A spun hub on an outboard motor happens when the propeller body becomes detached from the hub, causing the hub to spin futilely without turning the propeller. Because the prop is no longer firmly attached, it will eventually spin off and get lost. Then, you'll need to go to a prop shop and get a new hub and prop, which is a very common repair.
The most common reason for a spun hub is hitting something while the propeller is spinning, whether it be a rock or a piece of garbage that gets tangled up in the prop. If this happens, you will probably be aware of it and know to look for damage.
Sometimes, even if you have never hit anything, the bond between the hub and the propeller just lets go due to deterioration of the adhesive around the rubber hub. This is a relatively simple fix because the parts aren't actually broken -- just unstuck. Or, the rubber of the hub itself may have shrunk over the years, requiring a new hub.
The poppet valve is an important part of the engine's cooling system, and if it goes bad, it can cause a spun hub. This is because the excess heat vented by the engine can heat the hub and the propeller to the point of failure as the exhaust goes past the hub.
Some new propeller designs, such as the Mercury props with Flo Torque II hubs, are supposed to eliminate the spun hub problem entirely. Instead of a rubber hub, they use a plastic insert, which is easily checked and replaced by the owner of the boat. The only reason these hubs would spin is if you hit something hard enough to shear the plastic.
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