Re: Cracking around prop
After a day out in my boat today, I have a bit of info that will probably clear up the issue of why your corrosion line is so high, even though I don't think it makes much difference in terms of the issues that you are facing.
As is true of pretty much all of us, I have never paid that much attention to how deeply my motor sits in the water when the boat is at rest. Today I did, just because this thread was on my mind. As it turns out, the water level on mine in about an inch or so lower than yours. I have two large marine batteries under the splashwell, but no fuel tanks - mine are saddle tanks that are mounted further forward.
My motor is a 1972 Johnson 65hp (3 cylinder) but the midsection and gearcase are pretty much the same as yours. With that in mind, the comparison is close enough to be "apples to apples." Given that my motor is probably about 60 pounds lighter than yours, and you have three batteries in the stern, the lower position of your motor makes sense. Natural bouyancy of one boat v. the other could come into play too.
My point is that there may not be anything out of the ordinary about the height to which your motor was damaged. I don't know that I have ever seen one corroded that far up the leg, but I am now thinking we were all wrong about the motor beong on another boat. At this point, it just looks to me like the previous owner took lousy care of his motor.