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Changing A Boat Prop

Whatever the reason for changing a prop, it's an easy project that almost anyone can handle.

How To Change a Boat Propeller

There may be lots of different reasons why you might want to change out the prop on your outboard or sterndrive. It might be damaged, it may possibly be the wrong size or pitch for your boat, or you may be swapping from aluminum to stainless-steel. Regardless, the way you go about the job will be the same and is something even a novice DIY'er with a basic set of tools can accomplish in less than a half hour.

Tip

Wearing disposable gloves when doing this job will keep grease and dirt off hands and fingers.

You've got this! Here's all you need to know.

Tech Support

Degree of Difficulty: Easy

Tools:

  • Needle nose pliers
  • Prop wrench or socket set
  • Block of wood
  • Razor knife
  • Flashlight

Materials:

  • Prop
  • Cotter pin
  • Grease
  • Rags
  • Disposable gloves

Time: 15 minutes

Cost: Depends on prop (ours cost about $250)

Removing cotter pin

Photos: Mark Corke

Step 1: With the engine tilted up to a convenient working height, remove the cotter pin that prevents the prop nut from coming undone. Use a pair of pliers to straighten out the legs of the cotter pin before pulling it out. The cotter pin will not be reused so can be discarded.

Unscrew propeller nut

Step 2: Unscrew the propeller nut with a suitable socket. You'll need to wedge a block of wood between the prop blades and the cavitation plate to prevent the prop from turning as you loosen the nut.

Removing nut and washer

Step 3: Remove the nut and washer, then pull the prop off the splined shaft. The prop should slide right off, but if it's been in place for many years it may require a light tap with the wood block on the forward face of the prop to remove it.

Wiping splined shaft

Step 4: Put the prop to one side then wipe the splined shaft with a clean rag and check for any signs of wear.

Removing thruster washer

Step 5: Remove the thrust washer, noting that it often has a step in it which faces the propeller. This washer, like the prop, should easily slide off the shaft.

Checking for fishing line

Step 6: Now's a great time to check that there's no monofilament fishing line wrapped around the prop shaft that can damage seals and bearings. If fishing line is present, cut it free with a sharp razor knife.

Smearing on waterproof grease

Step 7: Smear on a little waterproof grease. Use one that's recommended by the manufacturer, and never use any grease containing graphite. (Graphite is one of the most noble of metals in the galvanic scale and can quickly lead to severe corrosion of the outboard leg.)

Changing a boat prop

Step 8: Replacement is a straight reversal of the removal procedure: Slide on the thrust washer followed by the new prop, prop washer, and nut.

Tightening nut and making sure of alignment

Step 9: Tighten the nut ensuring that the hole in the shaft aligns with one of the slots in the castellated nut. Either tighten the nut to the correct torque setting as specified by the manufacturer, or use a special prop wrench that will ensure that the nut is not overtightened. Use a new, suitably sized, cotter pin, passing it through the shaft before bending up the tails to lock the nut in position.

Author

Mark Corke

Associate Editor, BoatUS Magazine

A marine surveyor, and holder of RYA Yachtmaster Ocean certification, BoatUS Magazine Associate Editor Mark Corke handles the magazine’s very popular Practical Boater section, keeping it full of easy-to-follow how-to articles. Mark has built five boats himself — power and sail – has been an experienced editor at other top boating magazines, worked for the BBC, has written four DIY books, skippered two round-the-world yachts, and holds the Guinness World Record for the fastest there-and-back crossing of the English Channel, in a kayak! He and his wife have a Grand Banks 32.