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.
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.
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.
The prop nut secures the prop to the shaft and helps transfer reverse thrust to the shaft.
Normally made of brass, a nylon locknut is used in conjunction with a tab washer or else a castellated (castle) nut is used with a plain washer and a cotter pin on a drilled shaft.
Where used, the cotter pin passes through a hole in the propshaft after the castle nut has been tightened with its slots aligned on both sides of the hole.
With so few pieces, a prop assembly is not complicated. Because each of the parts in the prop assembly performs an important function, it is important to use the correct parts and to install them all as intended. To maintain the correct positioning, spacing and integrity, each part must be made to match both the propeller and the drive unit that it’s going on.
Whenever doing a propeller installation, use your knowledge and pay particular attention to any instructions that may be provided with your prop or hub kit, especially concerning which parts to use.