A shaft that's too short leaves the motor near the surface, where Minn Kota says it can cavitate and make noise. One measurement, plus the chart from your motor's maker, gets the length right.
Minn Kota says the wrong shaft length lets the motor cavitate, creating fish-spooking noise. Its rule of thumb is to have the top of the lower unit, the foot of the motor, covered by at least 12 inches of water.
Length matters most on a bow mount. Minn Kota points out that most transoms sit at a similar height above the water, so its standard transom shafts usually fit, while bow heights vary much more.
Use the spot on the bow or transom where the motor's mount will sit.
Measure straight down from that mounting surface to the top of the water. Minn Kota, MotorGuide and Garmin all start from this number.
Minn Kota adds 5 inches for rough water and 9 inches for a bow-mount Hand Control motor, plus another 12 inches if you will steer a hand-controlled motor while standing. Garmin adds 8 inches if you regularly run into a lot of waves.
Look up the total in the chart for the brand you are buying. Each maker publishes its own, so don't take one brand's chart to another brand's motor.
| Bow to waterline | Recommended shaft length |
|---|---|
| 0 to 10 in | 36 in |
| 16 to 22 in | 42 to 45 in |
| 22 to 28 in | 48 to 52 in |
| 28 to 34 in | 54 to 60 in |
| 34 to 52 in | 72 to 87 in |
| 52 to 72 in | 100 in |
| Transom to waterline | Recommended shaft length |
|---|---|
| 0 to 10 in | 30 in |
| 10 to 16 in | 36 in |
| 16 to 22 in | 42 in |
| 22 to 28 in | 55 in (Riptide Transom) |
MotorGuide uses a formula instead of a chart: measure from the top of the bow where the motor mounts to the waterline, then add 20 inches. Its catalog says the shaft should be at least as long as that total, so an 18-inch bow needs a shaft of 38 inches or more.
Once the motor is on, MotorGuide's X5 manual says to set the depth so the propeller blades are fully submerged 6 to 12 inches below the surface, to avoid ventilation.
| Motor | Bow to waterline | Recommended shaft length |
|---|---|---|
| Force and Force Pro | 0 to 19 in | 50 in |
| Force and Force Pro | 15 to 26 in | 57 in |
| Force Kraken | 0 to 19 in | 48 in |
| Force Kraken | 15 to 34 in | 63 in |
| Force Kraken | 26 to 46 in | 75 in |
| Force Kraken | 38 to 61 in | 90 in |
| Force Kraken | 53 to 81 in | 110 in |
Minn Kota says most of its shafts can be shortened, and recommends an Authorized Service Center do it: cutting the shaft yourself voids the warranty, while a service center job does not. The Ulterra, Ulterra QUEST and Riptide Instinct QUEST shafts can't be shortened at all. Fitting a longer shaft later is generally cost-prohibitive, so it pays to measure before you buy.
Minn Kota, Shaft Diameter and Shaft Specifications and Motor Selection Guide; MotorGuide, 2023 Catalog and X5 Owner's Manual; Garmin, How to Choose the Right Trolling Motor.