[Maxwell]

Cummins can give you a better answer but it doesn't sound right that your engines are slowly rolling over several/5 turns before lighting. I can't imagine any starter surviving long slow cranking that much.  Assuming the PO didn't install deep cycle batteries you may want to check the battery cables / connections as any one can cause the problem described. if they check out maybe load test the starter(s). By comparison if I don't parallel my Cats slow crank for maybe a second or two and then start right up. When paralleled they start right up with almost no hesitation even when cold. Good luck

Do you recall which batteries you are running in your boat?