This whole installation sounds 'Rub Goldberg' and I can't imagine any
professional builder or boat yard doing such an installation where there is
no proper thrust bearing.

As I stated earlier in this thread, I suspect someone, in the past, has
replaced the OEM hydraulic drive motor with this chain drive system.

I find no fault with the chain drive if there is no way to get a hydraulic
motor down into the space. It's hard to say without seeing the
installation..

However, to salvage this installation, I would recommend putting a thrust
bearing on the forward (inboard) end of the shaft and the bearing should be
mounted to some substantial member in the bilge. This would have to be
glassed in and should be able to absorb both the forward and reverse thrust
of the shaft.

Even though the engine is only 15hp the thrust is not to be taken lightly
(literally). If it move the boat, then there is a mechanical force against
the hull or in this case, the strut, IMHO.

Steve
s/v Good Intentions