I wrote this for i-Boats some time ago. See if it helps.
Probably95% of all starter problems is electrical. Is the battery fullycharged? Check all your connections. They must be very clean andtight. Check the lugs very closely for any signs of corrosion. Theycan corrode inside where you can't see it and cause the problem youare having. If you know how to do a voltage drop test you could checkthem that way. Another way would be to temporally substitute thecables with jumper cables. If all the cables have been confirmedgood, try jumping the starter solenoid. Just because it clicks doesNOT mean its good. If you are sure the solenoid is good have yourbattery load tested, regardless of age. This brings you to thestarter itself. Remove it and have it tested by your local autoelectric shop. They can rebuild it for you if needed and save yousome money. Let us know
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va