Joseph, I'm well aware of what can happen once water gets in the gear case,
perhaps more than you, since I've had it happen many, many times over the
last 25 years. In my little duck boat (w/a 25 Johnson) I've hit things and
cracked the case. I've hit things and shattered props. I've hit things and
bent the prop shaft. I've sucked in enough mud to not only wear out the
impeller, but also the ss housing and groove the drive shaft. Once I've
gotten water in, I've had it freeze and put more cracks in the case.
(Did'ja ever have to put a heater under the foot so you could melt the ice,
so you could turn over the prop??) In addition, I've wrapped so many weeds,
ropes, decoy lines (I use 100# mono on mine.), I have nicks in the housing
where I've cut these things off. The back side of the housing (junction to
the prop) is actually tapered and polished from wear of lines, weeds, etc.
BUT---I've NEVER found line that made it all the way to the prop shaft to
put pressure on the seal. Have you? Or, are you just repeating what you've
heard?
Incidentally, I also have a 200hp merc on an old V20 Steplift (only for
about a year). I've picked up line around the prop twice but none made it to
the shaft. So, I stick by my earlier statement, based upon my knowledge of
the design of these two engines (but I won't say it CAN'T happen, just that
there are many other possibilities). Maybe habbi's engine is different and
will easily allow line to wrap around the prop shaft and mash against the
seal which somehow opens it??
I will say that a "good" mechanic with analytical thinking abilities and
diagnostic abilities is extremely rare. I'm guessing there's maybe one for
every thousand parts replacers. BTW, it's NOT always evident that you've
got a leak just because you raised the engine , and as you said, "the owner
should notice a leakage of oil coming from the seal dripping on the ground".
There's a lot of other forces at work when you're rotating a 10# prop down
there at 2000 times a minute.

Bottom line--"habbi's" mechanic should NOT assume the lube and water
transferred through the prop shaft seal, replace that and send him on his
way. The mechanic should KNOW the cause. With a pressure and/or vacumn
check with or without prop/driveshaft rotation and shift shaft rotation the
problem should be located
LD