My suggestion would be 1st is to see how the current batteries perform pulling cranks. It's all about the quality of batteries and reserve capacity. I have an 18' glass and when I bought the boat it had two group 29 interstates designated for the 24v system. I installed my Ultrex and could pull cranks for 8 hours and still have juice left. I had to buy new batteries last year and got in a hurry. Got two group 31 fish hunters (deka) made for Ellis battery specs. My pulling time went to 6 or 7 hours depending on water conditions. I don't buy into the hype of cart batteries unless your maxed out on pounds of thrust already. I think you may be jumping the gun, unless the previous owner has made you aware of run time already. The US Batteries in group 31's boast the best reserve capacity I've seen and that will be my next set. Sounds to me like you may be over complicating matters before you know how the boat will perform. Just my opinion and hope this gives you a little food for thought anyway.

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