Trolling motor size comes down to your boat's fully loaded weight. Work out the thrust from that, and the voltage and batteries follow. Here is the motor maker's own rule and chart.
Minn Kota's rule is at least 2 pounds of thrust for every 100 pounds of fully loaded boat weight, with people and gear included. A 2,000 lb boat, loaded, needs at least 40 lb of thrust.
That rule assumes normal lake fishing. Minn Kota suggests more thrust if you often fish in heavy wind, rivers or rough weather, where extra power gives you more control.
Minn Kota's recommended minimum thrust and batteries by fully loaded boat weight.
| Boat weight (loaded) | Max boat length | Minimum thrust | Batteries |
|---|---|---|---|
| 1,500 lb or less | 14 ft | 30 lb | 1 (12 V) |
| 2,000 lb | 17 to 18 ft | 40 to 45 lb | 1 (12 V) |
| 2,500 lb | 20 to 21 ft | 50 to 55 lb | 1 (12 V) |
| 3,000 to 3,500 lb | 23 ft | 70 lb | 2 (24 V) |
| 4,000 lb | 25 ft | 80 lb | 2 (24 V) |
| 4,500 lb or more | Over 25 ft | 101 to 112 lb | 3 (36 V) |
Minn Kota points out that thrust is a static measure of pushing power, and more thrust does not necessarily mean more speed. A trolling motor's speed comes from its propeller pitch and the motor's RPM. What extra thrust buys is more control in wind and current.
You need one 12-volt battery for every 12 volts in the system, so a 24-volt motor runs on two batteries and a 36-volt motor on three. In Minn Kota's chart the 30 to 55 lb motors are 12-volt, the 70 and 80 lb motors 24-volt, and the 101 to 112 lb motors 36-volt. The trolling motor wiring diagram shows how to connect them.
Minn Kota says its motors run on any lead-acid, deep cycle marine 12-volt battery, and recommends at least 110 amp-hours for best results. More amp-hours means more run time. If a cranking battery starts your outboard, use separate deep cycle batteries for the trolling motor.
| Run time | Battery group size | Amp-hours |
|---|---|---|
| Good | Group 24 | 70 to 85 |
| Better | Group 27 | 85 to 110 |
| Best | Group 31 | 95 to 125 |
A motor with too short a shaft leaves the propeller near the surface. Minn Kota recommends submerging the center of the motor section at least 12 inches. Measure from where the motor mounts to the waterline before choosing a shaft, and add 5 inches to that measurement if you fish in rough water. The trolling motor shaft length guide has the full charts.
Minn Kota, Motor Selection Guide (Rev. 8.21.2020), Calculate Speed and Determine Required Thrust and Battery & Wiring Installation.