I have the same engine
. If you look closely at the factory wiring
diagram you'll notice there are several factory crimped connections in the starting circuit. Each one of these develops more resistance over time.
One and a half ohms at the switch probably isn't enough to cause trouble, but combined with the other voltage drops from resistance it can be. It takes about 10.5v to engage most common solenoids.
It's not uncommon to see a remote
starting switch in boats with 3gms for maintenance
reasons and to bypass this issue when it happens.
edit .... after rereading your post it seems maybe the trouble lies in the solenoid itself. It's "kicking in" but not passing high current
to the starter motor. This could be corrosion
or wear on the high current
contacts that connect the battery
cable to the starter motor inside the solenoid.
In fact this happened to me last year. The day after I told my friend the starter wasn't going to make it many more sails
before it failed to start, it didn't start. Changed to my spare starter and we were sailing in about 45 minutes. Still haven't replaced the solenoid on my now "spare" starter.