Charging question 9.8 tohatsu

I have a pair of 9.8 epuls on a sailing catamaran. Separate starting batteries in each hull are isolated from each other. Also have a house battery for electronics in the console, also isolated. BUT, for a Coast Guard certification, I am required to have a way to parallel the house to the starting system(s) and to parallel the starting systems, if needed. Seems easy enough, just 1- run a parallel wire from each terminal on the starting batteries, across the hull, connecting them and put an on off switch in the hot wire. and 2- run a pair of wires from the house battery in the console and connect them with a three way connector to the two new crossing wires, and put an on off switch in the new hot wire that comes off the house battery. That should allow me to use the three batts in any combo. QUESTION: will this muck up the alternators in any way? THANKS

Nope. Your alternators make 6 amps tops at WOT. The USCG requirement is ridiculous. But, having been there, done that with the CG, I would say, that is grounds are already common (as they probably are), just run one wire from one OB starting battery to the House Deep cycle, and forget it. Do you have any shore power charging? If so, charge all 3 as needed.

FWIW, I run a single group 31 deep cycle for everything, motor and all, on my 28-foot sailboat. Thing is, the 9.8 draws so little when charging, that a single bank for everything is fine, and no dedicated starting is needed. Deep cycle is fine. In your case, since you need to balance weight, just run two large deep cycle batteries (one in each hull) and tie them together, and forget about the one that is currently under the console.

Thanks Paul, Really appreciate your time spent here. Am still working on it, I have to figure out how to correctly size the cables etc, and need to show a load analysis to the CG. I can't find a number for the amperage draw when starting the 9.8. I realize this is dependant on many factors but is there a standard number I can plug in for a 9.8?

Most of these small OBs crank at less than 30 amps. So #10 AWG wire is sufficient. That's what I have on my 9.8... and remember that I run a single group 31 deep cycle for everything ... house loads and starting. If you prefer, #8 is good for 40 amps. You will never use it, but the loss will be less, so cranking may be a hair faster.

Personally, more batteries is a pain to manage and keep charged. If you want a second battery as backup (remember that you can pull start if there is no battery), you could just have two batteries (one for each side) that can tie together as desired with a cheap battery disconnect switch. Then, if you really want a separate house battery, also connect via a #10 or #8, and switch it as well.