What is this ? And what does it do ?

[RickK]

Double H,
A few questions.
The single battery has the positive lead running where? To the selector switch above it?
How many wires are running out of this (the single battery) switch?
The double battery switch has two wires running into it, one from each battery positive post?
Does this switch have two wires running out the bottom middle with one each going to each motor?

So it sounds like he has a house battery (single battery) with on/off switch and a starting battery for each motor  - now is where it gets weird.  If he has 2 wires coming off the center post of the switch he would be supplying juice to both motors from ONE battery at A OR B and then would switch in both batteries to both on "Both".  I'm thinking there is another A/B/Both switch missing - should be one for each engine.  Right now he can't combine both batteries to just one engine if necessary.
So if there is indeed a missing A/B/Both switch it would get weird again trying to combine to both from both (if you know what I mean....) - that is where the solenoid could come in - run a heavy wire from the center on both of the A/B/Both switches to one of the heavy terminals (one on each side) on the solenoid. Then when you activate the solenoid you'd finally be supplying both starting batteries to both motors at the same time.  And this doesn't include bringing in the house battery.

What would really help is some pictures.
Never having had a twin engine boat, I'm not sure how it is wired to start but.... Based on my wiring, I came up with one that would work for both engines (I think) off a single switch (two batteries) and use the solenoid to tie in the third (single) battery for emergency starting. Rick is right, no matter what color wire was used on the solenoid posts, it didn't go from the positive post of one battery to the negative post of another so....
Pics man, we need pics.

I'm not sure pics would even help at this point - my head hurts.

Edit:Here is a pic of what I think may have been happening: House battery was thrown in parallel with the other 2 with the solenoid?

[RickK]

Maybe this will help?
Thinking out loud here - Assume (ouch) that each AB switch is in the A+B position and then you activate the solenoid - would this work?  Would it put the starting batteries in parallel?  Maybe you don't even need the solenoid - having 2 AB switches both switched to A+B should put the batteries in parallel anyway.... I don't think it will hurt anything though.

[Capt. Bob]

Rick,
your first drawing is what I came up with also.
I assumed he send current to the two motors from the switch by the two batteries and used the solenoid to connect the third battery just for starting emergencies. Thing is, you can (as you showed) do the same thing without the solenoid by using the switch.

[RickK]

I think the 2nd drawing is the proper way to go, with 2 AB switches, but the solenoid is not needed in that application because 1) the ABs parallel both starting batteries and 2) I wouldn't tie the house battery in with the starting batteries - if both batteries are dead, you have bigger problems then one more battery will help.
On my 230 I have the house and single starting battery isolated, with an ACR between them for charging.  I do have a combine switch for emergencies.

[Capt. Bob]