A Guide to Trolling Motor Amp Draw
Navigating the complexities of trolling motor amp draw can be daunting. As does the challenge of understanding the relationship between volts, amps, and watts.
If you are looking to buy a trolling motor, this guide will help you separate the wood from the trees, so to speak.
Drawing from a long career in the marine industry, I’ve distilled the topic of amp draw in trolling motors using straight-talking English.
Now, let’s begin starting with the primary measurement of electricity.
🎓 Understanding Amp Draw in Trolling Motors
The four primary measurements used to measure electricity are:
- Voltage
- Amps
- Watts
- The fourth is ohms, which is not included in my explanation below.
When your energy bill arrives, the consumption is shown in kilowatt-hours. When you buy light bulbs, they show a measurement called “watts.”
Batteries always show a voltage (i.e., 9-volt batteries). Vacuum cleaners and other household consumers show a metric called amps (i.e., 15 amps of power that is drawn by the device.)
What are the purposes of so many measures of electricity?
The measure of each unit measures is explained in the following illustration.
Imagine the water at the top of a waterfall. When the water reaches the lip of the waterfall, gravity affects what happens to it.
When the water reaches the lip, gravity represents the potential force that is available to cause the water to fall. In this example, the top of the waterfall is similar to the negative terminal on a battery.
There is a direct relationship between each of the measures. If two measures are known, the third can be calculated.
Voltage
What do volts mean for electric trolling motors? Let’s go back to our imaginary waterfall. At the top, the energy is only potential as the water hasn’t started to fall. Voltage is the energy available to move the electrical current from the negative to the positive terminal.
The measurement of that available energy in trolling motors is given as pounds of thrust. I won’t overload you about thrust right now, but you can read this trolling motor thrust guide to explore the topic.
Voltage = Amps/Watts
Amps
As the water begins to fall, gravity takes effect, and the water drops downwards, increasing speed until it reaches the terminal velocity.
If a measuring device were placed in the stream, it would record the speed at which the water falls. This is equivalent to the amperage in a circuit. The official measure of one ampere is as follows.
One coulomb (equals 6.24 × 1018 electrons) of electrical current going past a given point in one second.
Amps = Watts/Volts
Watts
It would be called work if you made the water work on the way down by placing a turbine in the flow.
If we could find out the pressure of the water falling past a specific point, we could determine how much energy was available for this work.
Assuming we know how much energy is available (Volts) and we can measure the speed (or number of) molecules passing a specific point (Amps) per second, this would allow us to calculate the pressure (watts) of the water.
The “pressure” watts can be changed in the following ways.
1️⃣ If the water flow remains constant, but we widen the top of the waterfall, the pressure of the water would reduce (because the same amount of water would fall, but over a wider area.)
In the same way, if the voltage (potential energy) is reduced, the watts available to run the motor will reduce.
2️⃣ If you increase the pressure of the water (by increasing its flow rate), more water will flow over the edge, and the pressure will rise. In the same way, if the number of electrons passing a specific point in the circuit increases (amps), the pressure will increase.
This will increase the number of watts (pressure) in the circuit and so increase the size of the motor (amount of work) that the system can power
Watts = Amps * Volts
⚡ Factors Affecting Trolling Motor Amp Draw
Using the above illustration, the number of amps (electrons passing a certain point in 1 second) increases the battery draw and the amount of energy available to run the motor.
Motor Power and Efficiency
The optimal situation is to draw as few amps as possible and produce as much power (watts).
The difference between the two is the measure of the motor’s efficiency.
Electric motors are generally much more efficient than the equivalent gas-powered motor.
Electric outboard motors typically convert 35% to 50% of the battery’s energy into propulsion power. The equivalent figures for trolling motors are between 18% and 22%.
Battery Voltage
The amount of battery voltage available (the water at the top of the waterfall) determines the amperage available for the battery.
As can be seen from the formula to calculate amps (Amps = Watts / Volts), there is a direct relationship between each measure. If one is increased, so will the other two.
Therefore, if you double the voltage from the batteries, the amperage available will also double.
This provides two opportunities for the boat owner:
- Theoretically, the motor can run twice as fast.
- Or it can run at the same speed as with a single battery but run for twice as long (double the range).
Boat Weight and Water Conditions
In our example, we state that watts measure the amount of work a motor (electrical circuit) can do.
If the boat’s weight is increased, or the weather conditions make progress more difficult, the amount of work (watts) the engine has to provide will increase.
The amperage draw will increase (remember the relationship is direct), so the battery will discharge more quickly.
🔣 Estimating Amp Draw for Trolling Motors
The amp draw of a trolling motor is easily calculated because the relationship between volts, amps, and watts is direct.
Therefore, you can work out the amp if you know the number of watts the motor uses and the battery voltage.
To do this, divide the number of watts by the voltage being used by the system. An example is as follows.
If the motor is drawing 300 watts (0.3 kW) and the motor is using 12 volts, the calculation is 300 watts/12 volts = 12 amps.
The manufacturers generally provide the amps value as the maximum drawn when the motor runs at full power.
Real-World Scenarios
This is not realistic. Owning an electric trolling motor quickly teaches the user to use full power as rarely as possible.
The amperage draw will be governed by the power being deployed at that moment, affected by the following factors:
- The strength of the currents and whether the motor has to fight them.
- The prevailing wind conditions and how much power is being used to counteract them.
- The speed that is selected.
With these factors in mind, the only way to increase the potential run time of the trolling motor is to increase the battery capacity available. Batteries used for marine purposes (actually all batteries!) are rated on a value called – “amp hours.”
Amp hours is the maximum number of amps the battery will produce in one hour, after which it is discharged. A 100-amp hour battery can supply the following values:
- 100 amps for 1 hour
- 25 amps for 4 hours
- 10 amps for 10 hours.
📈 Implications of Amp Draw on Trolling Motor Performance
The number of amps drawn will impact two areas of the performance of a trolling motor:
- The available range
- The power that the motor produces.
These two are inversely related to each other, as can be seen in the following table. The higher the power rating, the lower the run time, and vice versa.
The typical draw of trolling motors is listed in the table below.
| Trolling Motor Thrust | Amps Typically Drawn | Run Time in Hours | ||||
| Max Speed | Med Speed | Slow Speed | Max Speed | Med Speed | Slow Speed | |
| 30lbs | 30.0 | 15.0 | 5.0 | 3.3 | 6.7 | 20.0 |
| 40 – 70 lbs. | 40.0 | 20.0 | 5.0 | 2.5 | 5.0 | 20.0 |
| 90 -100 lbs. | 50.0 | 25.0 | 5.0 | 2.0 | 4.0 | 20.0 |
🔋 Battery Life and Usage
Batteries for trolling motors are a pivotal part of the equation. You should be aware of the following factors that determine battery life.
The Number Of Discharge/Recharge Cycles
Modern Lithium-ion batteries are designed to be discharged and recharged for several cycles. If a battery is designed for a 1000-cycle life, it can be discharged and recharged 1000 times before a noticeable drop in performance.
The more often the boat is used, the more the discharge/recharge cycles occur; therefore, the time frame the battery is expected to last for will be reduced.
The Depth Of Discharge
All battery technologies have a limit on what they can be discharged to. Lead acid batteries must not be discharged to less than 50%, while Lithium-ion batteries can generally be discharged to 15%.
If the batteries are discharged to a value lower than the manufacturer’s indicated limit, it will cause damage to the batteries, which cannot be reversed.
The Conditions Under Which The Battery Is Kept
The batteries must be kept in a watertight position and should always be kept clean.
The electrolyte level must be maintained correctly if the batteries are lead acid or similar.
When the battery is stored, it should be kept in a cool place and on a trickle charge.
Motor Lifespan and Maintenance
Modern electric trolling motors need minimal maintenance. All that is required is to adhere to the manufacturer’s maintenance checklist. This will ensure that the motor lasts.
🎯 Choosing the Right Trolling Motor and Battery
With the above information, you can make an informed choice as to the correctly sized trolling motor and battery.
Balancing Power and Efficiency
Trolling motors are sold with ratings for 12V, 12/24V, 24V and 36V batteries.
The most powerful trolling motors produce over 100 lbs. of thrust, which is sufficient for a vessel such as a pontoon boat.
As a rule of thumb, the more powerful the motor, the better. More power allows the boat to counter wave action, wind, and current.
Selecting the Appropriate Battery Capacity
The size of the motor, the typical cruising distance, and the time on station will determine the battery you choose.
Once you have worked out your requirements, you can choose a battery (or series of batteries) that provides you with sufficient amp hours for your needs.
❓ Frequently Asked
How does temperature affect the performance of trolling motors?
Temperature can impact the efficiency of trolling motors. Extreme cold or hot temperatures can affect the battery’s performance, leading to reduced run time.
Are there any safety precautions to consider when using trolling motors?
Yes, safety is paramount when using trolling motors. Users should ensure that the motor is securely mounted to the boat, avoid overloading the boat, and regularly inspect the motor for any signs of damage or wear. Keep hands and clothing away from the propeller when the motor is running.
How do saltwater and freshwater impact trolling motors?
Saltwater is more corrosive than freshwater, which can lead to faster wear and tear on trolling motors. Choosing a trolling motor designed specifically for saltwater use is essential if you plan to use it in such environments. After each use in saltwater, it’s a good practice to rinse the motor with freshwater to remove salt deposits and prolong its lifespan.
