You're leaning the right way, in my opinion.

While I cannot speak to battery locations on a pre-Mk II boat, I believe that your best (and most economical) near-term improvement would be to parallel the two house batteries. You've already stated that they are fairly new, and that they check OK with a specific gravity tester. Why not go with them, and simply parallel them into what is, in effect, one great big house bank?

Your new electronics will draw quite a bit of power, but the autopilot is a large current suck. I suspect that you ran one of your single house batteries down, and the re-start and motoring in brought them back to near full voltage.

Two Group 27's are not a lot for a house bank nowadays given the amount of electronics on boats. AFAIK the best of these will give about 95 amps each so at most 190 amps in total. We had two Group 31's for house bank on our 28 that were 115 each and with no refrigeration and minimal electronics it was OK.

Going to a single pair of 6 Volts isn't going to improve on that much, T-105's are 225 amps each but wired in series to get 12v that's all you get. US2200's are a bit more at 232 amps. More likely you'd want 4 X 6V to get you up around 450 amps. There are bigger 6Vs capacity wise but the size gets to be an issue.

4D's might fit and will give about 170 amps each. But I could not find true deep cycle 4D's (other than AGMs which break the bank), they are all just heavy duty truck batteries and given the number of receipts for new batteries we found in this boat, they don't last, PO was getting about two years max out of them. The ones in there when we bought it are pretty much dead. From what I could find out, 8D's won't fit in there and they will break your back trying to do anything with them, the 4D's were bad enough to get out.

I am currently working on conversion from 4D's to 4 X 6V's but it's a Mark II so I'm not sure if this is relevant to the Mark I. Space is tight but they will fit under the seat in front of nav station were the 4D's lived. We have about 22 1/2" X 18" wide. The US2200's are 10 1/4" x 7 1/8" each. Biggest problem is a bundle of wires coming out of the battery compartment at the hull wall, it takes 3" of space and has to be re-organized to be able to get a proper battery tray in place. The height is a bit tight at 11 1/4" but should clear, just. I built a battery tray for them to sit in and it adds about 1/2" inch to the length and width battery dimensions and 1/4" to the height so it is tight.

It is not an insignificant job but not the most difficult either and should be a lot better than what was there giving us 464 amps. We tend to be pretty conservative on power consumption so as long as there are no big surprises with the fridge unit it should be fine (he says hopefully).

Tom Grover put me onto this setup and he has a Mark I, perhaps he could fill in the details on that aspect of it?

Chris:

I have a C36 MK1 and have replaced the two group 27 batteries that came with the boat with four 6volt golf cart batteries, the US 2200's which will give you 464 amp hours of capacity. I installed them under the starboard side aft seat, forward fo the nav table. To make them fit, I cust the sides off of the battery tray that housed the two group 27 batteries and added a plywood floor to the compartment. I fiberglassed it in place to ensure there was no way any leaked battery acid would find its way into the bilge. the ifrst picture is of the original battery installation and the second is of the 4 golf cart batteries.

Chris,
A few years back, I also went to 6V AGM golf cart batteries in series. One bank of two under the Nav Seat (up against the cabin bulkhead. Another pair under the after starboard seat and a 12V starting battery under the forwars startboard seat. All and all the two house banks have worked well. The switching networks allow any combination of the three battery banks.

I have 2 4D batteries which are generally plugged into shore power, except for an annual cruise where we try to anchor. I carry an automobile portable jumper battery(the kind that includes jumper cables, light and air compressor all in one handy case). I keep this on board as an emergency starter battery, but have never tested it to see if it will start my engine. Has anyone ever used one of these to start their boat??

Richard,

Yes, I too keep one of these aboard for the occasional emegency starting battery,it will do the trick.

I have fought some battery/electrical issues recently that could have left me stranded until the wind came back, l also like to note how effective the compression release is when your batteries are low. If your battery only makes your engine go rrrrrrrrrrrrrrrrruh rrrrrruh and no start. Pulling the compression release up and hitting the starter often will get the engine spinning fast enough for the engine to start when the lever is released. I have used the booster battery with and without using the compression release and it worked well. Though if you are down to your last couple of attempts at starting before you totally run out of cranking power from any source the compression release is a good tool.

I occasionaly single hand my boat and have considered putting in or carrying a remote starter switch as there is no way to use the compression release and the starter button at the engine panel when you are by yourself.

Thanks for the comments; this is good information.