How to Measure Boat Motor Shaft Length?

One of the better attributes of an outboard motor is its ability to adapt. If you do not know, how to measure boat motor shaft length then things might get challenging.

An outboard motor can continue operating almost any type of boat with the appropriate mounting. You can find outboards hanging from the sterns of anything from tiny canoes to substantial cruising sailboats, with the exception of motorboats designed to accommodate large outboards. Many single-engine boats are equipped with a tiny auxiliary motor that can be utilized during an emergency.

Determine the distance between both the bottom of the mounting bracket, which rests on the transom and the top of the cavitation plate, which is the horizontal surface of the lower unit straightforwardly just above the propeller, in order to determine the length of your outboard motor shaft.

Let’s learn how you can measure the length of a boat motor shaft on your own…

Boat Motor Shaft

Unquestionably, selecting a boat engine is challenging, and you must choose the type of propulsion before purchasing a boat. Engine type 2-stroke or 4-stroke, injection mode 2-stroke or electronic injection, propulsion 6hp, 25hp, 50hp, 100hp, or even 250hp engine, power 6hp, 25hp, 50hp, 100hp, or even 250hp engine, and even brand are all factors. The right shaft length for your boat must still be chosen once you have the answers to these questions.

Regardless of the size of the boat, you are operating, measuring boat motor shaft length is one of the most essential aspects. Every boat is dissimilar, and the motor automakers produce a variety of standard lengths. Finding the appropriate dimensions only necessitates a quick glance at your established possessions and the use of a measuring tape.

How to Measure Boat Motor Shaft Length?

What is Outboard Shaft Length?

The outboard motor manufacturer’s stated shaft length is matched by the vertical distance between the clamp’s interior and the cavitation plate. On the other hand, the cavitation plate is actually an insect ventilation plate and the shaft lengths range from 5″ and longer (127 mm).

If you are unsure of what is standard shaft length outboard, then working with it could be challenging for you.

The standard shaft length outboard is 15, 20, and 25 inches or 38, 50, and 63 cm. There are some exceptions to the industry standard for shaft lengths.

However, 20 inches is the most typical shaft length for mid-range engines (508 mm). Only protected waters should use 15 inches’ shafts.

Short Shaft Outboard Motor VS Long Shaft

If you want to know how to measure boat motor shaft length, then knowing the difference between short shaft and long shaft outboard motor is essential. Because outboard motors can be installed on a variety of boats, manufacturers have standardized their outboard engines so they can be mounted on almost any boat.

An outboard motor can be ordered in lengths ranging from 15 inches to 30 inches, depending on its power output and potential applications. For short shaft models, outboard motor shaft lengths are typically 15 inches, for long shaft models they are 20 inches, and for extra-long models they are 25 inches.

Due to the relationship between the length of your outboard’s shaft and the size of your boat, smaller engines typically have shorter shafts. Small and portable outboard motors with 2 to 20 horsepower are frequently sold only in short or long shaft versions.

In general, high-powered boats are only prepared to offer long and extra-long versions. For absolutely enormous boats, the “exceptionally long” shafts of the largest and worst six- or eight-cylinder outboard motors can reach a length of 30 inches.

How to Measure Boat Motor Shaft Length?

The following motor lengths are typical:

  • 15 inches short shaft;
  • 20-inch long shaft;
  • Elongated shaft, 25 inches;
  • 30 inches of extremely long shaft.

There are also available specialty motor shaft lengths. Small sailboat motors may have an unusual design with an unusually long shaft due to the fact that they are mounted higher off the water.

The same is true for the newest, most potent offshore fishing boats, which more often than not come equipped with triple or quadruple motor configurations.

It’s crucial to keep in mind that manufacturers give their models a variety of names. Some might describe it as being a “standard” length, while others might describe it as being a short or long shaft. Not just the name, but the actual measurement of the specifications is what matters.

Measure the Length of a Boat Motor Shaft

Now, let’s discuss how do you measure the shaft diameter and what things need to consider during doing it. It is equally simple to measure the length of your outboard’s shaft if you are traveling in the opposite direction and want to know how long it is. When buying or selling a used outboard motor for your boat, this is advantageous.

Find the motor’s location on the boat by using the mounting bracket on the motor. You will share the same point of reference with the bracket because, when mounted, its flat portion rests on the transom of a boat.

From there, the anti-ventilation plate, also known as the cavitation plate is measured. The lower unit’s horizontal section directly above the propeller is referred to as the “cavitation plate.”

How Long Should My Outboard Shaft Be (Listicle)?

Measuring outboard shaft length, you have to keep in your mind that, it will differ depending on a variety of variables. For the best fit, it is very vital to take accurate measurements of your boat first. Once you know what will fit on your hull, you can adjust your choice based on the performance you are after and how you plan to use the boat.

The anti-ventilation or cavitation plate on your outboard motor should, under perfect circumstances, be level with the bottom of your hull. It helps to ensure the highest level of performance. How the outboard is mounted on the boat’s back will actually rely on the transom’s design. Outboard motor brackets may be used on some boats in place of transom mounting.

The cavitation plate’s excessively low position will result in increased drag and poor performance at high speeds and is less crucial for slower boats too.

Below, I mention typical boat motor length:

  • Short shaft–15 inches;
  • Long shaft–20 inches;
  • Extra-long shaft–25 inches;
  • Extremely extended shaft–30 inches.

What Shaft Length Outboard Do I Need?

Motor Mount Surface Distance           Suggested Shaft Length

From 0 to 25 cm                                               76 cm

From 25 to 40 cm                                             92 cm

From 40 to 55 cm                                             107 cm

Over 55 cm                                                         Consult a reseller

Outboard Shaft Length Chart

Shaft Length Recommendation

Transom Height                                                         Shaft Length

30 cm / 11.8 inches – 40 cm / 15.7 inches                    Extra Short

40 cm / 15.7 inches – 50 cm / 19.7 inches                     Short

Higher than 50 cm / 19.7 inches                                      Long

How to Measure Boat Motor Shaft Length?


1. Could Your Trolling Motor’s Shaft Be Too Long?

Ans: On the other hand, if the shaft is too extensive, the propeller will end up creating turbulence. This turbulence will frighten any nearby fish deep beneath the boat. Besides that, a short shaft will make it virtually impossible for the prop to be completely submerged, which will cause loud and annoying chops in the water and scare off fish.

2. How Long is My Trolling Motor’s Shaft?

Ans: To determine the appropriate shaft distance for your boat, you need to calculate the distance from the mounting point on the transom to the water’s surface. However, the shaft length considered necessary can then be determined by multiplying this measuring system by 20 inches. So if you’re conflicted between two sizes, just choose the significantly bigger one.

3. What Should the Length of a Minn Kota Shaft Be?

Ans: Way of measuring down to the water’s surface from the mounting surface of the transom or bow, you need to add 5 “you must measure to the waterline when fishing in choppy water.

As a result, 12 “to use a standing hand control motor, measure up to the waterline using this standard measure, and the tables below determine the appropriate shaft distance.

4. How Do I Know Whether My Outboard Has a Short or Long Shaft?

Take a measurement from the bottom of your outboard’s engine compartment to the bottom of the cavitation plate and the horizontal metal wing above the propeller. Remember that a short shaft is one that measures between 19 and 21 inches in length. A long shaft, on the other hand, is approximately five inches long.


In this article, I already mentioned how to measure boat motor shaft length in detail. Selecting the accurate shaft length for your outboard is essential if you want your boat to perform at its peak.

If the cavitation plate of your outboard is positioned at the same level as the hull of your fast boat, fast top-end speeds and a quick hole shot are guaranteed.

Offshore fishing boats, RIB, and bass boat owners must do this correctly. You can be sure that the outboard’s trim will work properly, and you will also be able to save money on gas.

Displacement of hulled sailboats and workboats can gain from a small amount of extra room between the hull and cavitation plate.

Remember that a longer outboard shaft will end up causing a higher draft, so keep that in mind. The outboard may also be challenging to completely lift out of the water for storage. Repowering your boat is one of the most ridiculously priced significant improvements you can make. Ensure that you measure precisely and conduct extensive research before beginning. Safe travels!

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