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Learn How to
Diagnose and Repair
a Spun Outboard Motor
Propeller Hub

Mercury Inertia Propeller

Here's How to Definitively Determine When an Outboard Motor's Propeller Hub is Spinning in its Bore and How to Fix it

The lowly propeller provides forward motion. At least until bad things happen to a good propeller. You would think the punishment propellers endure when they whack into an inanimate object would be a death sentence. But even when a propeller hub spins, or a blade curls back at a sickening angle the damage

Beyond collision, another culprit is fouling the propeller with someone's abandoned dock line. When it wraps around the propeller the engine keeps turning the propeller shaft but the propeller is locked up tight. What happens next isn't pretty. The rubber hub breaks loose from the metal hub and begins to spin in its bore at 1000- to 2000 rpm. As a result not all the engine torque converts into forward thrust and you begin to notice decreased acceleration and top speed. The solution is as simple as the sea is salt: Replace the hub. Depending on its size a propeller can be reconditioned for as little as 30 to 100 dollars. Bent blades cost extra.

How Can You Tell The Propeller Hub Is Spun?

So how do you know definitively that the propeller hub is spun? Begin by removing the propeller and examining it. Look closely at the rubber hub for telltale clues, anything from subtle to dramatic signs of failure. The hub might be twisted or even look melted. Another method is to use a ball pein hammer and a center punch. Pein a dot on the metal splined hub and adjacent on the mating surface of the propeller proper. Reinstall the propeller and go for a ride. Run the engine under load. Merely idling dockside may not flow enough torque to challenge the hub. After a few minutes remove the propeller and examine your handiwork. If the dots no longer line up, then the rubber hub is spinning in its bore.

So how to you replace a spun hub? That's easy: Get out your debit or credit card. This is not a do-it-yourself project. Propeller shops use a hydraulic ram and many tons of pressure to press the bad hub out of of its bore. It is then unceremoniously tossed in the trash heap, or more properly, the aluminum or stainless steel recyling bin. A shiny, new hub is pressed in place, carefully. To ease the interference fit it is liberally lubricated with a spray of soap and water.

Some new wave propellers dispense entirely with the venerable pressed hub system. In its place are replaceable hubs with a different hub available to fit each one of the spline patterns required for the different brands of outboard motors and stern drives. In the unlikely event a hub is ever damaged simple take off the wheel, lift out the wrecked hub and throw it in the trash bin. Insert a new hub and you are back in the water in matter of minutes. Beyond the quick fix, what is really nice about the system is the way you can switch from one pitch propeller to another. That is invaluable for a boat that pulls double duty as a fast flyer or a ski tow boat. No matter what kind of propeller you have it's a good idea to have a spare onboard.

Marine Engine Lay-Up Book:

Marine engine layup the book

A Step-by-Step Guide to Decommissioning
Inboards, Stern Drives and Outboard motors

This small book features notes on how to service, diagnose and repair Outboard Motors, Stern Drives and Inboard marine engines. Coverage includes: winter lay-up and spring commissioning procedures; tune-ups, carburetor rebuilds and water pump repairs.

It also features a special section with forms to record work completed. Be advised this is not a shop manual, but instead a marine mechanic's notes on how a DIYer can accomplish these otherwise expensive procedures on his own.

About the Author

The author is a factory-trained marine mechanic and service manager who, as a contributing editor, wrote about marine engine technology and repairs for Motor Boating magazine for three decades as well as contributing stories to Popular Mechanics, and various Boating and Yachting magazines in the US, Australia, Singapore and Brazil. He has plied the trade in the Caribbean, Iowa, Arizona and California.

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