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Propellers

What is a "Spun" Propeller?

Article by Thomas P. courtesy of e-Ribbing.com

propeller hub, stainless-steel propeller hub

The red arrow points to the propeller's hub, which is designed to give way when the prop hits something or is over-revved.

The Achilles' heel of a propeller is often the rubber or composite hub. If the engine is too powerful and the boat is run hard, the hub can detach and spin freely inside the propeller. It’s easy to tell when a prop has a spun hub because the rpm will increase when the throttle is advanced and the boat doesn’t advance.

The propeller rubber a fortified ring made of durable rubber that is inserted between the propshaft and propeller's inner hub. If the hub detaches, the rotation of the propshaft cannot be transmitted to the blades.

The Usual Causes of the Rubber Hub Detaching

  • A violent strike of the propeller with a fixed object. When the propeller is forced to abruptly stop in a collision, the first thing to be sacrificed is the rubber hub.
  • Intense use of the engine, an overloaded boat and many hours of use (or, to put it right, the many miles covered) of the propeller increase the possibility of the hub detaching.

stainless-steel propeller, propeller hub kit

Most modern propellers have a hub that can be replaced if it’s damaged.

Made to be Sacrificed

Because the rubber hub is responsible for a spun propeller, it understandable that some would ask why single-piece propellers do not exist or are not made. This type of propeller can be made, but then we would lose the advantage of the rubber hub, which is the absorption of jolts in a potential collision. The hub protects the propshaft and the gears in the lower unit by absorbing the biggest jolts in a collision. The hub detaches and keeps the jolt from reaching the gears and engine, protecting them from more extensive damage.

Signs of a spun propeller include delays getting on plane or reduced speed at higher rpm. Knowledge and experience will help us recognize the above signs, reduce the engine rpm and head towards the closest possible land at idle. Once on land, replace the propeller with a spare that was on board the boat along with the tools to change it.

stainless-steel propeller, propeller hub kit

This cutaway image shows the hub inside the propeller and how it’s designed to be sacrificed if there is an unexpected impact.

Many newer propellers have removable rubber hub kits that can be replaced at a fraction of the cost of a new prop. A boat running a prop with a removable hub should carry a spare kit that can quickly be replaced. We would still recommend carrying a spare propeller as well.