Boat starter motor

Check and test for a failed bendix gear in starter --flywheel damage or motor seized

our school in 1964 this was covered. Old school.
we now just hot wire it on car, and bam the answer ,ask.?

first is torque reaction, get the starter held down or it will want to walk away, and ? danger can happen. (picture shorting battery and acid flying to eyes)

1: starter held FAST.

2 then connect jumper cable Plus RED to out side large, solenoid lug. and battery , +
3 connect jumper cable black to battery neg lug.then other end to starter casing (mount)
4: jumper the outside lug to the dinky lug, the solenod engages
the pinon flies out and the starter pinion spins fast.
when done remove battery minus lug first.
end.
in the olden days, we fixed bad starters.
and this test , finished that job.
new brushes, and or solenoid and bam.
i just did this on a 68 bug last week.

Sounds like it is not toughing 100% and making full gear contact with the fly-wheel. Replace starter or the misalignment can burr the flywheel causing rounding and subsequent damage to the device.

very good post, few like it.
this is the starter free spinning
the solenoids job is the engage the teeth the fly wheel (ring gear)

the pinon shaft in starter is dirty, clean it (easy fix)
bad starter solenoid. get a new starter , most solenoids cost more.
your suspisons are dead on.

Try turning it over with the spark plug removed. If it now turns over the starter is weak and needs to be replaced. Also check the oil level. Too much oil will will lock it up.

Have the battery tested for a dead cell. It may test at 12 volts but that doesn't mean there isn't a dead cell. Also, if the motor has good compression you probably shouldn't be able to turn it over by hand at the fly wheel. Maybe an old school motor with V belt, but not a newer motor with all those accessories attached to the front.

Without hearing it I would only be guessing, however I believe starter solenoid is a good guess.
When you turn the key the first position it applies power to the vehicles ignition coil and other operational components. The second position applies power to the starter solenoid which moves the starter gear against the fly wheel so the starter motor can crank the engine to start the combustion process. If the solenoid does not move the small gear against the big gear you will hear the starter motor running with no resistance producing a high pitch whine. Some of the gears could be missing on the fly wheel but it would not start unless you physically moved the vehicle forward to manually move the fly wheel.
Therefore, I would have the starter (starter and solenoid are usually together) checked or replaced. If you have some mechanical knowledge you could do it yourself with minimal tools. I believe turning the key back is not helping you. You just need to turn off the ignition and try to start it again. Sooner or later the solenoid will fail completely and not work at all. I would not want to be stranded so I would fix it soon but it could work intermittently for a while.

If you hand-turn the flywheel, and it fails to turn, it might be that your engine is 'frozen". Remove the starter-motor and use a sizable flat-end screw-driver to try to turn the flywheel, just to be sure! The starter-motor is behind the flywheel, and if it is worn, you should get at least a 'crink' sound to indicate that the starter-motor pulls out and spins a bit. Yes, it is a big job removing the flywheel. You must have oil in the engine at all times!

You may need to shim the starter and I would have the starter tested.It is not uncommon to purchase a faulty elecrical part of any type from a part store.