For Comparison's Sake: Prop Hubs | Boating Magazine

For Comparison's Sake: Prop Hubs

The evolution of prop hubs.

Here’s a quick look at the evolution of prop hubs, the vital assembly that connects an engine’s propeller to its drive shaft and simultaneously acts as a shock absorber should a prop strike an underwater object. Prop hubs have changed quite a bit over the years, but their function remains essentially the same today.

Shear-Pin Hub System
On the Scene: Depended largely upon manufacturer, but most engines built in the 1940s through ’60s used this system.

How it Works: A shear pin locked the propeller to a spinning drive shaft, which allowed the prop to turn. It was designed to break — or “shear” — off if the propeller hit an object. A sacrificial component of the old-school prop hub, a broken shear pin still allowed the drive shaft to spin unimpeded, often sparing damage to critical engine components, such as the drive train.

Tidbit: Shear pins were not always reliable. Their tensile strength was inconsistent. Sometimes they would shear too easily, while at other times they became hardened with wear and wouldn’t shear at all.

Rubber-Sleeve Hub System
On the Scene: Began to emerge in the mid-1950s and was used for decades.

How it Works: With the emergence of higher horsepower engines, shear pins became impractical because of the increased torque. Engine manufacturers turned to flexible rubber sleeves that coupled the splined hub to the shaft and absorbed impact, breaking loose or tearing from a prop assembly upon collision. This allowed a drive shaft to spin freely on impact, sparing the gear case.

Tidbit: This failure, known commonly as a “spun hub,” often protects vital gear-case components from damage. Once spun, the hub usually holds its grip only while the boater idles home. Since these rubber-sleeve hub systems are installed with a hydraulic press, replacement, especially on the water, is not a do-it-yourself task.

Replaceable, Plastic-Sleeve Hub System
On the Scene: Emerged within the last decade and is commonly used today.

How it Works: Not only were rubber couplings hard to replace in the field, but they also had a tendency to fail (they literally melted) in the high combustion and exhaust temperatures generated by today’s fuel-efficient engines. This led to the development of hard plastic sleeves capable of withstanding such extreme emissions. These plastic hub couplings break away upon propeller impact and rotational torque and are much easier for a boatman to change while on the water.

Tidbit: Federal emissions regulations led to this design evolution. The regulations produced much greater fuel economies but also generated hotter exhausts, which forced manufacturers to retool the hub system.

Safety Torque Hub System
On the Scene: To be introduced in 2011.

How it Works: You'll find no plastic or rubber sleeves on this new prop hub. Instead, the hub is preset to release at a specific torque load, according to manufacturer PowerTech! Propellers. When a prop strikes an object, the hub releases at the preset load, protecting the driveline systems from twisting and breaking loose. But here’s the kicker: PowerTech! says that within 10 seconds of hitting the obstruction, the hub automatically resets, allowing the vessel to continue operating at full power.

Tidbit: PowerTech! says that this system was designed to permanently replace standard square-cavity prop hubs. Torque-load setting never needs adjustment, the company says, and installation is easy. Downside is the cost at $399; safetytorque.com.

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