How to Measure Trolling Motor Shaft Length

Here is how to measure trolling motor shaft length for a new motor: with the boat loaded the way you fish, measure straight down from the spot where the motor will mount (the top of the transom, the bow deck, or the top of a kayak mount) to the waterline. Then add the underwater allowance your motor maker specifies and read the total against that maker’s shaft chart. For a motor you already own, measure the shaft itself from the bottom of the motor head (the control housing at the top) to the top of the propeller housing.

This guide is general product information, not repair or electrical safety advice. Follow the installation, wiring, and charging instructions in your product’s manual, stop using any battery, charger, or electronics that overheat, swell, or smell of burning, and contact the manufacturer.

The allowance is where most mistakes happen, because makers use different numbers for different mounts. This guide walks through the transom, bow, and kayak methods using the charts Newport and Garmin publish, then shows how to confirm the depth once the motor is on the boat. Choosing thrust and voltage is covered separately and only summarized at the end.

What Shaft Length Means on a Trolling Motor

Shaft length is not the overall length of the motor. Newport’s shaft length guide measures it from the base of the motor head down to the top of the motor compartment, and the chart in Newport’s buyer’s guide labels it from the bottom of the motor head to the top of the propeller housing. Garmin uses a similar definition for its saltwater bow mount: the chart in Garmin’s guide to choosing a trolling motor notes that the Force Kraken is measured from the bottom of the shaft cap to the top of the prop drive.

That matters when you check a used motor. Newport’s article on shaft length for inflatable boats says the measured length may vary by an inch or two, and to round to the nearest even number to get the motor’s rated shaft length.

Before You Measure

• Load the boat. Newport’s inflatable boat article notes that the waterline measurement varies with weight, sea conditions, and the type of boat. Put batteries, gear, and the usual number of people aboard, or at least add their weight near where they sit.
• Measure on calm water. A dock or a calm launch ramp gives a steady waterline. Wind chop makes the number jump.
• Use the real mounting surface. If the motor will sit on a mounting plate, a riser, or a quick-release bracket, measure from the top of that part, not from the bare deck. Newport’s kayak motor setup article makes the same point for kayaks: how high the motor sits once the mount is on the boat affects the shaft length you choose.
• Bring a straightedge. Lay a level or a straight board across the mounting spot and hang the tape from its edge so you measure straight down, not at an angle along the hull.

How to Measure Trolling Motor Shaft Length for a Transom Mount

1. Find the spot on the transom where the clamp bracket will sit.
2. Measure from the top of the transom at that spot straight down to the waterline.
3. Add Newport’s allowance. Its shaft length guide says to add 20 in to the transom-to-waterline measurement, and to add another 5 in if you are often in rough water.
4. Round to an available length. If you fall between two sizes, Newport’s guide says to go with the larger one, because you can’t make a short shaft longer but a longer shaft can be adjusted in its collar.

Newport’s buyer’s guide also publishes a transom chart, and it lines up with the 20 in rule:

An example with made-up numbers: a jon boat measures 12 in from the top of the transom to the waterline. Adding 20 in gives 32 in, which falls between 30 and 36 in shafts. Going larger gives 36 in, and the chart’s 10 to 15 in row agrees. If the same boat often runs in chop, 12 plus 20 plus 5 gives 37 in, which points to the next size up.

One Newport page uses a smaller number. Its inflatable boat article says the motor compartment needs to sit 10 to 12 in below the surface, so it adds 10 in to the waterline measurement for a recommended minimum and 12 in for a more comfortable fit, again moving to the larger length when the result falls between sizes. Treat that as a minimum for low inflatable transoms. The 20 in rule and the chart give a longer result, and when in doubt, follow the chart for the motor you are buying.

How to Measure for a Bow Mount

Bow decks usually sit higher above the water than transoms, which is why bow motors come in longer shafts; Garmin’s Force models start at 48 and 50 in. Garmin’s guide says to measure from the surface of the bow to the waterline and to add 8 in to the total if you regularly see a sizable number of waves. Its chart then gives a recommended shaft length for each range:

The ranges in Garmin’s chart overlap, so a measurement such as 17 in fits two rows. The chart does not say which row to pick in that case. Adding the wave allowance where it applies can move you out of the overlap; otherwise, ask Garmin support which length fits your boat. For example, a bass boat that measures 18 in from the bow deck to the waterline, plus 8 in for regular waves, totals 26 in, which sits in the 57 in row for the Force.

Measure at the actual mount location. Garmin’s Force installation instructions call for mounting the motor as close to the boat’s centerline as possible, with the mount bumper overhanging the gunwale. Deck height can change across the bow, so measure there, not at the point of the bow.

How to Measure for a Kayak or Canoe

Kayaks sit close to the water, so the numbers are small. Newport’s kayak motor setup article says to measure from the top of the motor mount location down to the waterline, explains that its measurements aim to put the propeller 12 in below the waterline, and notes that 24 or 30 in shafts are generally used on kayaks. Newport’s trolling motor FAQ says its Kayak Series motors come with a 24 in shaft.

As an example with made-up numbers, a stern mount whose top sits 8 in above the water needs about 20 in of shaft to reach that 12 in target, so a 24 in shaft leaves room to adjust. A side mount set higher on a sit-on-top, or a canoe with a tall stern, may need the 30 in length. Paddle-drive and pedal kayaks with built-in motor mounts follow the kayak maker’s instructions, which take priority over a general rule.

Confirm the Depth on the Water

Once the motor is mounted, set the final depth with the collar on the shaft. Newport’s Kayak Series owner’s manual says to position the depth collar so the propeller blades sit 6 to 12 in below the surface, and to make sure the motor is not too close to the bottom or in danger of hitting it. On a bow mount, Garmin’s Force owner’s manual warns not to set the motor so low that it compresses the coiled cable, which can damage the cable and cause unexpected movement in anchor lock.

Watch for the signs of a wrong length. Newport’s shaft length guide says a shaft that is too short gives shallow prop placement, and that the correct length helps avoid noisy cavitation bubbles that spook fish. A shaft that is too long can put the motor head and propeller into rocks, weeds, and logs, and adds drag. If the prop breaks the surface when the bow rises over waves, the shaft or the depth setting is too short for those conditions.

Keep your hands away from the propeller while you adjust. The Newport manual recommends disconnecting the battery before raising or lowering the motor, and the Force owner’s manual says not to run the motor with the propeller out of the water. Check your own motor’s manual for its depth and safety steps before you start.

Shaft Length Is Half the Sizing Job

The other half is thrust. Newport’s motor thrust guide starts with the boat’s weight plus its maximum load and suggests about 5 lb of thrust per 200 lb of that total, with more for wind, current, and chop. The thrust you settle on then sets the voltage. Choosing thrust and voltage for your boat has its own step-by-step walkthrough.

One note on scope: the transom-to-hull measurement used to size gas outboards isn’t for trolling motors. Newport’s inflatable boat article explains that gas outboards are measured from the top of the transom to the bottom of the hull, while trolling motors are measured to the waterline.

Frequently Asked Questions

How do you measure shaft length on a trolling motor you already own?

Measure the shaft from the bottom of the motor head to the top of the propeller housing. Newport says the result may be off by an inch or two and should be rounded to the nearest even number to get the rated length.

How do you measure a trolling motor shaft for a bow mount?

Measure from the bow deck at the mount location down to the waterline, add 8 in if you often fish in waves, and read the total against the motor maker’s chart. Garmin’s chart, for example, puts 0 to 19 in at a 50 in shaft and 15 to 26 in at a 57 in shaft for the Force.

What if my measurement falls between two shaft sizes?

Newport’s guidance is to go with the larger size, since a longer shaft can be raised in its collar and a short one cannot be lengthened. For other brands, check the maker’s own chart or ask its support line.

Is a longer shaft always better?

No. Newport notes that a shaft that is too long places the propeller too deep, adds drag, and can hit rocks, weeds, and logs. Aim for the length the chart gives, with room to adjust, not the longest one available.

Do I measure with the boat empty or loaded?

Loaded. The waterline moves with weight, so measure with batteries, gear, and passengers aboard, the way the boat sits when you fish.

For how AnglerTechLab evaluates gear, see How We Review, and browse the trolling motors section for related guides.

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