Background
We had been having intermittent issues with the starter not working. This was not an engine issue. If the starter did turn over, then the engine would fire up nicely. At first, this was a rare issue, but it was getting worse, and created a safety issue. When you need your engine, you usually need it right now. It was getting to the point that we were afraid to take her out of the slip.
Initial Fix Attempt
First I replace the starter, solenoid, panel switches, and wiring from the starter switch. Turns out this not only failed to fix the issue, but seem to make it worse. After all these upgrades, the 20A fuse just before the solenoid would get hot, and if you kept the switch engaged for more than a few seconds, it would blow the fuse.
Problem Diagnosis
Turns out the solenoid on the new starter is rated to pull 32A. This along with the heavier wire from the switch allowed more current than the fuse would support. I was surprised that the solenoid would pull that much current, but then I learned that it is not just connecting the load contacts, but it also drives the pinion into the flywheel. If the solenoid cannot pull enough current on the signal side, it is unable to hold the contacts closed, and it cycles on/off very quickly and fails to provide sufficient current to the starter.
Real Solution
Rather than just putting in a bigger fuse, I decided to remove all that current from the engine panel wiring completely and add a relay in front of the solenoid. This would pass those 32A from the battery though the relay and on to the solenoid all within the engine compartment. The wire from the panel starter switch would now only need to carry enough current to engage the relay.
Now the panel starter switch engages the relay (~4A), which passes current to the solenoid (~30A), which then passes a large amount of current (~80A+) to the starter itself.
While I was at it, I added a 20A blade fuse on the 12V supply to the panel and a 35A blade fuse on the load input to the relay.
Success!
This completely solved the starting issue, and is now a much safer setup.
See the engine wiring page of the electrical schematics to see where the relay fits into the overall engine electrical system.
