Don't go 8D!
Quote:"I had discussed with him the option of adding a starter battery on one circuit and putting the two stock 4D's in parallel for the house battery. He asked his instructor for his opinion and the instructor suggested buying a single new 8D and using one of the 4D's as a starter. The 4D's are being used now. I would have to look at the 8D's to compare AmpHours and reserve, etc. It merits some further discussion."
One 8D battery (Exide Marine Deep Cycle) weighs roughly 134 pounds and delivers only 200 amp hours while two 4D's weigh 196 pounds combined yet deliver 320 amp hours. 130 amp hours is a lot more punch than 200ah and besides you already have the 4D's. I personally don't like 12V batteries and much prefer 6V batteries in series/parallel to create a 12V bank. This is only if they will fit as 6V batts are generally taller than typical 12V batteries.
The ONLY reason we have the 4D's on our boat is because they were brand new and came with the boat, and I could not justify throwing them away. When they reach the end of their useful life I'd replace them with 4 Trojan T105 6V golf cart batteries. Four T105's weigh only about 204 pounds yet yield an amazing 450 amp hours. Compare that to an Exide Gold Marine Deep cycle 8D at 200 amp hours and 134 pounds a lot more punch for not a ton more weight. Plus a human being can actually handle a single 50 pound battery yet a single 134 pounder is nearly impossible to carry out of a boat each winter!
Trust me those 4D's are a bear compared to four individual 6V's. and they are 35 pounds less than an 8D. You're getting more than double the out put for very little extra weight with 6V. Plus the 6V batteries will last a lot longer. Thicker plates mean longer life. My brother has a bank of Sam's Club 6V golf cart batteries in his boat going on 6 years now and he paid less than $200.00, at the time, for four of them! Keep the 4D's until they need to be replace then consider your replacement options with either two 6V(225ah) or four 6V (450ah).