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Understanding Trolling Motors

This post was written by Andrew on September 24, 2010
Posted Under: Fishing Tackle,Fishing Techniques

Trolling motors can be an incredibly helpful – even indispensable – fishing tool, as long as you have the right one for your boat. There’s nothing worse than an underpowered trolling motor, or a battery that dies half-way through a great day on the lake. With the large number of options and motors available, it can be difficult to know which motor to pick. In this guest post, we’ll discuss a few of the most important things to consider and understand when it comes to trolling motors.

Thrust

Trolling motors are rated by pounds of thrust, and the question of how much power is needed is likely the most common trolling motor question people have. Motors range in power for around 30 lbs of thrust for entry-level models to up to 100 lbs upper-end models designed for heavy, large boats.

The weight of the boat is the most important determinant of how much power is needed.   While boat length can be used as a proxy, boat weight is the best factor to use in order to accurately pick a motor with ample power.

A good rule of thumb is that you’ll want 2 lbs of thrust for every 100 lbs of boat weight. For example, for a 2,000 lbs boat, you’ll want a motor with at least 40 lbs or thrust (2000 / 100 * 2). When calculating, you’ll want to factor in the weight of a fully loaded boat with people, fuel and equipment.

If you’ll be relying on your trolling motor to perform frequently in strong winds or currents, you’ll want to get a stronger motor than otherwise indicated. The previously mentioned rule of thumb is a great baseline for calm water operations, but use in stronger waters will require more power.

Shaft Length

Shaft length often introduces confusion into the trolling motor selection process as many motors come with multiple options. Getting the shaft length right is important. If the shaft is too short, the trolling motor propeller won’t be properly submerged and power will suffer. If the shaft is too long, it makes trolling in shallow waters difficult.

The proper shaft length differs depending on whether a transom or bow mount motor is being used:

Transom Mount: 20” to 25” of the shaft should be submerged

Bow Mount: 25” to 30” of the shaft should be submerged

To determine the proper shaft length for your boat, you’ll need to measure the distance from the bow/transom to the waterline, and then add the previously mentioned “submersion factor”. For instance, if the distance from the bow mounting platform to the waterline in 20”, you’ll want a shaft length in the 45” to 50” range ( 20” plus 25” to 30” submersion factor).

It’s always a good idea to err on the long side (vs the short side) when it comes to shaft length. While you can usually raise the depth of the propeller/shaft with the motor’s sleeve adjustment, it’s impossible to lengthen a shaft that’s too short.

Battery Life

One thing you want to make sure of before pulling the trigger on a new motor is that it has enough power to last all day! Nothing is more frustrating than running out of power when you still want to fish. There are two important things to know when it comes to estimating battery life – battery ratings and amps drawn.

Trolling motors use 12 volt marine batteries, which are rated by amp hours. An 80 amp hour rated battery is capable of delivery a total of 80 amp hours of power on a single charge. This could either be 80 amps delivered for 1 hour, or 20 amps delivered for 4 hours.

While it can sometimes be difficult to find, manufacturers (and good retailers) will be able to tell you how much power a motor draws at maximum speed. A trolling motor that pulled 50 amps at top speed would run for about 2 hours using a 100 amp hour battery before running out of power. Approximating, you could assume the motor would run for about 4 hours at medium speed (pulling 25 amps) and around 14 hours at slow speeds (pulling 7 amps).

A few suggestions to get the most run time possible:

  • At a minimum, get a battery rate for at least 100 amp hours
  • Variable speed motors are more efficient that fixed speed motors, and will run longer
  • Purchasing a more powerful motor than you need will extend your run time. While a 50 lbs motor may draw 50 amps at top speed, an 80 lbs motor may only require 35 amps to produce the same 50 lbs of thrust
  • Make sure you never let your battery become completely depleted as this will significantly shorten it’s life and ability to hold a charge.
  • Once you have an idea of the thrust rating and shaft length you want, you can start looking at individual motors. The two most popular manufacturers or fishing trolling motors are Minn Kota and MotorGuide. While Minn Kota is better known and more popular, MotorGuide usually offers a little bit more motor for the money.

Specific Models

With dozens of different motors available, we’ve recommend a few of our favorite motors in different categories:

Best Value – Bow Mount

It doesn’t have many bells and whistles, but the Edge series from Minn Kota is a well-built bow mount motor at a really affordable price point. While MotorGuide usually offers more value, the Minn Kota Edge series beats out MotorGuide’s comparable Freshwater bow mount line in terms of bang-for-the-buck. Edge only offers thrust up to 70 lbs, so this isn’t a great choice for larger boats.

Best Value – Transom Mount

For the money, you can’t beat the VariMax line from MotorGuide. The entry-level transom mount motors are priced similarly to Minn Kota’s Endura line, but also include variable speed motors and battery life indicators (Endura offers neither). Stepping up to similar variable speed models within the Minn Kota line-up will cost $50 to $100 more.

Best Features

Trolling motors have evolved with the times, and now offer some pretty amazing features. The Minn Kota PowerDrive V2 and Terrova series offer AutoPilot, wireless steering and GPS technology. The AutoPilot feature lets the operator set a course, and the motor stays on it without continual control or monitoring. CoPilot is an accessory that offers complete wireless motor control. The Minn Kota i-Pilot accessory uses a built-in GPS to control motor speed and direction, making it possible to re-trace routes or stay in place despite winds and waves. These features are also available on the Minn Kota Riptide ST and SP series, which are similar saltwater version motors.

Best Built

It may not boast fancy Minn Kota i-Pilot technology or wireless control, but the Fortrex line is built like a tank and designed to last. Using heavy-duty materials and single-pivots, the Minn Kota motor is designed to be used, abused, and counted on.

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    Reader Comments

    “Using heavy-duty materials and single-pivots, the Minn Kota motor is designed to be used, abused, and counted on.”
    Are you sure that this is so?

    #1 
    Written By maccprocte on December 19th, 2010 @ 7:19 pm

    MG v MK is a lot like the Ford v Chevy argument. They both work!

    I have an electric boat (16′) powered by a MK 74 and am building a larger (20′) one powered by two MG 67s. The choice is dictated by what I found cheap.

    In a yard sale lot I got a good 67 and 74 and two junkers for $20! I used the MK74 on my 16′ boat and have a lot of experience with it.

    My use is mostly on the Escambia River, above Pensacola. My typical outing is 20 miles, though with care it should easily do 60 on a charge. (I am planning a 50 mile trip when I can convince the wife to bring me home.) This is my only power. The river is full of submerged logs and such and the motor gets to hopping and jumping in its unpinned bow mount on the stern. The steering linkage takes a beating and the Kipawa prop gets skint-up, but it always gets me back.

    When I got the motor, I took it for a test drive. It had never been serviced and was quite old. The bearings were squeeking, but it got me back. New brushes and bearings made it right. The brushes are huge.

    Before I understood the nature of displacement hulls, I just kept pouring on the power, attempting to go faster. A displacement, non-planing, hull goes as fast as it goes and no more, regardless of power. The result is that 74 lbs of thrust is about 50% more than I need. I have run the motor flat out going upriver for 4-5 hours at a time. The armature shows signs of that heat, but there were no problems. At full hull speed the motor is loafing.

    So, my opinion is yes, the Minn Kota is built to be used and abused and counted on.

    As far as the two MGs go, we’ll see. Their shafts are not as heavy as the MK’s, but they seem well built. Tales from the field say that MG tends to pull a little harder for a given rating. That’ll be good because a preliminary test of the hull, before modification to add a fantail, indicates it’ll need all the push it can get.

    Before you ask, power comes from golf car batteries. The hulls of both boats started out as sailboats and have been stretched with the addition of a fantail. Keels and masts are gone. Mounts are stump jumpers, made by putting bow mounts aft and removing lock pins.

    #2 
    Written By Neal on June 24th, 2012 @ 12:02 pm

    efore I understood the nature of displacement hulls, I just kept pouring on the power, attempting to go faster. A displacement, non-planing, hull goes as fast as it goes and no more, regardless of power.

    ithink so!!!

    #3 
    Written By fishingtalking on February 21st, 2013 @ 2:59 am

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