How to Measure Trolling Motor Shaft Length

ByMathew F Updated On:

Selecting the proper shaft length for your trolling motor is a critical step to ensure optimal performance and control of your boat with the propeller submerged at least 12 inches below the waterline.

To determine the correct shaft length for your trolling motor, you need to measure the distance from the mounting point on the boat to the waterline.

This measurement is typically taken where you intend to install the motor, whether it be on the bow or the transom.

The variety of boats and where you choose to mount your trolling motor directly influences the necessary shaft length.

đź’ˇ Pro Tip: A trolling motor shaft isn’t determined in the same way an outboard shaft is measured.

Measurement Fundamentals

The two main aspects are measuring the shaft length accurately and mastering waterline measurement techniques.

Measuring the shaft length requires you to know the distance from where the motor is mounted down to the water’s surface.

First, measure the distance from the top side of the bow’s horizontal mounting point straight down to the waterline.

Add approximately 20 inches to this number to accommodate the length of the motor head above the deck and the optimal submersion depth of the propeller.

This is a general guideline; however, different conditions may require adjustments. For rough-water fishing, an additional 5 inches might be necessary for optimal performance.

To derive the waterline measurement, use a tape measure or a ruler to find the vertical distance from the intended mounting spot on the bow to the water’s surface.

Make sure your boat is in a normal, level floating position to get an accurate reading.

For transom-mounted trolling motors, assess the height of the transom relative to the water.

This distance is critical for determining the shaft’s length, with typical recommendations being a 30-inch shaft for transoms up to 10 inches high, and a 36-inch shaft for transoms up to 16 inches high.

đź’ˇ Pro Tip: Standard practice suggests that the propeller should run at least 12 inches below the water surface to perform effectively.

The Role of Shaft Length

The shaft length directly impacts how effectively your motor operates in different water conditions.

It’s essential to match the shaft length of your trolling motor to the waterline of your boat.

The waterline is the point on the boat’s hull that sits at the water’s surface when the boat is afloat.

To determine the correct shaft length, measure the distance from where you will mount the motor to the waterline.

This measurement ensures that the propeller is sufficiently submerged, following the submersion rule, typically 12 inches under the waterline for effective thrust and control.

An accurate shaft length is also pivotal in preventing cavitation.

Cavitation occurs when rapid pressure changes create air bubbles around the propeller, leading to loss of thrust, excessive noise, and potential damage.

To prevent this, you must ensure the propeller remains below the water’s surface during operation, which is, in part, a function of having the correct shaft length.

If the shaft is too short, it may cause the propeller to be too close to the surface, risking cavitation, especially in choppy conditions.

Boat Types and Mounting Options

Choosing the right type of trolling motor for your vessel involves understanding the types of boats and their respective mounting options. Your choice will affect handling, control, and the overall fishing experience.

These are ideal for medium to large boats, where directional control is crucial. They pull the boat through the water for improved steering and less wind interference. Typically, they require a flat surface at the bow for mounting and an adequate shaft length to reach the water.

Suitable for:

  • Pontoon boats
  • Bass boats

These are suited for smaller watercraft. They are easier to install and operate from the stern of the boat, pushing the craft forward. Since these boats sit closer to the water, a shorter shaft length can be sufficient.

Suitable for:

Factors Affecting Shaft Length Choice

Your selection should be based on the specific water conditions you expect to encounter and whether you will be operating in saltwater or freshwater environments.

Water conditions heavily influence the appropriate shaft length for your trolling motor.

In calm water, a standard shaft length may suffice. However, when you anticipate rough water, a longer shaft is necessary to prevent the propeller from emerging from the water due to bow rise.

If you often fish in varied conditions, erring on the side of a longer shaft can help accommodate unexpected changes in water state.

The environment you plan to fish in—whether saltwater or freshwater—also determines the shaft length.

Saltwater often equates to more turbulent conditions, requiring a long shaft trolling motor to ensure stability and performance.

Conversely, freshwater conditions are typically less demanding on the shaft length.

Remember, saltwater can be more corrosive, so choosing a shaft that’s not only the correct length but also built to resist this harsh environment is crucial.

Your choice of shaft length will balance between ensuring deep enough submersion of the propeller in various water conditions and considering the location of your fishing primarily in terms of water roughness and composition.

Adjustment and Customization

Proper adjustment of your trolling motor is crucial for optimal performance and customization options allow you to tailor your motor to your specific needs. This section will guide you through making precise modifications to the depth collar and steering, and how to customize hand-control and bow-mount motors for enhanced control and efficiency.

The depth collar on your trolling motor enables you to set the exact depth below the water’s surface that the propeller operates. To adjust the depth collar:

  1. Loosen the collar clamp screw.
  2. Slide the motor to your desired depth.
  3. Tighten the screw to secure the position.

For steering adjustments, most trolling motors offer a tilt/extend tiller that can be positioned for ease of use:

  • Extend or retract the tiller to match your reach.
  • Tilt the head for more comfortable steering angles.

Remember to regularly check these settings to ensure they’ve not shifted due to vibration or use.

Hand-control trolling motors give you direct manipulation over thrust and direction. The key aspects to consider for hand control customization are:

  • Mounting location: Ensure it’s within a comfortable reach.
  • Extendable handles: Some models offer handles that extend for better control and convenience.

For engine mount trolling motors, it is essential to check compatibility with your outboard engine and the mounting area, adjusting the unit for balanced steering.

With bow-mount motors, consider these customization tips:

  • Foot pedal adjustments: Position the pedal where you have quick and easy access.
  • Shaft length: For a bow-mount, remember the guideline from Minn Kota, encouraging adding at least 22″ to the bow-to-water line measurement for your shaft length, ensuring the propeller is submerged in varying water conditions.

Practical Guides and Resources

Shaft length recommendation charts and reference tables can help you take the guess work out of it. Although having read up to here you shouldn’t need to guess any longer, hopefully!

Shaft length recommendation charts are invaluable resources for choosing the right shaft for your trolling motor.

These charts consider factors such as the bow height, type of water conditions, and boat size. For instance, if your boat has a higher bow, you will need a longer shaft to maintain proper propeller depth.

Reference tables provide a condensed view of recommended trolling motor shaft lengths based on specific boat measurements.

These tables help you quickly cross-reference the height of your boat’s bow or transom to the waterline with a recommended shaft length.

An example is a table that suggests adding 20 to 22 inches to your measurement from the bow to the waterline for ideal clearance.

Frequently Asked Questions

To select the right shaft length for a bow mount trolling motor, measure the distance from the bow’s mounting point to the waterline and add 20 inches to this measure to accommodate the motor’s clearance and propeller depth.

For a deep V boat, consider that the shaft needs to reach deeper waters to provide stability and control. Use a longer shaft to ensure the proper immersion of the trolling motor even in choppy conditions; a general rule is the deeper the hull, the longer the shaft required.

Yes, if your trolling motor shaft is too long for a jon boat, it may cause improper stowage and affect boat control by creating drag. Ensure the length allows the propeller to be submerged by about 12 inches under the water surface for efficient thrust without being excessive.

For bass boats, shaft length is critical for maneuverability. Measure from the mounting platform to the waterline while the boat is under typical load conditions. Add enough length to ensure the propeller operates at least 12 inches underwater for efficient propulsion.

To match a trolling motor’s shaft to a 16-foot boat, attention to the vessel’s transom height and bow-to-waterline measurement is key. As a starting point, consider using a shaft that keeps the propeller 12 to 18 inches below the waterline when the boat is loaded.

Written by:

Mathew F

I’m the founder and chief editor here at Kite Ship. The electrification of boating is the most exciting thing to happen to the marine industry in a generation! Welcome, and I hope that we can provide the portal you need to dive into the world of electric propulsion and power.