Trolling Motor Shaft Length Guide
Selecting the proper shaft length is critical. You want to make sure the motor's shaft is long enough to keep the propeller submerged in varying water conditions, but not so long that it catches on the bottom or is a hassle to stow.
A Few Rules to Remember
Submersion Rule: You'll want the center of the propeller submerged at LEAST 9" under the water to ensure adequate power and to prevent noisy cavitating which will scare fish. The charts below will help you determine what shaft length achieves this (given different distances to the water, the shaft portion about the mount, etc) but it's still a good idea to keep this 9" figure in mind.
Always Go Long: If you're struggling to decide between two lengths, pick the longer one. If the shaft ends up being a bit too long, you can always adjust the shaft depth upward with the depth collar adjustment found on nearly all trolling motors. But if you pick a shaft that's too short, you're simply out of luck.
Shaft Length on the Bow
Proper shaft length selection is is more critical with bow mount motors (as compared to transom mount). Bow-to-water distances tend to vary more from boat to boat, and the bow moves much more on the water than the transom. As such, it's important to make sure you select the proper shaft length. Start by measuring the distance from the horizontal mounting point on the bow to the waterline, then add 20" to this measurement. Generally, you will want a shaft length that is equal to or greater than this sum, but not less. The following factors may be further considerations:
If you frequently fish in rough waters: Then add 5" to the recommended lengths below. This will help the propeller stay submerged despite the additional deck bob of choppy or windy waters.
If steering a hand controlled motor standing up: Then add 12" to the recommend lengths below. This will make it easier and more comfortable to steer by brining the motor tiller higher up.
Shaft Length on the Transom
Shaft length on transom mount motors is less critical than on bow mount motors, but is still important to double-check.
How the Shaft is Measured
If you're trying to determine how long your existing shaft is - or take measurements on your own boat - it's helpful to know exactly how the shaft is measured. Shaft measurements are taken from the base of the motor head to top of the propeller housing, as seen below: