In this blog post, we’ll guide you through the process of measuring your outboard motor shaft size. Let’s dive in!
What Is an Outboard Motor Shaft?
An outboard motor shaft connects the engine to the propeller and is responsible for transferring the engine’s power to the propeller blades. It’s the part of the motor that sits inside the water and propels the boat forward. Depending on your boat’s design and the type of water you're navigating (e.g., shallow or deep), you'll need a shaft of the appropriate length.
Why Does Shaft Length Matter?
The correct shaft length is crucial for several reasons:
- Proper Water Propulsion: A shaft that’s too short can cause your motor to be partially exposed to air, reducing its efficiency. A shaft that's too long may cause drag and unnecessary fuel consumption.
- Avoid Cavitation: Incorrect shaft length can cause the propeller to be too close to the surface or drag in the water, leading to cavitation (air bubbles that reduce efficiency).
- Boat Handling: The right shaft length ensures that your boat handles well and provides the best performance for different water conditions.
Measuring the outboard motor shaft size is straightforward, but accuracy is key. Follow these simple steps:
Step 1: Gather Your Tools
To measure the shaft length, you’ll need:
- A measuring tape or ruler (preferably a flexible one to work with curved surfaces).
- A straight edge (like a level or ruler) to ensure you measure perpendicular to the transom.
Step 2: Identify the Shaft Type
There are typically three types of shaft lengths for outboard motors:
Short Shaft: Generally around 15 inches (38 cm).
Long Shaft: Typically about 20 inches (51 cm).
Extra Long Shaft: Usually 25 inches (64 cm).
Ultra Long Shaft: Often 30 inches (76 cm).
Understanding these categories will help you determine the standard shaft size for your motor type before measuring.
Step 3: Position Your Boat on a Level Surface
Make sure your boat is on a flat, level surface before you begin measuring. If the boat is angled, the motor may not be sitting straight, which can affect your measurements.
Step 4: Measure from the Transom to the Anti-Cavitation Plate
The key to measuring an outboard motor shaft size is the anti-cavitation plate (or the “trim plate”). This is the metal plate that sits just above the propeller, and you’ll measure from this plate up to the transom of the boat (the back wall where the motor mounts). Here's how to do it:
- Place your measuring tape or ruler at the bottom of the anti-cavitation plate.
- Measure straight up to the top of the transom of your boat. Make sure your measuring tape is perpendicular to the boat's transom, as any angle could throw off the measurement.
- Record the measurement in inches or centimeters (depending on the unit you're using).
*See photo at the photo showing exactly what to measure*
Step 5: Check the Manufacturer’s Specifications
If you’re unsure about your measurement or need a second opinion, consult your motor’s owner’s manual. Most manufacturers list the recommended shaft length for your specific model. This can be particularly helpful if you’re buying a replacement motor or shaft.
What Should Your Shaft Length Be?
- Short Shaft (15 inches): Best for small boats, typically in shallow water.
- Long Shaft (20 inches): Suitable for most general-purpose boats.
- Extra Long Shaft (25 inches): Ideal for larger boats or those operating in deeper water or areas with fluctuating tides.
- Ultra Long Shaft (30 inches): Designed for high-performance boats and extreme water conditions.
If you measure your shaft length and it doesn’t match one of the standard sizes, consider consulting a marine professional. You may need to adjust the motor height or swap out for the correct shaft length.
Common Mistakes to Avoid When Measuring Shaft Length
- Measuring at an Angle: Ensure your measuring tape is straight and perpendicular to the transom.
- Not Using the Anti-Cavitation Plate: The plate is the correct reference point, not the bottom of the propeller or other parts of the motor.
- Rushing the Process: Double-check your measurements for accuracy to avoid mistakes when ordering replacement parts or accessories.
Conclusion
Measuring the outboard motor shaft size is a simple but essential task for boat maintenance and upgrades. Accurate measurements ensure you have the right setup for optimal performance on the water, whether you're cruising on calm lakes or navigating choppy ocean waters. By following the steps outlined above and avoiding common mistakes, you can confidently measure your shaft length and select the best outboard motor configuration for your needs.
Don’t forget to consult your motor's manual for specific guidance, or give Great Lakes Skipper a call at 262-898-1855 for expert advice on replacing or upgrading your outboard engine.
Browse Our Selection Of Outboard Engines
Great Lakes Skipper is the ultimate source for replacing or upgrading your outboard engine. We have the lowest prices, and everything we show is ini stock and ready to ship to anywhere in the world!
Take a look:
Happy boating!
