I've been recently trying to fix my dad's boat, and wanted to poll the aldeer knowledge before diving too deep and forking out big bucks on parts.

Basically you turn the key and nothing would happen at all. Maybe 1 time out of 20 the starter would engage if that. Troubleshooting with the multi meter showed input voltage to the solenoid from the battery and voltage from the switch signal to the solenoid. I wasn't getting anything out of the solenoid to the starter though.

My dad replaced the solenoid this weekend and the starter engages every time now, but does not have enough power to turn the wheel over enough to crank the motor ....

Reading different forums it could be that the starter now needs rebuilt/replaced, there could be other mechanical failures binding the motor, or still an electrical problem not delivering enough current to the starter?

Next troubleshooting steps:

1. Remove spark plugs and turn wheel by hand. If it's still hard to turn, then there a mechanical issue binding the motor. How hard/easy should this turn with plugs in/out?
2. Bypass the solenoid with jumper cables and directly send current to the starter to see if it behaves differently.
3. How hard is it to rebuild a starter vs. buying a new one? (Context for skill level: I'm a SW Engineer with some EE classes under my belt back in the day; Worked on vehicles enough to be dangerous Chilton manuals and youtube videos.)

Thoughts and help please