Hi Roger,

I have series 31s for the reason you stated.  You probably can get two batteries in the same location as one 4D battery.  Do measure but that should be the the solution.  In terms of power two 31s equals one 4D battery.  

Good luck,

We've used Tractor Supply 4DLT's starter batteries for our light cruising/day sailing. Get 4-5 years of life and they are about 15 pounds lighter than  a regular 4D. Current cost is ~$220.

I'm surprised that you have not received more responses.  I switched out my Interstate 4D batteries a number of years ago for 4 Trojan 6 volt wet cell batteries.  Not only are they significantly lighter and easier to handle, they collectively have more amp hours and last longer.  I replaced my first set after 7 years.  During this time we would cruise 4 to 6 weeks each spring, do day sails and also go on shorter trips of 3 to 7 days.  They also readily fit in the battery compartment, although I removed the original battery trays and am using a standard battery case for each one.  You will need new cables but this can be readily done at your local WM or other boat store. 

Roger, We use three group 31 AGM's in our 1990 Catalina for house batteries. A fourth battery located under the sink is our emergency start battery. Last set lasted eight years here in the Florida sun. Remember that you've got to keep in mind the ability of your alternator to supply the necessary current to charge this battery load. Many folks use golf cart batteries down here but I found them to heavy to deal with.

Battery discussions is a lot like discussing religion (not possible).  I have 3 banks: House w/ 2 Lifeline 31 TX (12vdc/125AH ea.), Aux. w/ 2 Lifeline 31 TX (12vdc/125AH ea.), and Staring w/ 1 Lifeline 2700T starting battery (12vdc/95AH).  The starting battery's only job is to start the engine.  This original discussion was about the weight of the batteries.  31 series vs. 4D batteries.

I also have solar panels, 170 watts total.

Good luck,