How to use it
- Pick your motor's thrust. The amp draw and voltage fill in for you. If your manual lists different numbers, type them in.
- Choose how you fish. Each choice sets a typical speed and how much of the time the motor runs. Pick "My own settings" to set them with the sliders.
- Enter your battery. Choose the battery type and enter the amp-hour (Ah) number from its label. Not bought one yet? Leave 100 Ah and see what the results recommend.
- Enter your wire run. Measure from the battery to the motor plug, one way, along the path the cable takes.
- Read your results. You get hours on the water, the battery size for your day, the wire gauge and breaker, a shopping list, and a picture of how it all connects.
Tips: The page opens in the detailed view with every control. Switch to Simple for just the essentials. If you have a clamp meter, enter your measured amps for the most accurate run time.
Run time
Battery for your day
Wire and breaker
What to buy
How it connects
Make the last battery connection with the breaker switched off.
Run time at each speed
| Speed | Amps | Motor on | On water |
|---|
Planning estimates, not a wiring diagram for your exact boat. If your motor's manual gives a different wire gauge or breaker size, follow the manual. Undersized wire and missing breakers cause fires. Have marine electrical work checked by someone qualified.
How the numbers are worked out
Amp draw
Full speed uses the maximum amp draw the manufacturer publishes. That figure is a peak. A motor pushing a moving boat usually draws less, so full-speed run times here lean short.
Variable-speed motors: draw at a partial setting is max amps × (setting ÷ 100)^1.6. The curve is fitted to a posted clamp-meter test of a 45 lb variable-speed motor (1, 4, 10, 20 and 31 A at settings of 10, 30, 50, 75 and 100).
5-speed motors: speeds 1 to 5 are taken as 17%, 25%, 40%, 55% and 100% of maximum. Only the low and high ends come from a published figure, so these are the least certain numbers on the page. Enter a measured reading if you have one.
The fishing-style choices are starting assumptions, not measurements. Casting: 40% speed (speed 2) with the motor on 35% of the time. Steady trolling: 50% (speed 3), on the whole time. Holding position: 60% (speed 3), on 60% of the time.
Run time
Lead-acid batteries deliver fewer amp-hours the harder you pull on them. Peukert's law accounts for that: hours = H × (Ah ÷ (amps × H))^k, where H is the hour rate the battery was rated at.
- Flooded: k = 1.25, rated at the 20-hour rate
- AGM: k = 1.15, rated at the 20-hour rate
- Lithium (LiFePO4): k = 1.05, rated at the 5-hour rate
The result is multiplied by the share of the battery you are willing to use, and never credited above the rated capacity. Hours on the water is motor-on time divided by the share of the time the motor runs.
Wire gauge
Wire is sized with the ABYC formula circular mils = 10.75 × amps × round-trip feet ÷ allowed volts dropped, at maximum amp draw, with round-trip length equal to twice the one-way run.
The limit is 3% voltage drop. A result that passes with almost nothing to spare (over 2.9%) is moved up one size, because connectors and the motor's own leads add drop the formula does not see.
The wire is also never smaller than the gauge that can carry the full breaker rating with 75°C insulation: 8 AWG on a 50 or 60 A breaker, 10 AWG on a 30 A breaker. Figures assume stranded copper marine wire, no more than two conductors bundled, routed outside engine spaces.
Breaker
Breaker size follows Minn Kota's sizing table: 30 A for motors drawing up to 30 A, 50 A for motors drawing under 50 A, 60 A for 50 to 60 A. Use a manual-reset marine breaker rated for your system voltage, mounted close to the battery on the positive lead.
What it leaves out
- Wind, current, weeds and boat weight, which change how hard you have to run the motor
- Cold water and air, which cut lead-acid capacity
- Battery age, since an older battery holds less than its label says
- Fish finders, pumps and lights on the same battery