Buyers Guide: Transom Mounted Trolling Motor
Before making a selective decision, you need to consider some vital considerations. Answering the five questions below will help you choose the best option for your needs.
1. What Is Thrust, And How Much Does My Boat Need?
Thrust (measured in pounds) is the amount of power a motor uses to propel a vessel through water. How much your trolling motor needs depends on the size and weight of the boat.
The general rule of thumb is that you need at least 2 pounds of thrust for every 100 pounds of total vessel weight.
Total vessel weight means the heaviest potential weight. So, you need to calculate it loaded with people, gear and full fuel levels. What if I don’t even know my boats weight? You can find it in the NADA vessel directory.
Acquiring as much thrust you can afford is great for battling winds and current. But remember, more thrust underwater can create vibrations that scare of fish.
2. How Much Runtime Do I Need?
To answer this question you need to consider two factors. First is the battery capacity measured in amp-hours. The second is the motors current draw, which is a measurement in amps. Now you do an easy equation with these two factors:
- Take a battery’s amp hour rating (AH) and divide it by the “average amperage draw” of the motor. Most companies state amperage draw in maximum amps, so get the average by dividing the max rating by 2.
Example: 100 AH Battery divided by 25 A (average amp draw) = 4 hours battery runtime
Keep in mind, you need to further consider the type of battery you choose to use. As it’s vital, you never want to deplete a lead-acid battery to 0% of its capacity. In fact, it’s best to use between 20-80% AH of a battery’s capacity to ensure longevity.
Another consideration that effects runtime is where are you operating your boat? Adverse weather and water conditions will also lessen how long your motor will run. Strong current, choppy waters, or wind gusts, creates more current draw and short run times.
3. Which Speed Control System Is Best?
The best way to control the speed of your transom mount trolling motor is a personal decision. There are three modes of speed control, ranging from basic to sophisticated:
- The absolute basic is a simple 2 speed, high or low button or switch on the motor head. This is too simplistic for trolling but fine for small tender boats.
- Classic multi-speed motor found on the tilling handle. You twist the handle in either direction. In general, you get 5 Forward and 3 Reverse speeds. It’s an improvement for trolling and making basic docking maneuvers.
- New variable speed controllers. Often combined with micro-processors to extend battery life. Actual control can be selectable via twisting the handle. Otherwise, newer versions are remote control or wired foot pedal controls. Best for making precision movements during fishing or docking.
4. Which Shaft Length Does My Boat Need?
The shaft length you need, depends on the depth of your transom. Making assumptions or mistakes risk harm to both the performance and the propeller.
A good rule of thumb is for the motor unit meant to be underwater to be at least 10″ submerged. Doing this helps to reduce prop cavitation, vibrations and noise.
To determine the correct shaft length for your motor do the following measurement. Measure between the bow (for bow mount) or transom mounting surface and the waterline. Write down this measurement, then compare to the trolling motor brands (if available) shaft length table.
5. Does Saltwater Ruin Trolling Motors?
Yes! This is a critical factor to ensure long term use of you trolling motor. Salt is much more corrosive than freshwater so combating it requires a different construction. If you are fishing on saltwater, a rust-resistance motor is a must-have.
Overall, a freshwater motor will last longer than salt version. You take considerable risks running a freshwater motor in oceans. Expect oxidation, rust and dead motors if you choose to do this.
In fact, many manufacturers claim their product is dual purpose, many aren’t. The best way to determine saltwater rating is to look for these specific features.
- Built with premium-grade alloys coated with zinc.
- Painted or powder coated with corrosion-resistant finish.
- Includes a sacrificial anode fixed to the prop.
If you use a freshwater trolling motor in saltwater, often this will void your warranty!
Budget For Use
If money is a concern, maybe a freshwater trolling motor is the easiest selection. This version is often more affordable than a saltwater option. The reason for this is because freshwater doesn’t harm the internal.
Lake, pond and river boaters tend to worry about cutting through weeds. Which is why freshwater products feature heavy-duty props. Saltwater models don’t have this worry, so these props don’t feature.
In contrast, dual and salt specific trolling motors demand extra rust protective additions. As a result they ten to be more expensive, yet are also more powerful to fight against ocean currents.