The Prop is stuck from just before the shear pin hole back.
I’d like to see what condition the L U is in inside. Not seeing
anything in there could be a bad sign. There are no seals so
some grease goes out the shafts. This lubes the bushings but
water enters replacing the grease. If water was left in there
to sit for decades you can have a prop gear that is fine ¾
of the way around and the teeth rusted off to some degree
on the rest of it. Maybe the last person to run it used gear
oil and it has all run out.
Using it without inspecting it first? Well if it’s junk you can’t
hurt it. Try moving the prop shaft up and down & side to side
to check the condition of the bushing. It is what it is but this
can give you some indication of what you might find inside.
Turn the prop back and forth to check the gear lash. Do this
every ¼ turn of the prop. Expect some slop but if it isn’t
excessive filling the L U with grease and using the motor may
free the prop up some. If the prop nut is tight up to the prop
I’d take out the cotter pin and back the nut off a couple holes.
You want to use lubriplate 105 or John Deere corn head
Grease in the gear case.
Looking at your L U picture, the steel driveshaft tube and
not having a cavitation plate indicate its early cast tank
Johnson. If you don’t want to disclose your serial number
would you say if it is in the hundreds or what thousand’s?
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.