I used to have an 18 ft Hewescraft Searunner with a 24 volt Minnkota bowmount with 60 or 70 pounds of thrust and I felt that was the minimum for what I was doing, trolling in lakes and using it for bow control in rivers. I think it is more of a battery charge issue than thrust since if you are under powered you will tend to run it on a higher speed and run the battery down faster. The 12/55 would probably move the boat just fine but you may only get 4-6 hours of trolling on a charge depending on speed, wind, current ect. I would often take that boat on 2-3 day trips where there was no way to charge the batteries so battery life was important. One advantage of a 12 v motor is that you can bring an extra battery to change out if needed, depending on how you plan to use it.