Batteries are sized by group. 31's are the biggest deep cycle battery then you go up to golf cart batteries. Some boats come with group 27's which are fine for some situations but pulling cranks require quite a bit of reserve capacity. The larger the group, the more reserve capacity they have. 27's are a little light for that. 29's are adequate and 31's are better. Golf cart batteries are the best if you have the room in your boat and pocket book. Lol