Different starter issue than just posted
 

Got a twin 454s. One engine is fine. When I go to crank the engine its like off and on when the starter kicks on. I have checked all the connections, cleaned the terminals on the starter switched batteries and it still does it. Engine fires right up when starter works. When it doesnt work there isnt any noise click anything. Leads me to beleive its not the starter it may be something else. Any clues

Slave solenoid failing.

I was also thinking the ignition swtich. Where is the slave solenoid at.

The boat is at the shop getting some interior work out and I was going to check everything with a meter as well.

Usually on the same mount as the mercathode controller. You won't be able to check this with a meter most likely as the coil/relay is failing intermittantly. It could be a corrosion issue though.

isnt actually the part by the red breaker button

Yes that's it. When they go bad they act just like you described.

thanks yeah i did some research and that sounds like it. When I get back Im going to test every wire and connection again and order this part

Mine did the same thing, I'm curious why you would need a slave solinoid when there is a solinoid on the starter.

i read today something with amps and how merc built the engine not know exactly how things were going to go or something so added this extra peice

Its too long a run from the ignition switch (voltage drop) to energize the starters solenoid. You'd need huge gage wire and ignition switch. An auxilary contactor (slave) was the right design. The exact same solenoid is on the trim pumps for the same reason.