What Size Trolling Motor Do I Need? Thrust and Shaft Length

The short version

Plan on at least 2 lb of thrust for every 100 lb of loaded boat. That's Minn Kota's rule. A boat that weighs 2,000 lb with people, fuel and gear needs at least 40 lb of thrust.
Count everything that rides in the boat. Hull, outboard, fuel, batteries, people, coolers and tackle all count. Most people guess low, and then the motor feels weak.
Add extra if you fish wind, current or big water. Minn Kota says to go up when wind or current matter. Our calculator adds 25% for that.
Thrust decides the voltage. On Minn Kota's brushed motors, up to 55 lb runs on 12 volts (one battery), 70 to 80 lb on 24 volts (two) and 101 to 112 lb on 36 volts (three).
Measure for shaft length, don't guess. Measure from where the motor mounts down to the water. The prop should sit at least 12 inches under the surface. If you're between sizes, go longer.
Bow mount for control, transom mount for small boats. A bow mount pulls the boat and steers far better. A transom mount is simple and cheap for jon boats, canoes and small tin boats.
Fishing salt or brackish water? Buy a saltwater model. A freshwater motor used in salt can wear out fast and may lose its warranty.

How much thrust you need

Thrust is how hard the motor pushes, rated in pounds. Minn Kota's sizing rule is simple: pick a motor with at least 2 lb of thrust for every 100 lb of boat weight when it's fully loaded.

That rule is a minimum for calm water. Wind pushes on the hull, and current pushes on everything under the water. If you fish open lakes, rivers or tidal water, give yourself room. Minn Kota only says to add "extra." Our Trolling Motor Power Calculator adds 25% when you tick the wind and current box.

Here's how that works on a made-up boat. Say the hull, outboard, gas, batteries, two people and gear add up to 2,200 lb:

Calm water: 2,200 ÷ 100 × 2 = 44 lb of thrust. The calculator rounds up to the next common size, a 45 lb motor.
With wind and current: 44 × 1.25 = 55 lb. The calculator picks a 55 lb, 12-volt motor.

Extra thrust doesn't hurt you. You don't have to run a big motor at full speed, and Bass Pro notes that a stronger motor handles wind and waves better and runs more easily at low speeds. The cost is a higher price, more weight, and on bigger motors, more batteries.

Thrust by boat size

Minn Kota and Bass Pro both publish the same sizing table. It runs a little higher than the 2 lb rule on mid-size boats, which builds in some of that margin for you.

Loaded boat weight	Typical length	Minimum thrust	Voltage
Up to 1,599 lb	Up to 14 ft	30 to 40 lb	12 V
1,600 to 2,599 lb	16 to 17 ft	40 to 55 lb	12 V
2,600 to 3,599 lb	17 to 18 ft	70 to 80 lb	24 V
3,600 to 4,599 lb	18 to 19 ft	80 to 90 lb	24 V
4,600 to 5,599 lb	20 to 23 ft	112 to 115 lb	36 V
5,600 lb and up	24 ft and up	112 to 115 lb	36 V

From Minn Kota's motor selection and boat size guide and Bass Pro's trolling motor buying guide. These are minimums. Go up a step for wind, current or rough water.

Length is only a rough stand-in. A 17 ft fiberglass bass boat can weigh far more than a 17 ft aluminum boat, so go by weight when you can.

Getting your boat's real weight

Add up these, using the owner's manual and spec sheets where you can:

Hull: the dry weight from the boat maker. It's often in the manual or on the maker's website.
Outboard: the weight on the engine's spec sheet.
Fuel: gasoline weighs roughly 6 lb a gallon, so a full 12 gallon tank adds about 70 lb.
Batteries: every battery you carry, including the starting battery. Lead-acid batteries are heavy, and a 24 or 36 volt setup adds two or three of them.
People: everyone who usually fishes with you.
Gear: coolers, tackle, rods, anchors, water in the livewell, and the trolling motor itself.

If the number falls right on the line between two sizes, take the bigger one. A fully loaded day with a buddy in a stiff wind is the day you'll notice.

Voltage and batteries

Trolling motor batteries are 12-volt batteries. A 24-volt motor runs on two wired in series, and a 36-volt motor runs on three. On Minn Kota's brushed motors:

30 to 55 lb: 12 V, one battery
More than 55 up to 80 lb: 24 V, two batteries
More than 80 up to 112 lb: 36 V, three batteries

Minn Kota's brushless Quest motors break that pattern. They make up to 90 lb on 24 volts and up to 115 lb on 36 volts. Other brands draw the lines in different places too, so check the voltage on the motor you're looking at.

Voltage matters because it sets how many batteries you buy, charge and carry. Bass Pro suggests 24 volts is often a better fit for long days and boats 16 ft and up. Our trolling motor batteries guide covers how many and how big, and the calculator shows how long your batteries will run the motor you pick.

Shaft length

A shaft that's too short lets the prop come up out of the water in waves. It churns, makes noise and loses its grip. Minn Kota's targets:

Keep the prop tip at least 12 inches below the waterline.
Keep the middle of the motor (the lower unit) at least 12 inches under the surface.

To find your number, measure from the spot where the motor mounts (the bow deck or the top of the transom) straight down to the water. Do it with the boat floating and loaded the way you fish. Then read the tables.

Bow to waterline	Bow mount shaft
0 to 10 in	36 in
16 to 22 in	42 to 45 in
22 to 28 in	48 to 52 in
28 to 34 in	54 to 60 in
34 to 52 in	72 to 87 in
52 to 72 in	100 in

From Minn Kota's boat size guide. The chart skips 10 to 16 inches. Bass Pro's advice for in-between sizes is to choose the longer shaft.

Transom to waterline	Transom mount shaft
0 to 10 in	30 in
10 to 16 in	36 in
16 to 22 in	42 in
22 to 28 in	55 in

From Minn Kota's boat size guide and Bass Pro's buying guide.

Then add for how you fish, per Minn Kota:

Add 5 inches to your measurement if you fish rough water.
Add 9 inches for a bow mount you steer by hand.
Add another 12 inches if you stand while steering a hand-control motor.

A shaft that's a little long is easy to live with, because you can slide the motor up in its mount. A shaft that's too short can't be fixed.

Bow mount or transom mount

	Bow mount	Transom mount
Where it goes	On the front deck	Clamped to the back of the boat
How you steer	Foot pedal, remote, or by hand on some models	Tiller handle
Boat control	Best. It pulls the boat, so it tracks and holds position well	Fine for trolling and slow moves
Best for	Bass boats, open-bow boats, casting along banks	Jon boats, canoes, small tin boats, dinghies
Install	Bolts to the deck, longer wire run to the batteries	Clamps on, short wire run

From Minn Kota's selection guide and Bass Pro's buying guide.

A bow mount's power wires run the length of the boat, so they often need to be extended. Read our trolling motor wire size and breaker guide before you buy wire.

Brushless, saltwater and other choices

Brushed or brushless

Brushless motors cost more but use power better. Bass Pro puts the run time gain at about 20% to 50%, along with quieter running and less upkeep. A brushed motor is cheaper and simple, and it works fine for most small-boat fishing.

Saltwater

Minn Kota says a freshwater motor used in salt water can have a much shorter life, and it may void the warranty. Its saltwater Riptide line uses stainless hardware and sealed electrical parts. Rinse any motor with fresh water after a salt trip, and never leave one sitting in salt water while the boat is moored.

Spot-lock and GPS

Many bow mounts now hold the boat in place by GPS. It's a big help on wind-blown water, but holding position all day takes power. Budget your batteries with the calculator before you count on it.

When you're ready to shop, our trolling motor picks are sorted by thrust and voltage.

Questions people ask

Is a 55 lb trolling motor enough for a 16 or 17 ft boat?

Often, yes. Minn Kota's table puts 16 to 17 ft boats of 1,600 to 2,599 lb at 40 to 55 lb. Once a loaded boat reaches about 2,600 lb, or you fish lots of wind, the table moves you to a 70 to 80 lb, 24-volt motor.

Will a bigger trolling motor make my boat go faster?

Not much. Top speed depends mostly on how fast the prop spins and its pitch. Minn Kota's own example works out to about 5 mph. Extra thrust mostly helps you hold that speed into wind and current. In a Bass Resource forum thread, one owner reported about 3.0 mph with a 30 lb motor and 3.6 mph with a 50 lb motor on a 14 ft jon boat.

Can I have too much thrust?

Not really. A bigger motor at low speed handles wind better, and Bass Pro says higher-thrust motors run more easily at low speeds. The trade-offs are cost, weight, and the extra batteries a 24 or 36 volt motor needs.

How do I measure shaft length if the boat is on the trailer?

You'll be guessing at the waterline, so measure on the water if you can. Forum members on iboats give the same advice: float the boat, measure from the bow to the water, and buy on the long side.

What size trolling motor for a kayak or canoe?

By the 2 lb rule, even a loaded kayak needs very little thrust, so almost any small 12-volt motor will push it. The bigger questions are the battery's weight and how long it will run. A light lithium battery and our calculator will tell you more than the thrust number.

Can I use a freshwater trolling motor in salt water?

You can, but Minn Kota warns it may shorten the motor's life a lot and void the warranty. If you fish salt or brackish water often, buy a saltwater model and rinse it after every trip.

Should I get a 12 V or 24 V motor?

Let thrust decide. If your boat needs more than 55 lb, you're in 24-volt territory on most brushed motors. A 24-volt motor needs two batteries, a two-bank charger and the space to carry them.