Batteries: (2) Group 4D or (3) Group 31 ?

[Whimsy]

The boat (1972 Morgan Out Island 41, on the hard, new to me) has a battery 'tray' with two dead 4D batteries in it. Was going to replace them when I noticed that it seems three Group 31 will fit that space also(?).

Aside from having to make some jumpers and lengthen a few wires that go to the terminals, what are the pros and cons of having three batteries, vs two, feeding a standard OFF-1-2-BOTH battery switch?

I guess with three, two will have to be in parallel and one on its own? Will this create problems? Or is this better so two can feed the house and leave one in reserve for starting the engine?

Both batteries are of the same brand and line:
"EXIDE NAUTILUS MARINE & RV BATTERIES - DEEP CYCLE"

Spec sheet covering both batteries:
http://www.exide.com/portal/server.pt/gateway/PTARGS_0_0_405_221_16527_43/http%3B/WINAPAQLOG1.Inside.Exide.ad%3B7089/publishedcontent/publish/exide_corporation__public_/transportation/north_america/images/specs_marine.pdf

All the advantages in the specification department seem to be in favor of the Group 31s:

Price:
Group 31: 3 x $89 = $267
4D: 2 x $149 = $298

20 hr Ah rating:
Group 31: 3 x 115 = 345
4D: 2 x 160 = 320

Marine Cranking Amps:
Group 31: 3 x 900 = 2700
4D: 2 x 850 = 1700

Reserve Capacity:
Group 31: 3 x 205 = 615
4D: 2 x 250 = 500

Questions:
- How can the three Group 31s seemingly offer so much better specs over the two 4Ds, when their footprint is about 25% smaller and their height is over an inch less?

- Why does a Group 31 DC have more MCA than a 4D DC?

- Less cost, less size, less weight, more MCA, more Ah, more RC... How can this be - am I missing some important factor here?

Any comments will be appreciated.

[djmarchand]

I am amazed that boat builders continue to install batteries (4Ds and 8Ds) that require supermen to R/R.

What you need is a group 31 STARTING battery connected to one terminal of the 1,2,all,off switch and two group 31 DEEP CYCLE batteries connected in parallel to the other terminal. Use the starting battery to start- it will have pleny of oomph, then switch to all to charge both sets and when you are at anchor switch to the house batteries to protect the start battery from running down.

It may take some searching to find a true deep cycle battery. I suspect that many brands label a battery deep cycle when it is really not. A clue is that the spec will be in CCA or MCA for a fake deep cycle. The only spec that matters for a deep cycle batteries is amphour capacity, usually measured at an amperage draw of 1/20 of capacity.

David