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Understanding Boat Propeller Assemblies

rubber hub

Sometimes a pressed rubber hub assembly and the spacer are manufactured as a single unit.


Like the thrust washer, the spacer assists alignment of the prop to the shaft when the inner hub is unable to hold its shape, only this time at the aft end of the prop.

The spacer bears reverse thrust loads and prevents the inner hub from moving aft, with its large surface area. But it differs from a thrust washer in that it in turn depends on the final washer and nut to prevent it from moving on the shaft.

prop spacers

Spacers come in various corrosion-resistant materials and shapes.

Many spacers are splined and some (for numerous Mercury, Mariner and Mercruiser applications) are also cogged or pinned to work in conjunction with an outer tab washer to prevent the nut from turning loose.

tab and plain washers

Solas tab washers and a plain prop washer


The washer helps fill space between the spacer and the nut, where the splines on the propshaft end and the threads begin. This helps prevent fore and aft play on the shaft by the prop.

A tab washer can be used if the spacer is designed for one. Otherwise, a large traditional washer would be expected on an assembly that uses a cotter pin to retain the nut.

When used, a tab washer’s primary function is to prevent the nut from rotating in relation to the spacer.

prop nuts and cotter pin

Nylon lock nut, castle nut and cotter pin.


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!