Hi Bill You're going to receive lots of answers, for this is a favourite TWL subject. I just went through all the questions you pose as my 4-D's died last summer. After lots of help from TWL members and advice from paid professionals, I offer the following comments: A 4-D is the right start battery for your Perkins (I have 120 Lehmans), and I recommend you stay with one 4-D and switch the other to a couple of Golf Cart batteries as a house bank. They'll fit in the same space. Wire one alternator to charge each battery group. Start the engine that charges the start battery first. Although I wanted to convert my 4 4-D's to all Golf Carts and had actually bought 8, every professional including the battery vendors recommended an 8-D or 4-D for starting, and argued strenuously against anything smaller. Since an 8-D was too big for the space, I returned a pair of the GC's for one new 4-D. You need a lot of amps to start, and you're cranking two engines one immediately after the other. The remaining 6 golf carts serve as a house bank. BTW, I was surprised at your estimate of 130 lbs for a 4-D, so I looked it up and you're right! However, I'm 60 and was able to remove all 4 and carry them off the boat and down the deck - one at a time of course - and bring the new ones in without much difficulty. The carry handles make a big difference. BTW, wear old clothes. You WILL get acid on them. Finally, my 4-D's died because I didn't check and maintain the electrolyte levels. I recommend you relocate your new batteries to somewhere that you can get to them routinely. Cheers, Garrett