Diagnose and Repair a Spun Propeller Hub

The lowly propeller provides forward motion. At least until bad things happen to a good propeller. You’d think the punishment propellers endure when they whack into an inanimate object would be a death sentence. But even when a spun hub spins, or a blade curls back at a sickening angle the damage usually be remedied and for less money than you may think. Beyond collision, another culprit is fouling the propeller with someone's abandoned dock line. When it wraps around the prop the engine keeps turning. What happens next isn't pretty.

Depending on the size of the wheel and the extent of the damage inflicted a propeller can be reconditioned for as little as 30 to 100 dollars. Step one is to determine whether a hub is spinning in its bore, and incapable of transferring engine energy into forward motion. Find out in minutes by removing the propeller and peening a dot, both in the hub and the mating propeller surface. Use a hammer and a center punch.

Next re-install the propeller and run the engine under load. Remove the propeller again and look at your handiwork. If the dots no longer line up, the rubber hub is spinning in its bore. More obvious symptoms are melted rubber, where the hub has begun to disintegrate.

So how to you replace a spun hub?Prop shop use a hydraulic ram and many tons of pressure to unceremoniously press the bad hub out of of its bore. It’s tossed in the trash heap. A new one is pressed in, carefully. To ease the interference fit it’s liberally lubricated with spray of soap and water

Some new wave propellers dispense entirely with the venerable fixed 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’s really nice about the system is the way you can switch from one pitch propeller to another. That’s invaluable for a boat that pulls double duty as a fast flyer or a ski tow boat.

Straightening bent blades is a very delicate matter. Savvy prop techs never use force. Instead, they reach for a bigger hammer. To reform the blade to its original profile, the wheel is placed on mandrel, which is just another name for a fancy shaped blacksmith’s anvil. Each mandrel is calibrated to shape a different pitch. Sometimes the blades are heated before hammering because softer metal is malleable and returns to shape without stressing the metal.