Interesting, I guess weight is the main issue. Almost sounds like you needed to get one with a higher thrust that uses 3 batteries instead of 2.
I have a 17 ft aluminum boat with 40 hp, with a newer 80 lb Terrova, and I've trolled as much as 10 hrs on a charge before, pulling cranks all day, and still had some to spare. I have the Walmart EverStart Marine size 29 batteries.