Re: Replacing 12 volt batteries with 6 volt
"OK. I got 2 6v batteries. I saw some diagrams that showed 4 batteries ...Can I do that with just 2 6v and is it a good idea?"
You really want to get and read "The 12 volt doctor" or "the 12 volt bible" or a similar good book on boat electrics before you fry something.
It was set up for FOUR now you have TWO pretty obviously, no, you cannot set up 2 the way 4 were set. The only thing you can do with two 6v batteries is set tme up in series to make one 12v battery bank.
"I can set it charge to any rate I want to. I guess that I should change it to a different voltage for the Trojans! "
Rate is AAMPERAGE and has got nothing to do with VOLTAGE. Again, buy and read nice little book to avoid major problems. You will leave the charger set for 12v wet acid batteries, and set the rate to match the capacity of your battery bank. Which will be clear after reading the little book.
Your tied 12v battery probably will be good enough to keep as a starting battery. You may find that after equalizing it, and possibly topping up electrolyte, it is good enough for a starting battery. If not, go to Walmart, buy a properly sized starting battery and enjoy a 3-year replacement warranty and a good price.