How To Size A Trolling Motor Correctly For Your Boat (Detailed Guide)
UPDATED 17 MAY 2023
When buying an electric trolling motor, it’s essential to get the right size for your boat.
Trolling motor size has two main components: thrust (or motor power), and shaft length.
Both of these need to be correctly matched with your boat, and getting it wrong can result in major headaches on the water.
In this article we’ll walk you through how to choose trolling motor size, thrust and shaft length to match your boat size.
What size trolling motor do you need?
As a rule of thumb, a trolling motor should provide about 2 pounds thrust for every 100 pounds of fully loaded boat weight.
However, if you expect to encounter strong winds or currents, you’ll need more power.
In a case like that, it’s necessary to add about 10 to 20% additional thrust in order for the motor to perform adequately.
But keep in mind that boats with a deep draught provide more resistance to a water current than boats with a shallow draught, and thus require a larger trolling motor.
Also, boats with a large deck height above water (such as pontoon boats), provide a lot of surface for the wind to push against, and therefore require additional thrust to deal with strong winds.
So the specific amount of thrust you’ll need for your specific situation could require more thrust than you expected.
When in doubt, it’s usually best to get an overpowered trolling motor, since that’s easier to compensate for than an underpowered one.
Trolling motor size chart
| Boat weight (fully loaded) | Boat Length | Minimum thrust required | Thrust for strong wind or current |
|---|---|---|---|
| 1500 lbs (or less) | 14 - 15 ft | 30 lbs | 40 lbs |
| 2000 lbs | 16 - 17 ft | 40 lbs | 50 lbs |
| 2500 lbs | 18 - 19 ft | 50 lbs | 60 lbs |
| 3000 lbs | 20 - 21 ft | 60 lbs | 70 lbs |
| 3500 lbs | 22 - 23 ft | 70 lbs | 80 lbs |
| 4000 lbs | 24 - 25 ft | 80 lbs | 90 lbs |
| 4500 lbs | 26 - 27 ft | 90 lbs | 100 lbs |
| 5000 lbs | 28 - 29 ft | 100 lbs | 110 lbs |
| 5500 lbs (or more) | 30 ft (or more) | 110 lbs | 120 lbs |
The table above shows the average pound trolling motor thrust required for different boat weights and lengths. Column 3 shows the minimum thrust you need for each boat size, while column 4 shows the higher thrust levels required for dealing with strong wind or current.
In general it’s better to err on the side of having too much thrust, rather than too little, as that gives you more flexibility to deal with unexpected circumstances.
But keep in mind that the table above is an approximation for an average boat, and in addition to wind and current, your boat type may require different amounts of thrust (more on that below).
If you’re currently in the market for a new trolling motor, check out our article on the cheapest spot lock trolling motors reviewed.
Trolling motor thrust guide for different boat types
Let’s look at the thrust required for some of the most common recreational boat types, to help you choose the right trolling motor size for your boat.
What size trolling motor do you need for a bass boat?
| Boat weight (fully loaded) | Boat Length | Minimum thrust required | Thrust for strong wind or current |
|---|---|---|---|
| 1000 lbs (or less) | 14 ft (or less) | 30 lbs | 40 lbs |
| 1250 lbs | 15 ft | 40 lbs | 50 lbs |
| 1500 lbs | 16 ft | 50 lbs | 60 lbs |
| 2000 lbs | 17 ft | 60 lbs | 70 lbs |
| 2500 lbs | 18 ft | 70 lbs | 80 lbs |
| 3000 lbs | 19 ft | 80 lbs | 90 lbs |
| 3500 lbs | 20 ft | 90 lbs | 100 lbs |
| 4000 lbs (or more) | 21 ft (or more) | 100 lbs | 110 lbs |
The table above shows the recommended thrust for different bass boat sizes.
While bass boats are usually lighter than other boat types of comparable length, most bass anglers prefer to use bigger trolling motor sizes, from 80 lbs all the way up to 120 lbs thrust.
The reason for using high-powered trolling motors for bass boats is that it enables you to fish anywhere, including spillways, river currents, or wind swept flats.
Also, bigger trolling motor sizes come with 24V or 36V batteries, which have a longer lifetime, and thus work well for bass fishing tournaments.
What size trolling motor do you need for a jon boat?
| Boat weight (fully loaded) | Boat Length | Minimum thrust | Thrust for strong wind or current |
|---|---|---|---|
| 500 lbs (or less) | 12 ft (or less) | 30 lbs | 40 lbs |
| 750 lbs | 13 ft | 40 lbs | 50 lbs |
| 1000 lbs | 14 ft | 50 lbs | 60 lbs |
| 1250 lbs | 15 ft | 50 lbs | 60 lbs |
| 1500 lbs | 16 ft | 50 lbs | 60 lbs |
| 1750 lbs | 17 ft | 50 lbs | 60 lbs |
| 2000 lbs | 18 ft | 60 lbs | 70 lbs |
| 2500 lbs (or more) | 19 - 20 ft (or more) | 60 lbs | 70 lbs |
The table above shows the recommended thrust for different jon boat sizes. Jon boats are usually very lightweight, which means you’ll be fine with a 45 to 55 lbs thrust trolling motor.
The main exception to this is if you plan to take a lot of passengers on board larger sized jon boats. In that case it’s better to use a bigger 70 to 80 lbs thrust motor.
What size trolling motor do you need for a pontoon boat?
| Boat weight (fully loaded) | Boat Length | Minimum thrust | Thrust for strong wind |
|---|---|---|---|
| 1000 lbs (or less) | 14 ft (or less) | 30 lbs | 40 lbs |
| 1500 lbs | 15 ft | 40 lbs | 50 lbs |
| 2000 lbs | 16 ft | 50 lbs | 60 lbs |
| 2500 lbs | 17 ft | 60 lbs | 70 lbs |
| 3000 lbs | 18 ft | 70 lbs | 80 lbs |
| 3500 lbs | 19 ft | 80 lbs | 90 lbs |
| 4000 lbs | 20 ft | 90 lbs | 100 lbs |
| 4500 lbs (or more) | 21 ft (or more) | 100 lbs | 110 lbs |
The table above shows the recommended thrust for different pontoon boat sizes. Since pontoon boats have a very shallow draught, they won’t have a lot of problems with a strong current.
However, since pontoon boats usually have a large deck height above water, strong winds do pose a problem, especially if you’re using a spot lock trolling motor to keep your pontoon boat in a specific location. In that case, it’s better to go with a bigger trolling motor size.
What size trolling motor do you need for a canoe?
Since canoes rarely exceed 500 lbs in fully loaded weight, a 30 lb thrust trolling motor is strong enough for most situations without much current, while a 40 to 55 lb thrust is a better choice if you are expecting to encounter a stronger current (on a river, for example) or wind.
What size trolling motor do you need for a kayak?
A 30 lb thrust trolling motor is plenty strong enough to power a kayak, which rarely exceeds 300 to 400 lbs in fully loaded weight.
The main exception to this is ocean kayaking, where you can expect to encounter a strong current. In that case, a 40 to 55 lb thrust trolling motor is the best choice.
Trolling motor shaft length
It’s important to choose the right shaft length for your trolling motor, to ensure the prop is submerged deep enough in the water to function properly.
But the problem with this is that different boats have different deck heights above water, and in addition there are different mounting locations for a trolling motor, the most common of which are bow mount and transom mount.
As a rule of thumb, the prop of your trolling motor should be submerged 15 to 20 inches below the water.
So you need to measure the distance between the deck and the water, and then add 15 to 20 inches to that to calculate the ideal shaft length for your trolling motor.
Shaft length for a bow mount trolling motor
| Distance from bow to waterline | Recommended shaft length |
|---|---|
| 0" to 16" | 36" |
| 16" to 22" | 42" to 45" |
| 22" to 28" | 46" to 52" |
| 28" to 34" | 53" to 62" |
| 34" to 44" | 72" |
| 44" to 64" | 87" |
The table above shows the recommended shaft length for bow mount trolling motors, depending on the height of the bow above the waterline.
Since the height of the bow above the waterline varies a lot from boat to boat, it’s important to measure this before you buy a trolling motor.
Also, keep in mind that the bow can be much higher above the waterline than the transom, and so often requires a longer shaft length.
Shaft length for a transom mount trolling motor
| Distance from transom to waterline | Recommended shaft length |
|---|---|
| 0" to 10" | 24" to 30" |
| 10" to 15" | 30" to 36" |
| 15" to 20" | 36" to 40" |
| 20" to 25" | 40 to 42" |
The table above shows the recommended shaft length for transom mount trolling motors, depending on the height of the transom above the waterline.
Since most boat transoms are usually quite close to the waterline, you need to choose a shorter shaft length for transom mounted trolling motors than for bow mounted ones.
This also means that you can’t switch a trolling back and forth between bow or transom.
Once you’ve chosen your new trolling motor, you may also be interested in our guide on where should you mount a trolling motor?
How is trolling motor shaft length measured?
The length of a trolling motor shaft is measured from the bottom of the motor head down to the top of the motor housing (which is the part with the propeller on it).
This can be a little confusing, because some trolling motors (especially transom mounted ones), have their mount positioned in the middle of the shaft.
Can a trolling motor shaft be too long?
Yes, a trolling motor shaft is too long if the prop protrudes significantly below the deepest point of the hull. If the shaft is too long for your vessel, the prop will tend to churn up mud and weeds in shallow water, so it’s important to choose the right shaft length.
Trolling motor battery size
| Trolling motor thrust | Required battery voltage |
|---|---|
| 30 to 55 lbs | 12v |
| 55 to 80 lbs | 24v |
| 80 to 120 lbs | 36v |
The table above shows the required battery voltage for different trolling motors, depending on their thrust rating. If you’re not sure which thrust rating is right for you, check out our article: is it better to use a 12v or 24v trolling motor?
Once you’ve chosen the right trolling motor size for your boat, you also need to get the right battery for it. Depending on the thrust rating of the trolling motor, you’ll need either a 12v, 24v, or 36v battery system for it.
You can wire two 12 volt batteries together in series to generate 24 volts, and 3 in series will generate 36 volts. However, keep in mind that a single 24v or 36v battery weighs less, and occupies less space, than 2 or 3 12v batteries wired together.
And finally, a few trolling motors (including the Lowrance Ghost and Garmin Force), are compatible with either 24v or 36v.
The nice thing about this is that you can adjust the thrust of these trolling motors up or down by changing your battery voltage from 24 to 36, and vice versa.
Another factor that’s worth mentioning is that a battery system with higher voltage has a longer runtime than a system with lower voltage, which means you get a longer runtime.