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What Size Trolling Motor Do You Need: Simple, Clear Answers!

Picking the right size trolling motor for your boat can be a bit confusing, but getting this right is critical.

First, you need to calculate how much thrust you need from your motor; second, you need to measure to ensure you select the right shaft length; and third, you need to match the power output of your motor to the right battery or batteries.

Neither of these three is rocket science, but getting any of them wrong is going to seriously reduce your motor’s effectiveness.

If you want to know how to pick the right size trolling motor for your boat, keep reading!

Also Read:

Thrust Options: Getting the Right “Size” Trolling Motor

Unlike outboards which measure their output in horsepower, electric trolling motors have thrust ratings measured in pounds.

choosing a trolling motor size

When picking a trolling motor, you need to measure and calculate, not guess!

Already, it’s easy to see why anglers get confused.

But don’t guess - measure.

The experts at Minn Kota note that a good rule of thumb is 2 pounds of thrust for every 100 pounds of boat (fully loaded, including fuel and anglers).

The calculation is pretty easy.

You need to know how much your fully loaded boat weighs, then divide that number by 100 and multiply that answer by 2.

(Boat weight/100) x 2 = minimum thrust rating

For instance, for a 2500 pound boat (fully loaded):

2500/100 = 25

25 x 2 = 50

50 pounds of thrust is what you need.

This chart, provided by Minn Kota, offers ready calculations for recommended minimum thrust ratings:

minimum thrust ratings and calculations for sizing trolling motors

For offshore or inshore angling, you might want slightly more power to help you contend with tides, currents, waves, and wind.

Shaft Length: Getting the Right “Size” Trolling Motor

To ensure excellent performance, you need to keep the top of the prop section of your trolling motor 12 inches under water.

If it’s higher than that, the prop will suck air down into the water, creating cavitation. Not only is that very, very loud under water, but it’s also terribly inefficient.

You’ll spook nearby fish and have less power than you paid for: a lose-lose if ever there were one.

Kayaks and other small boats: transom-mounted trolling motors

For dinghies and other small boats that use a transom-mount trolling motor as primary propulsion, typical shaft lengths will be more than adequate.

Trolling motors are excellent options for small boats.

They can even be too long for kayakers who want to maintain a shallow draft.

Ideally, you’ll want a very short shaft to allow you to maneuver into tight spots, and the good news is that Newport Vessels has you covered with their Kayak Series. Whether you prefer 36 or 55 pounds of thrust, both motors sport short, 24-inch shafts that will pair perfectly with your ‘yak. Check out other recommendations for the best kayak trolling motors.

Bow-mounted trolling motors

It’s safe to say that most bow-mounted trolling motors are equipped on bass boats, and the good news is that there are plenty of perfectly-sized shafts designed for just this purpose.

bass angler using a trolling motor

Bass anglers are spoiled for options.

Assuming that you’re using a foot pedal or electronic control, measurement is easy:

Carefully measure from the mounting surface bow to the water level. Add 12 inches to ensure the prop is submerged and another 5 inches to this waterline measurement to create a margin for rough water.

Bow to waterline measurement + 12 + 5 = proper shaft length

For instance, if your bow to waterline measurement is 24 inches:

24 + 12 + 5 = 41”

A 41” shaft or slightly longer will be perfect for your boat.

Minn Kota adds even more length, probably as a margin to cover mis-measurement:

0" - 10" 36"
16" - 22" 42" - 45"
22" - 28" 48" - 52"
28" - 44" 54" - 72"
45"+ 87"

As you can see, even larger saltwater boats can be equipped with trolling motors, allowing for precise control.

big boats use trolling motors

Even big boats can be equipped with trolling motors.

Batteries: Getting the Right “Size” Trolling Motor

All trolling motors are designed to be powered by a 12v, 24V, or 36V system of deep cycle or dual-use batteries.

As we’ve discussed before, your best bet is a dedicated battery or batteries for your electronics, unless you simply can’t afford the weight and need to start an outboard as well.

To learn more about why we make this recommendation, check out this article:

Best Trolling Motor Battery: Dedicated Batteries To Keep You Moving In 2022

How many 12V batteries you need is directly correlated to the thrust rating of your trolling motor.

battery for trolling motor

Small trolling motors producing 55 pounds of thrust or less will need a single 12V battery.

Typically, trolling motors rated to 55-pound thrust or less are powered by a single 12V battery.

From a thrust rating of 55 pounds to about 80 pounds, expect to need two 12V batteries.

And for the most powerful trolling motors, those rated to more than 80 pounds of thrust, you’ll need three 12V batteries.

The most powerful trolling motors will require three 12V batteries.

There’s no way to “cheat” here: more batteries than you need won’t make the trolling motor more powerful, but fewer may not even run it at all!

To learn more about choosing a battery size for your trolling motor, check out our full guide: What's The Right Size Battery For Your Trolling Motor?

Final Thoughts

Keep in mind that all three steps are critical to getting the right size trolling motor for your boat.

First, calculate minimum thrust.

Second, measure and calculate the ideal shaft length.

Third, know how many batteries you’ll need to power your trolling motor.

If you follow these steps, you’ll be sure to get the performance you really want.

As always, we’re here to help, so please leave a comment if you have any questions.

About The Author
Pete Danylewycz
Born and raised in Cleveland, Ohio, Pete grew up fishing on the Great Lakes. Whether he's casting a line in a quiet freshwater stream or battling a monster bass, fishing is his true passion.