Skip to Content

First my apologies to our loyal readers. I’ve been traveling and under the gun with some deadlines that just could not be put off. So, I’ve been a little lame in the posting area for the last week or so.

Well as usually happens, one of our regular readers and a good friend of mine shot me an email last week talking about a boat he had visited recently that illustrated a problem that can have dire consequences if you miss what is a really important maintenance point. The issue has to do with the electrical termination points on the back of your engine(s) starter motor or alternator. I’ve talked in the past here about the importance of keeping these terminal studs booted (insulated) but what Wayne observed on his buddy’s boat could easily turn into a horror show.

On the boat in question, the owner had recently installed high output alternators. Not a bad thing necessarily. But, in this case the higher output units were a bit larger physically that the original equipment alternators. No surprise there either. Well anyhow the installer had mounted the new units and adjusted the drive belts to an appropriate tension. In the process of doing all of this the stud on the back of the alternator that is normally labeled B + had a boot installed, but to achieve proper belt tension, the alternator had been pulled into a position that wedged this boot up tight against another metal component mounted on the engine.

Due to vibration, its just a matter of time before that boot gets chafed through, exposing a bare metal stud to this engine part. Look at the photo below and in the photo, the terminal actually got bent a bit and forced contact with the engine. The point of contact is identified by the obvious corrosion shown in the photo.


engine stud

So, the question is so what? Well, in the example above the net result of this corroded connection between the DC positive conductor and the case of the starter motor solenoid in the example shown cost the boat owner thousands of dollars in loss. The propeller shaft that is attached to the engine shown here is shown below:


wasted shaft

The really interesting thing about this particular boat was that it apparently didn’t take even a week’s time for the propeller shaft to end up looking like this. The cause? Not a sea monster chewing on the metal folks but DC stray current corrosion. Even with the obvious low quality connection at the alternator, enough battery level current was able to enter into the engine and migrate right out of the boat via the engine block, transmission and propeller shaft.

So the moral of the story? Carefully, and I mean very carefully, check all of the electrical connections on your engine and make sure they are properly booted or other wise insultated from the engine block itself. And, make sure they are positioned and located such that chafe due to normal engine vibration can’t damage this insulating barrier. You do not want to have to go through what the owner of the boat in the two photos above went through. Believe me.

Special thanks to my friend Dudley Gibbs at Dudley’s Marine Electric in Puerto Rico for the photos. They were actually found on a boat he was working on about six years ago.

Comments are closed.