What is the most common outboard shaft length in Ontario?

20-inch (long shaft) is the most common on Ontario freshwater boats. It fits most 14 to 19 ft aluminum fishing boats and open runabouts. But measure your specific boat before ordering.

Can I use a longer shaft than recommended?

You can, in the sense that the motor will mount. But an oversized shaft lowers your top speed, burns more fuel, and may cause the motor to load up at cruise. Use the right shaft for your transom.

Does shaft length affect speed or performance?

Yes. A shaft that is too long adds lower-unit drag that costs top speed and fuel economy. A shaft that is too short allows the prop to ventilate under load, causing cavitation and power loss.

What if my transom height is between standard shaft lengths?

Do not apply a universal round-down rule. Measure the transom, account for hull design and any setback, and confirm the exact motor and mounting height. Verify on the water before treating the install as final.

How do I know what shaft length my Mercury currently is?

Read the complete model suffix as a unit. ELPT is electric start, long (20-inch), power tilt. EXLPT is electric start, extra-long (25-inch), power tilt. A letter L inside a longer code is not enough by itself. A model code with no length letter is usually a 15-inch short shaft.

What is the difference between a long shaft and a short shaft outboard?

A short shaft is 15 inches (usually no length letter in the Mercury model code) and a long shaft is 20 inches (code L). The difference is how far the gearcase hangs below the mounting bracket, which sets how deep the propeller runs. Match the shaft to your measured transom height, not to the boat's overall size. Most Ontario aluminum fishing boats take the 20-inch long shaft; small tinnies with low transoms take the 15-inch.

What is the outboard shaft length chart for 15, 20, and 25 inch shafts?

Measure your transom height, then match it: 15 to 16 inches takes a short shaft (usually no length letter, 15 inch), 19 to 21 inches takes a long shaft (code L, 20 inch), 24 to 26 inches takes an extra long shaft (code XL, 25 inch), and 29 to 31 inches takes an ultra long shaft (code XXL, 30 inch). If you land between sizes, use boat- and engine-specific mounting guidance rather than a universal round-down rule.

How do I measure transom height for outboard shaft length?

For a conventional hull, with the boat level, measure vertically from the top of the transom where the motor bracket sits to the bottom of the hull at the transom centreline. For a pontoon or separate motor pod, confirm the measuring reference with the boat manufacturer and exact engine installation instructions. Confirm the shaft configuration and mounting position before ordering.

What shaft length do I need for a 25 inch transom?

A transom measuring 24 to 26 inches takes a 25-inch extra long shaft, Mercury code XL or EXL. Deep-V hulls with steep transom angles sometimes need the 25-inch even when the tape reads slightly under. If your boat sits close to the line, we lake-test borderline cases before rigging is finalized. Build a quote at mercuryrepower.ca.

Outboard Shaft Length Guide: 15, 20, 25 Inch (2026)

Common nominal outboard shaft lengths: short 15 inches, long 20 inches, extra long 25 inches. Confirm the exact motor and hull.

Last reviewed: 2026-10-16

Mercury Premier Dealer·Gores Landing, ON·Quote builder available

Hero graphic: common nominal shaft-length labels, not installation measurements. Use the exact motor and hull instructions to confirm fit.

Quick answer

Most Ontario aluminum fishing boats need a 20-inch (long) shaft. Pontoons can require a 20-inch (long) or 25-inch (extra long) shaft, depending on the exact hull and transom design. Overall boat length does not determine the correct shaft. Small tiller boats with low transoms typically take a 15-inch (short) shaft. For a conventional hull, measure vertically from the top of the transom where the motor bracket sits to the bottom of the hull at the transom centreline. For a pontoon, confirm the measuring reference and shaft configuration with the boat manufacturer and the exact engine installation instructions. Mercury's shaft code on the motor (L, XL, XXL, or no length letter for a 15-inch short shaft) tells you the rest. The full decoder and a measurement guide are below.

Ready to price it? Build your quote, shaft length is a step.

Video: outboard shaft length, get it right

Watch: where the prop should sit, what too short and too long do, how to measure your transom, and how 15, 20 and 25 inch shafts match up. 81 seconds, captions available.

Why shaft length matters more than most people think

Shaft length determines where the propeller sits in the water column relative to your hull. The anti-ventilation plate, often called the cavitation plate, is the flat plate just above the prop. Roughly level with the hull bottom is a starting point for some planing-boat installations, not a universal mounting rule. Mercury's 2.5/3.5 HP manual and 4/5/6 HP manual, page 12 in each, specify the plate 25-50 mm (1-2 inches) below the boat bottom. Follow the exact engine and hull installation instructions, including any setback. A dry mounting measurement and the plate's position in running water are different checks.

Too short, and the prop rides too close to the surface, pulling air and cavitating under load. Too long, and the lower unit drags unnecessarily through the water, costing you speed and fuel economy.

Neither is a minor problem. We see wrong-shaft installs regularly, sometimes from dealers who did not measure, sometimes from online purchases made without measuring. The fix is either returning the motor before it is rigged, or living with a motor that never performs the way it should.

Get the measurement right before you order. It takes five minutes.

Mercury shaft length options

Short shaft, 15 inch (usually no length letter)

Best for: Small aluminum tinnies with low transoms, typically 12 to 14 ft
Common Mercury models: Portable motors in the 2.5 to 9.9 HP range
Transom height to match: ~15 inches
Less common than it used to be. Most modern aluminum boats are built taller than this.

Long shaft, 20 inch (Code: L)

Applications include aluminum console fishing boats and open runabouts. Some pontoons require a 20-inch shaft; confirm the exact hull and transom design rather than choosing by overall boat length.
Choose the exact model listed with a 20-inch shaft in its specification. Read the complete model suffix and confirm it against the model’s specification sheet; an XL designation belongs to the 25-inch category.
Transom height to match: ~20 inches
This is the most common shaft length on Ontario freshwater boats. If you own a standard aluminum fishing boat on Rice Lake, Ontario, this is probably what you need, but measure first.

Extra long shaft, 25 inch (Code: XL)

Applications include deep-transom hulls, sailboat auxiliaries and some center-console boats. Some pontoons require a 25-inch shaft; confirm the exact hull and transom design rather than choosing by overall boat length.
Common Mercury models: 90 EXLPT, 115 EXLPT, 150 EXLPT, plus extra-long options on many 9.9 ProKicker, 15, 25, 200, 250, 300 HP, and Verado lines. Confirm the exact current SKU.
Transom height to match: ~25 inches
Pontoons can require a 20-inch or 25-inch shaft, depending on the exact hull and transom design. Follow the boat manufacturer and exact engine installation instructions. Measure; do not assume.

Ultra long shaft, 30 inch (Code: XXL)

Best for: Sailboat auxiliaries and deep-transom commercial applications
Less common in Ontario freshwater. Special order on most models.

Confirm the measurement before choosing a motor. Use the installation instructions for the exact engine and the boat manufacturer's guidance. Have a qualified installer confirm the transom measurement, shaft configuration and mounting position. A generic illustration is not an installation specification. Find the correct engine instructions through Mercury's official owner manual lookup.

How to measure your transom height

For a conventional hull, use these reference points with the boat level. For a pontoon or other hull with a separate motor pod, first confirm the measuring reference with the boat manufacturer and exact engine installation instructions. Do not substitute a generic motor-well measurement.

You need a tape measure:

With the boat level, find the top of the transom where the motor mounting bracket sits.
Measure straight down to the bottom of the hull at the transom (the keel line).
Use that transom height as a starting point for the nominal shaft-length chart below. Confirm the exact hull, engine and mounting instructions before ordering.

Transom Height	Shaft Length Needed	Mercury Code
15 to 16 inches	Short	No length letter (15")
19 to 21 inches	Long	L (20")
24 to 26 inches	Extra Long	XL (25")
29 to 31 inches	Ultra Long	XXL (30")

Between chart sizes, do not apply a universal round-down rule. Shaft length depends on the exact hull, transom, setback, motor, and mounting height. Measure, then confirm the boat- and engine-specific mounting guidance before you order.

For a conventional hull, our Shaft Length Picker is a starting point only. It does not account for pontoon motor pods. Pontoon owners should confirm the measuring reference and shaft configuration with the boat manufacturer and exact engine installation instructions.

Five things that change the answer

Hull design. Deep-V hulls with steep transom angles sometimes need a 25-inch shaft even when the transom height measures 22 inches.
Motor mounting position. Where the motor mounts on the transom affects the effective working shaft length.
Setback brackets. Some boats extend the motor 5 to 10 inches behind the transom with a setback bracket. Setback changes the effective shaft-length requirement.
Hull condition and waterline. A heavily loaded or unevenly trimmed boat rides differently than the same hull at factory spec.
Intended use. High-speed applications sometimes run a slightly shorter shaft than recommended for reduced drag. Not advised for typical recreational use.

How to tell if your shaft length is wrong

Shaft too short

Prop cavitates (ventilates) under load: RPMs spike, the boat will not accelerate cleanly
Choppy ride at speed, the motor lifts in waves, the prop breaks the surface
Overheating, the cooling water intake can rise above the waterline
Cavitation plate is visibly above the water at cruising speed

Shaft too long

Reduced top speed, the lower unit is dragging more water than it should
Worse fuel economy, same cause
Motor sounds "loaded" at cruise, working harder than the throttle position should require
Cavitation plate is well below the water surface at speed

If you are seeing any of these symptoms, shaft length is a strong candidate. HBW's acceptance checks follow the specific motor, hull, rigging, work order, and safe seasonal conditions.

Decoding Mercury shaft length codes

Code	Meaning
No length letter	Short shaft (15 inch)
L	Long shaft (20 inch)
XL or EXL	Extra Long shaft (25 inch)
XXL	Ultra Long shaft (30 inch)
ELPT	Electric start, Long shaft, Power Tilt
EXLPT	Electric start, eXtra Long shaft, Power Tilt
ELH	Electric start, Long shaft, tiller
ELHPT	Electric start, Long shaft, tiller, Power Tilt

Examples:

Mercury 90 EXLPT FourStroke = 25-inch shaft, electric start, power tilt
Mercury 9.9 MH = Short shaft (15 inch), manual start, tiller
Mercury 60 ELHPT = 20-inch shaft, electric start, tiller, power tilt

Common shaft length mistakes we see every season

Ordering online without measuring. Customer assumes "long" based on looking at the boat, orders a 20-inch shaft, hull actually needs a 25-inch.
Trusting the old motor shaft length. The old motor may have been wrong, or it may have been compensated with a different mounting height. Measure the boat, not the old motor.
Splitting the difference. "My transom measures 22 inches, I will just get the 20-inch and make it work." You cannot make it work.
Forgetting setback effects. Setback brackets change the effective shaft requirement.
Short-shaft sailboat installs. Sailboat transoms are typically deep, 20-inch is the minimum, and 25-inch is often correct.

How HBW gets shaft length right

Three things we do on every repower that prevent wrong-shaft mistakes:

We measure the transom during the walk-around. Every repower starts with a tape measure, not a guess. We see the hull in person before we order.

We know the hull patterns. Three generations of rigging experience means we know that a typical 16 to 18 ft Lund console takes a 20-inch, a Princecraft pontoon runs 20 or 25 depending on model year, and a Crestliner Tournament series may need 25.

We lake-test borderline cases. If the transom height is between sizes, we install and run the boat before signing off. Wrong shaft gets caught and corrected before it leaves our lot.

Ready to spec the right shaft?

Build a quote at mercuryrepower.ca. The configurator includes shaft length selection by Mercury model.

Checking a conventional hull? Our Shaft Length Picker offers a nominal starting point. It does not account for pontoon motor pods or replace the exact hull and engine installation instructions. Pontoon owners should confirm the measuring reference and shaft configuration with the boat manufacturer and installer.

Sizing a 9.9 to your transom? Our Mercury 9.9 tiller, kicker, and ProKicker guide lists shaft length for every 9.9 variant.

What we see at HBW

The single most common rigging mistake we see on used-boat purchases: wrong shaft length on a repower. Owner pulled a 15-inch short-shaft, ordered a 20-inch long-shaft expecting "more boat coverage", and the prop is now too deep to plane efficiently.

For a conventional hull, measure vertically from the top of the transom where the motor bracket sits to the bottom of the hull at the transom centreline. Pontoons can require a 20-inch or 25-inch shaft; use the boat manufacturer and exact engine installation instructions to confirm the measuring reference, shaft configuration and mounting position. Do not choose from overall boat length.

Frequently Asked Questions

What shaft length do I need for a pontoon boat?: Pontoons can require a 20-inch (long) or 25-inch (extra long) shaft, depending on the exact hull and transom design. Confirm the measuring reference, shaft configuration and mounting position with the boat manufacturer and the exact engine installation instructions. Measure; do not choose from overall boat length.
What is the most common outboard shaft length in Ontario?: 20-inch (long shaft) is the most common on Ontario freshwater boats. It fits most 14 to 19 ft aluminum fishing boats and open runabouts. But measure your specific boat before ordering.
Can I use a longer shaft than recommended?: You can, in the sense that the motor will mount. But an oversized shaft lowers your top speed, burns more fuel, and may cause the motor to load up at cruise. Use the right shaft for your transom.
Does shaft length affect speed or performance?: Yes. A shaft that is too long adds lower-unit drag that costs top speed and fuel economy. A shaft that is too short allows the prop to ventilate under load, causing cavitation and power loss.
What if my transom height is between standard shaft lengths?: Do not apply a universal round-down rule. Measure the transom, account for hull design and any setback, and confirm the exact motor and mounting height. Verify on the water before treating the install as final.
How do I know what shaft length my Mercury currently is?: Read the complete model suffix as a unit. ELPT is electric start, long (20-inch), power tilt. EXLPT is electric start, extra-long (25-inch), power tilt. A letter L inside a longer code is not enough by itself. A model code with no length letter is usually a 15-inch short shaft.
What is the difference between a long shaft and a short shaft outboard?: A short shaft is 15 inches (usually no length letter in the Mercury model code) and a long shaft is 20 inches (code L). The difference is how far the gearcase hangs below the mounting bracket, which sets how deep the propeller runs. Match the shaft to your measured transom height, not to the boat's overall size. Most Ontario aluminum fishing boats take the 20-inch long shaft; small tinnies with low transoms take the 15-inch.
What is the outboard shaft length chart for 15, 20, and 25 inch shafts?: Measure your transom height, then match it: 15 to 16 inches takes a short shaft (usually no length letter, 15 inch), 19 to 21 inches takes a long shaft (code L, 20 inch), 24 to 26 inches takes an extra long shaft (code XL, 25 inch), and 29 to 31 inches takes an ultra long shaft (code XXL, 30 inch). If you land between sizes, use boat- and engine-specific mounting guidance rather than a universal round-down rule.
How do I measure transom height for outboard shaft length?: For a conventional hull, with the boat level, measure vertically from the top of the transom where the motor bracket sits to the bottom of the hull at the transom centreline. For a pontoon or separate motor pod, confirm the measuring reference with the boat manufacturer and exact engine installation instructions. Confirm the shaft configuration and mounting position before ordering.
What shaft length do I need for a 25 inch transom?: A transom measuring 24 to 26 inches takes a 25-inch extra long shaft, Mercury code XL or EXL. Deep-V hulls with steep transom angles sometimes need the 25-inch even when the tape reads slightly under. If your boat sits close to the line, we lake-test borderline cases before rigging is finalized. Build a quote at mercuryrepower.ca.

Harris Boat Works · 5369 Harris Boat Works Rd, Gores Landing, ON · (905) 342-2153

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