Thank you guys for the replies.
There is one problem. The propeller shaft has a roller chain coupling
that connects it to the hydraulic drive. I think that this type of
coupling does not take any axial load too well. The only place left to
take the axial load is the strut in front of the impeller. When I look
at the plastic washer I can see where the axial load eats away at the
plastic thrust washer. That is why I was thinking to replace the
plastic thrust washer with something more robust.