How to Measure Shaft Length for a Trolling Motor

Co-authored by Hunter Rising

Last Updated: January 13, 2023

So you want to add a trolling motor to your boat, but how can you know if one is long enough for your boat? Choosing a motor with the right shaft length ensures that it reaches the water and has the best performance, and it's really easy to figure out what size you'll need. Keep reading to find out how to measure for a trolling motor so you know what shaft length will work best for you!

1

Measure from the top of your boat deck to the waterline.

The distance is the length you need to reach the water from your boat. Put your boat in the water on a clear, calm day to take the best measurement. You can mount a trolling motor either on the bow (the front of your boat) or the transom (the back of your boat). Extend a tape measure from the top edge of the boat deck down to the surface of the water and write down the measurement.^{[1]
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- The distance between the boat deck and waterline will change depending on how much weight you have in the boat, so try to measure when you have all of your equipment with you.

2

Add 20 in (51 cm) to your measurement.

The length accounts for the area above the mount and the propeller depth. The head of the trolling motor, which is the top part of the shaft with the handle, usually extends around 8 inches (20 cm) above the boat deck. The propeller works best about 12 inches (30 cm) below the water’s surface. Sum up your measurement and the added length for the approximate shaft length you need.^{[2]
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3

Trolling Motor Size Charts

Bow Mount Motor Shaft Lengths^{[3]
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Bow to Waterline Measurement	Shaft Length
0–10 in (0–25 cm)	36 in (91 cm)
16–22 in (41–56 cm)	42–45 in (110–110 cm)
22–28 in (56–71 cm)	48–52 in (120–130 cm)
28–44 in (71–112 cm)	54–72 in (140–180 cm)
45 in (110 cm) or more	87 in (220 cm)

Transom Mount Shaft Lengths^{[4]
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Transom to Waterline Measurement	Shaft Length
0–10 in (0–25 cm)	30 in (76 cm)
10–16 in (25–41 cm)	36 in (91 cm)
16–22 in (41–56 cm)	42 in (110 cm)

4

Include an extra 5 in (13 cm) if you fish on saltwater or rough waters.

The added shaft length keeps the propeller submerged if there are waves. If the body of water where you’re fishing frequently has waves or choppy water, the propeller could break the surface if you have a shorter shaft. With a little extra length, the changes to the waterline level won’t affect your motor.^{[5]
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- When the propeller breaks the surface, it causes “cavitation,” which is when the propeller makes a loud noise when it slaps the water’s surface and makes loud noises that scare fish.

5

Tack on 9 in (23 cm) for hand-control motors.

The longer shaft makes it more comfortable to hold and control the motor. While some motors are remote- or pedal-controlled, you have to bend down to steer a hand-control motor. Add a few extra inches to your measurement so you don’t have to reach as far to reach the motor controls.^{[6]
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- It’s okay for a trolling motor to be a little shorter if you’re mounting it to the transom since most boats have rear seats or a bench you can sit on while you steer.

6

Opt for a the next longest size motor if there’s not one in the size you need.

Motors shorter than your measurement could pop out of the water. If you can’t find a trolling motor that’s the exact length you need, pick one with the closest shaft length to your measurement. You can always shorten the shaft length of your motor later on.^{[7]
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- If you need to make a small adjustment to the depth after you mount your trolling motor, loosen the depth collar knob on the top of the shaft until it slides freely. Then, pull or push the shaft so the propeller is 12 in (30 cm) below the surface before tightening the knob again.^{[8]
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