Understanding Trolling Motor Shaft Length
When shopping for a trolling motor, the three most important considerations are the thrust generated by the motor, the length of the shaft, and whether it’s cable or electric steer. We’ll focus on shaft length here.
Why Trolling Motor Shaft Length is Important
Choosing a trolling motor with the right shaft length for your boat is essential, with the goal being making a choice that favors stealth over power. In other words, if the shaft is the wrong size, the trolling motor alerts fish to your presence.
However, if the shaft is too long, the propeller creates turbulence deep beneath the boat that will spook any nearby fish. And if the shaft is too short, the prop won’t be adequately submerged, which will create noisy chop in the water and also scare away the fish. Basically if you bought a trolling motor for fishing purposes, the wrong shaft length defeats the entire purpose of investing in one.
Calculating the Correct Shaft Length
A trolling motor’s ideal propeller should be at a depth that keeps about 6 inches of water above the blades. In other words, the centerline of the motor and the prop shaft should be about 12-18 inches below the waterline, depending on the make, model and dimensions of the trolling motor.
To calculate the ideal shaft length for your boat, select the point on the transom or bow where the motor will be mounted, then measure down to the waterline. Having established the mount-to-waterline distance, you’ll need to factor in wave action. The rise of a boat due to waves will affect the depth of the trolling motor, so a longer shaft is needed to keep the propeller fully submerged.
The transom doesn’t move too much in relation to the waterline, so for transom-mounted trolling motors you’ll probably need to add about 6 inches to the mount-to-waterline measurement. However, the shaft of a bow-mounted trolling motor may need to have 12+ inches added to the mount-to-waterline measurement to account for the bow bobbing up and down.
If you do your fishing on perfectly flat lakes or backwaters, you don’t need to worry about adding extra shaft length to your mount-to-waterline measurement.
Armed with your final measurement (the initial static mount-to-waterline measurement plus any additional measurement to account for boat movement), you’ll have a good idea of the trolling motor shaft length needed. The chart below gives you approximate shaft lengths.
NOTE: These figures are only for guidance, and are not exact figures. Bear in mind that the exact dimensions of trolling motors vary depending on the make and model.