Trolling motors come in many shapes and sizes. Learn which size trolling motor you need for your own vessel.
Last updated: September 25, 2022
By: Brandon Sanders
When purchasing a new boat, you often will be buying a trolling motor to go with it. While this may seem like a simple task, one look at the wide array of options will quickly reveal that it is much more complicated.
Truth be told, there really isn’t a black and white, one-size-fits-all solution to the proper size of a trolling motor. However, we’ll cover all the information you will need to make that determination for yourself.
No matter what brand, size, or mounting style you choose, the biggest consideration you must take into account is boat safety. Though a trolling motor is small and electric, it is still a motor.
Having one that is oversized, improperly mounted, or miswired can result in catastrophic failure or injury. It is of paramount importance that you select the most appropriate motor for your application and mount it correctly.
The first aspect to consider is the type of boat you’re using. For instance, there’s a drastic difference in the way trolling motors are used between kayaks and bass boats. For a bass boat, the trolling motor is to precisely position the boat so that the proper presentation can be made. With kayak fishing the trolling motor fills that role but is also the main source of propulsion.
But before we dive into the details, let’s cover the basics, starting with shaft length. Before you know it, you’ll be out on the lake with the right size trolling motor, moving around in a stealthy and simple manner. Ready? Let’s dive in!
First, you need to calculate thrust. Divide your total boat’s weight by 50, and you have the pounds of thrust needed for your vessel.
Most 16 - 18ft boats require just 55lbs of thrust.
Keep the depth of the prop between 6 - 18 inches below the surface of the water. On average, the 12 inch mark (below the surface) is a good starting point. Ensure your trolling motor has a shaft that accommodates these measurements.
Thrust, shaft length, and battery requirements will depend on the type of boat (jon, kayak, bass boat, pontoon, etc.) and your desired length of use per outing.
The weight of the vessel is of paramount importance in trolling motor selection. Generally for every fifty pounds of weight of a fully loaded boat one pound of thrust is required of the trolling motor. However, this is a rule of thumb and can vary depending on hull design and the conditions the boat is operated in.
As a rule of thumb, the blades of the trolling motor should be at least six inches below the surface and no more than 18”. If the blades are not sufficiently low they will breach the surface of the water causing a loss in thrust and pose a hazard to anyone around it.
If it is too low, the shaft will bend resulting in a loss of thrust from the shaft bending. Therefore, it is important that the shaft length is appropriately selected.
Measure the trolling motor shaft from the bottom of the head to the top of the motor housing. This will include the length of shaft that is covered by the mount.
Do not measure the shaft from the mount to the top of the motor housing as different conditions may call for adjusting the trolling motor shaft on the fly. It is important that you have the ability to move the trolling motor up and down while fishing to maintain proper depth of the prop.
This table provides a rule of thumb for shaft length. However, depending on your boat's configuration and build, these lengths may be adjusted. The main consideration should be to keep the depth of the prop below six inches and above 18 inches. If you are able to maintain that, the shaft length should only have to accommodate for slight adjustments while fishing.
Understanding thrust is critical to selecting the thrust you need from your trolling motor. Thrust from a trolling motor can be fully understood by being familiar with Newton’s second law.
The force needed to achieve acceleration will be determined by the mass that it is called to move. More simply, the bigger the boat, the bigger the trolling motor. It really is that simple.
A rule of thumb for this is to calculate your trolling motor needs by dividing your total boat’s weight by 50. The result is how many pounds of thrust you will need from a trolling motor. A typical 1848 aluminum boat will be around 1,000 pounds with a motor.
Therefore, it will only need 20lbs of thrust. Since the most commonly sold trolling motor is 55lbs of thrust, it will be more than enough to move the standard boat.
The most important factor when selecting a trolling motor’s thrust is the weight of the vessel it will be attached to. Calculate this with max occupants, gear, fuel, and then add around 10% of that final number for a safety margin.
Once you have that number, then select the trolling motor that provides the lowest thrust possible while not going under the weight you calculated using the rule of thumb previously mentioned.
For instance, if your boat is 3500lbs you will need a 70lbs thrust trolling motor. However, they typically come in 55lbs, 80lbs, and 120lbs thrust models. Select the 80lbs thrust model over the 120lbs thrust.
Another consideration in trolling motor selection, but specifically thrust is hull design. If you have a large river boat with a square front, the lackluster streamlining will slow the vessel down significantly.
However, if your boat is narrow and pierces through the water with ease, less force will be required to achieve the same speed. While there is no solid way to calculate this mathematically, it is prudent to bear hull design in trolling motor selection. Having a proper understanding of boats and their needs from a boater education course can be helpful with this if you have no experience.
There are some caveats to this. Fishing for bass during rainy weather, in high current, or wind will require more from a trolling motor to keep it where the angler wants it. This means that a more powerful trolling motor will be required to compensate for the added force acting against the force of the trolling motor. You should consider where you intend to be fishing when selecting a trolling motor.
Given their size, jon boats sit between kayaks and bass boats. Therefore, many common pointers for fishing from a kayak translate very well to jon boats. This includes most of the advice surrounding kayaks in terms of size and employment. However, mounting is a vastly different concept since the two boats are built differently.
The one difference between the two is that a jon boat has a pronounced transom and handles like a traditional boat. Therefore, the trolling motor needed for a typical jon boat is small and transom mounted. Though it is very possible to have a bow mounted trolling motor, it is less conducive to being the primary mode of propulsion and typically will overpower the narrow boats.
In terms of the size of a trolling motor needed on a jon boat, smaller will typically be better. There simply is no need for a large trolling motor on a boat that only weighs as much as a human. Further, there is no room or place to run a large amount of wires on the typical jon boat. Therefore, always default to the small and simple when putting a trolling motor on a jon boat.
Putting a trolling motor on a kayak can open many doors for the kayak angler. The good news is that since the boat is made to be extremely light and efficient in the water, there simply is no need for a large trolling motor.
While there is only one kayak specific trolling motor on the market, many of the common models of kayak trolling motors will suffice. The best rule of thumb is to pick the smallest trolling motor that will meet your needs.
A word of legal caution on kayaks. When putting a trolling motor on a kayak, many states will change the class of vessel it is and require registration. Registration requirements change with every state and there is no common thread between them.
It is best to do sufficient research on them before you go about mounting a trolling motor on your kayak. While the top fishing kayaks are equipped with a trolling motor, it’s important for anglers to be aware of the safety and legal requirements that come with adding a motor to a kayak.
There was a time in life when trolling motors were reserved for anything but pontoon boats. However, today things are different. Pontoon boats are catered to with trolling motors made specifically for them and many of the standard models will also fit. With as many options as are afforded on other boats, there is sure to be an increase in the amount of trolling motors being mounted on pontoon boats.
The main concern for pontoon boats is how high they sit out of the water. With the platform on the pontoon boats sitting several feet out of the water, it is essential that the trolling motor prop be sufficiently submerged to be effective. Following the shaft length table from above as well as following crucial safety rules can help ensure success with pontoon boat trolling motors.
The bass boat is not where the trolling motor was born, but it is where it came to maturity. Largemouth bass fishing is the biggest driver in trolling motor innovation and therefore, has the most options.
When selecting a trolling motor size, it is easy to get lost in the different options that are available. However, no matter the make, model, or fancy gadgets that are attached to the trolling motor, the thrust to weight rule remains the same. You only need 1lbs of thrust for every 50lbs of gross boat weight.
The good news is a bass boat can float a significant amount of weight. Therefore, they typically will allow for the bigger, more advanced trolling motors.
Understanding basic electrical terminology will help in making sense of finding the right trolling motor battery size. First, one must understand ohms, volts, and amps.
Ohms is the amount of electrical resistance in a system. Think of this like friction the electricity has to overcome to flow effectively. Volts is the amount of electric potential in a system. Think of it as pressure looking for a place to escape to. Finally, amperage is the rate in which electricity flows through a system.
The classic way to understand these terms is to envision a water hose. Volts would be the amount of water that can come out of the hose at any one point in time. Amperage is the rate of flow of the water as it comes out. Finally, ohms is the diameter of the water hose.
As you envision that, think of what would happen to the rate of flow if you had a bigger or smaller hose or different water pressures.
The hour ratings on batteries will be stated like this “100 amp hours”. That means it can supply 100 amps for one hour. However, it can also supply one amp for 100 hours. It all depends on the size of the hose the electricity in the battery has to flow through and the pressure it has pulling on it. This concept is vital to understanding what the most appropriate size trolling motor you need.
Why does this matter? The higher the voltage the trolling motor takes, the larger the battery bank you will need.
Simply increasing your thrust from 55lbs to 80lbs will require a second battery and you may wash the 25lbs of thrust gained by increasing the boat’s weight. Not to mention, it will cause a drastic increase in complexity of your electrical system. Therefore, understanding your boat's electrical needs and how the system works is vital to understanding what size thrust you will need onboard.
Trolling motor selection is often a stressful part of buying a boat or redoing an older vessel. However, it is a very useful thing to have alongside the required items on your vessel.
Selecting one that has the appropriate amount of thrust is crucial to having a safe, effective, efficient, and fun boat that you will enjoy for years to come. Follow the shaft length and thrust to weight rules for selecting an optimal size trolling motor for your vessel.
By doing this your boat will have a trolling motor that is tailored to fit your needs, your operational environment, and the activities you are likely to engage in.
Most 18ft boats will require only a 55lbs thrust trolling motor. However, you should consider the weight of the vessel, current and wind it will be operated in, and average time you will be on the water.
A 16ft boat will often need a trolling motor under the lowest available size. Therefore, little concern should be exercised in selecting a trolling motor for a 16ft boat unless it is of exceptional design. Jon boats, mud boats, and bass boats are all light enough to only require a standard 55lbs thrust trolling motor.
A 55lbs thrust trolling motor is less than 1HP. A horsepower is equal to lifting 550lbs vertically one foot off the ground in one second. Most boats do not require that much force from their trolling motors to move forward.
A 50lbs thrust trolling motor can go as fast as the weight of the hull it is attached to will allow it. Speed in a trolling motor is largely a function of the type of boat it is attached to rather than the force of the motor.
In terms of trolling motor thrust, 65lbs is the rough equivalent of one horse power. A horsepower is equal to lifting 550lbs vertically one foot off the ground in one second. Since it moving a boat forward slowly takes much less effort than that there is much less required of trolling motors.
A trolling motor should be no more than 18 inches below the water. This allows for six inches of water to be above the blades and limits loss of thrust by bending shafts. Most fishing kayaks take this into account and allow for trolling motors to be mounted at the optimal depth.
Possibly the best type of motor that can be used on jon boats is a trolling motor. However, due to the size and weight of most jon boats, trolling motors can still propel the boat forward at considerable speed. It is important to follow a boater's guide and operate the jon boat just as safely as you would if it were equipped with an outboard.
A trolling motor battery will last as long as the amp hours it is rated for. This will significantly change based on the size of your motor and the intensity of its use. If you are planning on spending a significant time over typical bass habitat, it is best to have two batteries on board wired in parallel for maximum time on the water.
About the author: Brandon Sanders, who goes by BBSanders, is a freelance outdoors writer that enjoys hunting and fishing across the world. He is a combat veteran of Iraq, Afghanistan, and many other smaller deployments. He lives in East Texas in a small cabin with his wife and two dogs. You can learn more about him on his own website, here.