Shaft Length Guide

Trolling Motor Shaft Length Guide

Selecting The Proper Trolling Motor Shaft Length Is Critical

Determining the proper trolling motor shaft length is one of the key aspects of finding the right motor for your boat. It will help ensure that the motor's shaft is long enough to keep the prop submerged at least 12" below the water line; sometimes more depending on conditions. You also want to prevent the prop from coming out of the water during normal operation which creates noise and reduces thrust (aka "cavitation").

On the flip side, while an adequate shaft length is important, you don't want it to be so long that the motor's control head sticks way up or hangs over the rub rail when stowed. When in doubt always go longer but make sure that the stowed dimensions of the motor will fit on your bow. This is particularly important for some of the longer shafted motors.

Shaft Length Measurement for Bow Mounted Trolling Motors

Proper shaft length selection is more important with bow mount motors (as compared to transom mount). Bow-to-water distances tend to vary more from boat to boat, and the bow rises and falls much more on the water than the transom.

Start by measuring the distance from the mounting surface on the bow to the top of the water (a tape measure works well for this). For most small to medium sized boats in calm conditions, we tend to be conservative and recommend adding 23"-25" to your measurement to be on the safe side.

The final measurement will provide you with the minimum required shaft length. If you are trying to decide between two shaft lengths for a certain motor, always go longer! The absolute last thing you want is to have a motor that is too short.

There are other important factors that you must take into consideration when determining the right shaft length. Due to the increase in demand for saltwater trolling motors on larger offshore boats, trolling motors with shafts in excess of 100" are becoming more common. Due to the much higher freeboards on boats such as 10,000 lbs center consoles, shaft length measurements must be adjusted accordingly.   

If you frequently fish in rough waters with moderate swell:  Add 27"-30" to your bow to water line measurement to determine your minimum required shaft length. 

For larger, offshore fishing vessels such as center consoles over 22' in length weighing between 5,000 lbs. and 12,000 lbs. we recommend adding 30"-40" to the bow to water line measurement.   

This will help the prop stay submerged despite the additional swell or chop. If you have the stomach to fish in 3'+ swells, your prop will most likely cavitate at one time or another. These trolling motors are not designed for significant swell but Spot-Lock can recover pretty quickly. 

If steering a hand controlled motor standing up: Then add 12" to the recommended transom lengths below.  This will make it easier and more comfortable to steer by bringing the motor tiller higher up. Of course, you can always add a telescoping extension handle to add some height and length as well.  

How To Correctly Measure Bow To Water Line

 

Minimum Trolling Motor Shaft Length Recommendations

Bow to Waterline (inches)	Recommended Shaft Length (inches)
0" to 10"	36"
16" to 22"	45"
22" to 28"	48" to 52"
28" to 34"	54" to 60"
34" to 52"	72" to 87"
52" to 72"	100"

 

Transom Mount Shaft Length

Shaft length on transom mount motors is less critical than on bow mount motors, but is still important to double-check.  

Trolling Motor Shaft Length Measurement Tips

Generally speaking, measuring for correct trolling motor shaft length is a simple process...really simple. However, there are some tips to getting a realistic measurement. While some of these tips may seem silly, there's nothing funny about your prop bobbing up (cavitation). A prop that is too shallow will significantly reduce thrust, make a bunch of noise and bubbles as well as shorten run time. A free spinning prop is also not "healthy" for your trolling motor.

1. Measure in calm water to get an accurate read.
2. Account for the extra weight directly over the bow. If you are in a light boat, the bow will dip more than you think. 
3. Try to measure with a full tank of fuel, gear etc. in order to get an idea of how much it will impact bow rise. You are looking to measure under "real world conditions."
4. Regardless of where you install them, you need to account for the weight of the motor as well as the batteries. A 24v motor and two 12v batteries can weigh as much as a well-fed, grown man. 
5. If at all possible, try not to fall overboard when taking your measurement...it's embarrassing. (See Below) 

 

A Few Important Rules to Remember

Submersion Rule:  You'll want the top of the motor submerged at LEAST 12" under the water to ensure adequate power and to prevent noisy cavitation which will scare fish. The charts above will help you determine what shaft length achieves this (given different distances to the water, the shaft portion above the mount, etc) but it's still a good idea to keep this 12" figure in mind.

 

Measuring The Shaft Length Of Your Current Motor

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 generally taken from the base of the control head (the housing on top of the motor) to the lower unit (motor housing) as seen below...basically the full length of the shaft itself.

However there are variations of from model to model in order to accommodate for the motor mount and shaft adjustment collar. As a result, this type of measurement will only give you the approximate length of your current motor.

Ultimately, you should use either the factory "rated" shaft length or take a bow to waterline measurement which are more reliable measurement methods. If you do not have access ready access to your boat, you can always call a local dealer or manufacturer for your boat brand; they might know what is typically installed on a model like yours.  

 

 

Ultimately, getting the correct measurement is CRITICAL. You can always raise the shaft of the motor but if you've gone too short, you can only lower it so much. Unfortunately, by the time that you determined you've gone too short, you're the official owner of that motor and that's not a good situation to be in...for any of us!  

 

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