It is usually recommended one lb of the motor power per 50 lb of the fully loaded boat weight. Mariner- 3 (about 50 lb) plus maximal load 600 lb, thus even 15 lb is quite enough, and I use 30 lb one. I may run 6 hrs on my Seahawk- II (older model of Mariner- 3) and still have 90% capacity left on my 78 AH AGM battery. Thus you may use smaller, lighter and cheaper battery. And according to Endura Minn Kota company, the greater thrust of the electric motor doesn't make a boat faster. May still runs 8.5 km/hr at the 5th gear.
30lb trust = approx 18-30amp/ah draw; then take into account lead acid batteries are supposed to be drained to 50% capacity. But yes these boats don't gain much in speed maybe half or 1km depending on conditions.
This reminds me of Mr. Weisser of RBI intro about you “Cesar can fixed everything except the heart of man”…. So proud of you… you don’t stop learning and practicing what you learned…
