More on the starter motor itself as cause of slow cranking:
In my case, all wiring had been completely upgraded and all engine upgrades recently performed. What's more, the batteries both later passed the load tests recommended here, meaning that they hold a charge fine and have plenty of life left.
And yet, the starter motor didn't have much oomph. Yes, diesels are hard to crank, blah blah, I'm no engine mechanic, blah blah, but still--you want the engine to turn over with some vigor. Is it worth pulling the starter off and taking it to Pep Boys, O'Reilly's or Autozone for "testing?" Yes, worth taking it off. Here are the three responses I got from those three stores, which are equipped with "starter motor testing benches" which show whether it runs or not.
1 "This motor can't be tested."
2 "This motor works fine"
3. "I have no idea what that test we just did shows."
"Well, can I talk to the manager, then?"
"I am the manager."
To pull the starter motor and solenoid requires a ratchet wrench and several extensions. It's probably held onto the block by two bolts. They're hard to access, but you can do it with patience and the correct wrenches--and probably without removing the manifold.
I took the motor, after the experience with the franchise outlets, to a local alternator shop. I suggest asking a boat yard what shop they use. Try to ask discreetly, not "In order to escape paying you, could you direct me to your subcontractor for an off-the-books cash deal."
A guy named Luis ripped my 28-year-old starter motor apart on his bench, blew it out with air, slapped grease on various parts, ground the bendix fair with a hand grinder, slapped it back together with his array of air tools, and handed it back to me in 15 minutes. "Give me 25 bucks."
Upon reinstallation on the M25, this same motor turned over the engine like pants on fire. I mean literally twice as energetically as before. I mean with gusto, baby, with emphasis and flair.
So, anybody with a 30-year old starter motor ignored since installation, I think we ought to at least take it out to dinner once, ask how it's doing, give it some appreciation for long uncomplaining loyalty, and then ballroom dancing.
Don't keep it out late and get it back on the boat in time for breakfast.
Work on the relationship!