What is the diode #9 used for and does it tie into the trim or neutral safety circuit?

http://www.mercruiserparts.com/Show_...PAGE+1+OF+2%29

it ties into the trim sys. it is used for triming twin engines with a single switch on the throttle control.

This may sound dumb but is it really necessary?

Only if you want it to work.

LOL somehow I knew that was the answer

Seriously, someone 'splain this to me like I'm a 2 year old. I know that a diode allows electric flow in one direction. If the circuit from 2 electric motors are in series controlled by a switch, why is that needed?

If you want to control two motors with one switch but still have individual control of each motor with it's own switch, you need the diode. If you don't have the diode, you're basically going to activate both motors with all three switches. The diode blocks the current from going to the opposite motor when using the single switch.

Okay, I see the sole purpose of it now. I'm helping out a friend with a 42' Excalabur. He is looking to use only the switch on the controls. Personally I think that is not the way to go. If you are using 1 switch for 2 engines you would not need the diode?

He can use just the one switch in the throttle for both drives. In order to retain the individual dash switches for each engine, it needs the diode.