Twice now, I have had issues with my batteries getting sucked almost dead when I have idled for a prolonged period of time. When I'm underway, both are charging at 13, and if I run hard for a bit, I have no problem at all restarting.

I have Optimas and run with my Perko switches in the "BOTH" position all the time. Any ideas?

Just guessing bad batteries have them tested. I would also put a meter on the batteries and see what they are really charging at. !3 volts is low.

Optimas that were put in last summer...

belt good and tight I'm assuming.

Yes. I am not a fan of them for that reason. Seen a LOT of them go bad fast.

Yup...both of them. And this is the big reason why I'm having trouble with this...BOTH are acting the same way. Even with the batteries...I just can't see having two fail that miserably...maybe its just my luck?

Run the switches so that each engine charges one battery, not both to both. That will help isolate the problem and is better for the alts and batts.

Yeah what he said
Picture two pumps trying to supply water through the same tube to two different tanks....while they do work properly in the begining over time you will see one pump eventually supply more then the other pump......in the case of alternators you will eventually back feed the other alternator and cause all sorts of problems...
Best thing to do is let each Alternator do it own thing with its own battery....

Check the charging voltage at idle. Also, make sure that your alternator has enough output to run all of the accesories that you are using at idle. If the voltage is below 12v at idle, I would consider increasing the pulley diameter or rebuild/replace your existing alternator. The only downside to increasing the pulley size is the possibilty of slinging the belt at high rpm. Hope this helps some.

Thanks guys.

I wonder if I may have already created the "backfeed" problem...I've been running this boat with the batteries on "Both" since I bought it.. Perhaps it's time for some rebuilds...