more guidance on fuel requirements for your engine, see the fuel section of your Mercury® operation and maintenance manual.

Preventing corrosion

Your engine lives and works in the water, where it is constantly exposed to elements that can cause corrosion.

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