I figured I'd post a followup to this thread as the starter problem seems to be widespread. In my case the problem was twofold.
When I put a voltmeter on the starter, I was in fact reading good battery voltage. As I delved deeper I figured out a way to monitor
the battery voltage as I tried to start the engine and an entirely different picture developed. With the voltmeter connected to solenoid and engine block, I depressed the start button and battery voltage at the solenoid dropped to 8 volts and the starter wouldn't turn. I then changed my ground lead to the voltmeter from the engine block to the battery bank and repeated the test. Voltage remained rock steady. This indicated a poor ground from battery bank to the Starboard engine. I spent a few hours replacing ground connectors and cleaning
ground cables. In repeating the original test, voltage to the solenoid increased from 8 to 11.5 volts under load. Ground obviously repaired, but still no joy. I then replaced the starter and everything has been working perfectly.
I think SkiprJohn was correct in that battery cable corrosion
is an important consideration. I probably had the poor ground for some time and it was responsible for insufficient voltage being supplied to the solenoid under load. This in turn probably contributed to the contacts within the solenoid becoming dirty and ultimately fail.