How to Select the Right Trolling Motor

Selecting the right electric trolling motor for your boat can be broken down into a few things.

1.How much power you need (What type and size of boat do you have?)
2.What shaft length do you need?
3.Do you need a bow mount (front) or transom mount motor (rear)?
4.How many batteries will you run on? (The more power, the more batteries)
5.Will you be using it in freshwater or saltwater?
6.Extra Features (Goodies)

The size of boat you have will determine the size of trolling motor you need. Obviously, the bigger the boat, the bigger the motor you need.
Most electric trolling motors are not rated on horse power like gas outboards, they are rated in pounds of thrust.
30 lb. thrust will work satisfactory for only the smallest of boats. I use a 30 lb. on my 8 ft. plastic bass boat which is pretty slow moving motor. This would probably work satisfactory on a canoe as well.
For boats under 16 ft long such as aluminum john boats and row boats, select something in the 40-50 lb. range. More power is always nice, but if you need a short shaft length you may be limited with the smaller thrust rated motors. These shorter shaft lengths typically range from 30” to 36”.
Moving up in both power and shaft length are the motors between 50 and 100 lbs. Motorguide recommends using 46-82 lb. motor for boats 16 to 18 ft. and 60-109 lbs for boats 19 to 22 ft. long. Motors in the 50-109 lb. range have shaft lengths approximately 36” to 60” long.
It’s important to know what length shaft you will need and this depends on the boat you have as well as if you need a bow or transom mount.
Motorguide offers this rule-of-thumb for foot controlled motors for determining the proper shaft length. Measure the distance from the boat's waterline to the mounting platform (boat gunnels) and add 24 inches and then round to the closest shaft length available.
The larger the motor, the more power you will need.  For a small boat, running more than one or two batteries may be unrealistic. Keep in mind that your trolling motor batteries should not be used as your starting battery for your outboard. If you have an electric start  outboard, it should have it’s own battery.
If you have a larger boat consider at least a 24volt system running at least two batteries for your trolling motor. If you fish in tournaments or are a heavy user consider a 36 volt system.
There is a wide variety of mounts available some of which include some type of shock  absorption, perfect if you fish in heavy cover areas. Consider the style boat you have and the type of mount you require.
Goodies:
Even the most basic trolling motor has adjustable speeds. Some motors offer a wider choice of speeds both forward or reverse.
A foot controller is standard on most bow transom mount motors while a hand controller is used on most transom mounts. There are some transom mount, foot controlled units available which is perfect for certain boats. Some higher-end units have cable-free foot controllers.
Today’s digital electronics offer innovative motors, some of which allow you to automatically set a direction or follow a depth contour or course. Some motors have built-in transducers that hook-up to certain sonar units. Digitals run quieter and smoother and hopefully will last longer.
Wireless remote control options are another nice feature to have as well as a battery strength indicator.
With each new boating season, manufacturers of trolling motors add more options and  sophistication to their new line of trolling motors, but do not let that dissuade you from buying used. Used motors if not abused that were not worked to death can last you a long time and will save on your pocket book.