I am running 2 batteries in my boat. I have a battery switch: 1,2, 1+2, and off. My batteries are different sized. Do i need an isolator. I plan to use starting battery ONLY for starting and cruising. Second battery for radio only while beached. Do i really need an isolator? If i need to recharge my radio battery, can i just start with starting battery and switch to #2?

An isolator makes it completely idiot proof.

I personally don't put battery switches in without an isolator. But I'm odd that way.

GT

But if i use a switch doesnt it act like a manual isolator? As long as i switch it to 1 when starting/ running and 2 for stereo only?

Not really.

What the isolator actually does is charge both batteries no matter what position the switch is in.

What the switch does is act like a set of permanent jumper cables.

To each his own. I just hate screwing with shit all the time lol.

GT

Ok. I dig the idea of charging both batteries no matter what position. Currently i have 1 wire alternator to starter. From starter to switch. Then from switch to both batteries. I also feed my dash from the alternator. Where do i install isolator? what needs to change?

Why are you feeding the dash with a wire off the alternator?  That is really dirty (noisy) power.

Your system will pretty much stay the same.

The big change will be the charge wire from the alternator.

Alternator power pole to center of isolator, number one of isolator to pole one (batt 1) on the battery switch,  number two of isolator to pole 2 (batt 2) on switch. Your dash should be fed from the starter power lead.

GT