The switches are Perko switches. Yes it does start much better with battery switch set to ALL. I'm sure the issue is that the voltage is dropping to 9+ volts under a load but what would be causing such a drop?

Reread the Duracell battery specs then buy starting batteries to start your BBC's and use the deep cycle batteries to run
your stereo.

http://www.driveduracell.com/assets/...image/1853.pdf

Sorry but your logic is Bass Ackwards.

I would start by replacing the heavy leads from starter to switch and from battery to ground. Had that problem in my old checkmate, unless batteries were completely fresh, engine would crank slow and not have enough voltage to fire MEFI1 computer. Replaced the leads and made sure I had a good clean ground. Spun right up good as new. The heavy leads even while insulated will get corroded over time, especially depending how the crimps are made. The factory crimp ends can be improved by sweating some solder into the ends before you install them.

Chances are it is a failing crimp or dirty ground somewhere.

Thanks. I was thinking that was where I might start. Not looking forward to trying to get at the starter cable though. One other note - I did have to replace the onboard charger this spring. Would only put out 11.5 volts to one battery bank but that shouldn't have anything to do with what i'm experiencing now.

Use cranking batteries for your starter and deep cycle batteries for your house loads. Deep cycle batteries are not designed for high amperage / short duration demands as in your starter.

First I would not call 12.7 fully charged. should be 13.2 at a minimum. I would check your charger and or alternators . Second make sure you have a ground wire coming off each battery going to the block. Not just a jumper black wire going from one neg terminal to the other neg terminal then to the block.

Chris

Boat only has room for two batteries. I've always run deep cycles and never had an issue.

I'm having the same issue as you. I have fresh batteries, 1 is an Interstate cranking that I bought last May, 1000 CCA. The 2nd is a Duracell AGM dual purpose, 800 CCA. I load tested both and they are good. But I get a bad hesitation when I start my 496 Mag. I'm worried that it's the started because it's hell to get to if I have to replace it. None of my cables are corroded and I use dielectric grease on the battery posts.

Dielectric grease is non conductive. I wouldn't put it on a battery post.

https://www.w8ji.com/dielectric_grea...ive_grease.htm.