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Understanding Boat Propeller Assemblies

propeller assembly

An interchangeable hub propeller assembly

On the Prop Shaft

A typical recreational boating propeller is mounted on a splined propshaft with a thrust washer, spacer, washer and nut, with some provision to keep the nut from turning loose, such as tabs on the washer or an additional cotter pin. There are exceptions, like the shear pin systems that were once widespread, but now most are as discussed here.

thrust washers

Various thrust washer shapes. Not all are splined.

Thrust Washer

The thrust washer is mounted on the propshaft, between the lower unit and the propeller, and it may or may not have splines. It’s an important component of a boat propeller’s installation, because it fits the shaft in such a way that will not allow either itself or the propeller to move forward under thrust, thereby transferring thrust to the shaft and protecting the gear housing and its seal from contacting the rotating propeller.

A matching taper on the propshaft and the washer is one effective design that stops a thrust washer from moving too far forward.

The thrust washer also helps to keep the propeller centered on the shaft when the soft inner prop hub fails to do so under adverse high-torque conditions.

interchangeable and pressed hubs

A Rubex interchangeable rubber hub (left) and a pressed rubber Solas hub.


Inserted in the prop is a splined inner hub, which serves as a shock absorber between the prop blades and the shaft. A good prop hub should keep the propeller precision-centered on the propshaft, dampen vibrations and help protect the shaft from the severe shock of striking a submerged object with the propeller.

There are two basic types of hub (in addition to some rare specialty hubs): a pressed rubber hub or an interchangeable hub. The pressed hub is semi-permanently forced into position by a high pressure press in a prop shop. By contrast, the interchangeable hub is easily inserted by the prop installer. Each propeller is designed to use one type or the other. While pressed hubs are made of rubber, interchangeable hubs are sometimes made of a more rigid material, but we find that rubber interchangeable hubs offer better vibration dampening.

What’s the difference between Rubex and Solas? Most Solas propellers are of the pressed rubber hub type and a few have specialty pin drive hubs. All Rubex props use interchangeable rubber hubs. There you have it!

Next: Spacers and more in Part 2.