Choosing the right outboard motor shaft length is crucial for safe and efficient boat operation. The two most common options are short shaft and long shaft outboards. Selecting the incorrect length can lead to propeller cavitation, poor performance, or even damage to your motor. This comprehensive guide will help you understand the key differences between short and long shafts, enabling you to make an informed decision.
What is the difference between a short shaft and a long shaft outboard?
The primary difference, as the names suggest, lies in the length of the shaft connecting the motor to the propeller. Short shaft outboards typically have a shaft length of approximately 15 inches (measured from the cavitation plate to the top of the anti-ventilation plate), while long shaft outboards boast a shaft length of around 20 inches. This seemingly small difference significantly impacts the application and suitability for different boat types.
How do I know if I need a short shaft or a long shaft outboard?
Determining the correct shaft length depends largely on the transom height of your boat. The transom is the back of the boat where the motor is mounted. Incorrect shaft length selection often leads to operational problems.
Measuring Your Transom Height:
Precise measurement is critical. Use a tape measure to determine the distance from the bottom of the boat's transom to the top of the transom, ensuring the boat is level. You'll then need to compare this measurement to the manufacturer's specifications for your outboard motor.
Common Transom Heights and Shaft Lengths:
- 15-inch transom: Typically requires a short shaft outboard.
- 20-inch transom: Typically requires a long shaft outboard.
Important Note: Always consult your boat's manufacturer's specifications to confirm the recommended transom height and corresponding outboard shaft length. Using an incorrectly sized shaft can severely impact performance and potentially damage your engine.
What happens if I use the wrong shaft length?
Using the wrong shaft length can have several negative consequences:
- Short shaft on a boat with a long transom: The propeller will not be submerged deep enough, leading to ventilation (air being drawn into the propeller) and a significant loss of power and efficiency. The motor may also be subjected to excessive stress.
- Long shaft on a boat with a short transom: The propeller will be submerged too deep, potentially causing the propeller to strike the bottom, leading to damage and hindering maneuverability. In extreme cases, the lower unit could be damaged.
What are the typical applications of short and long shaft outboards?
- Short shaft outboards: These are commonly found on smaller boats such as inflatables, jon boats, and some aluminum fishing boats with lower transoms.
- Long shaft outboards: These are better suited for larger boats, including fiberglass fishing boats, pontoon boats, and some smaller runabouts, that have higher transoms.
How can I tell if my current outboard is a short or long shaft?
The shaft length is usually clearly marked on the outboard motor itself, often on the lower unit near the propeller. Alternatively, you can find this information in your owner's manual.
What if I'm unsure about which shaft length I need?
If you're unsure which shaft length is appropriate for your boat, consult your boat's manufacturer's specifications or a qualified marine mechanic. They can accurately measure your transom height and advise on the correct shaft length. It is far better to be sure than to risk damaging your motor or compromising your boat's performance.
This guide offers general information and is not a substitute for professional advice. Always consult your boat and motor manuals or a qualified marine professional before making any decisions about your outboard motor.
