What size of boat for a 15.5 footer. I assume it's more than a aluminum boat with a 25 tiller? The highest thrust that is made for 12 volt power from MinnKota is a 55lb thrust model. In 24 volt setups, you can go up to 80lbs of thrust. A one battery (with the highest reserve minutes you can find for a deep cycle) 55lb thrust motor, will throw that boat around very well, and allow you to get a days use from it on a good walleye trolling speed. Do you want a bowmount or a transom model?
Chris K