One of the most important aspects to consider when choosing an engine is the length of its shaft. This can directly impact boat performance and handling in different conditions.
With so many outboards available and different boat sizes, it can be difficult to determine the ideal shaft length. That’s why this guide is here to help. It will provide an overview of outboard shaft lengths, explain their significance, and offer a step-by-step process for measuring your boat’s transom height and choosing the right one.
Outboard Shaft Lengths
Outboard motors come in three primary shaft lengths to accommodate different boat transom heights. Let’s take a look at each type:
Short shaft (15 inches): Ideal for boats with a transom height of up to 15 inches, short shaft outboard motors are commonly used on smaller boats, inflatables, and tenders.
Long shaft (20 inches): Boats with a transom height between 16 to 20 inches will need a long shaft outboard. These are often found on larger fishing boats, pontoon boats, and sailboats.
Extra-long shaft (25 inches): Designed for boats with a transom height of 21 inches will need an extra long shaft outboard. These are normally found on larger boats, offshore fishing boats, and cabin cruisers.
Why Measure an Outboard Shaft and Transom?
Optimal propulsion: Selecting the right shaft length ensures that the outboard’s propeller operates at the ideal depth, providing efficient propulsion and minimizing the risk of cavitation (air bubbles forming around the prop).
Safety and handling: A correctly matched shaft length contributes to better boat handling, stability, and overall safety on the water. An incorrect shaft length can lead to poor steering, reduced control, and potential damage to the boat and motor.
Preventing costly mistakes: Accurately measuring your boat’s transom height and selecting the corresponding shaft length can save you time, money, and frustration by avoiding the need for adjustments, repairs, or even a complete motor replacement.
Tools You’ll Need
Tape measure: Choose a measuring tape long enough to cover the distance from the waterline to the top of the transom.
Spirit level: Using a spirit level will help ensure the boat is level, providing consistent and exact measurements.
Helper: Enlist the assistance of a friend or family member to hold the tape measure while you take the readings. Keeping the tape straight and taut during the process is important for accurate measurements.
Measuring the Transom Height
Prepare the tape measure: With your helper, measure from the top of the transom and extend it to the waterline. If the boat is not in the water, estimate the waterline by measuring the bow and stern height and then average the two measurements.
Take the measurement: Make sure the tape is straight and taut, and measure the distance in the middle of the transom.
Repeat for accuracy: Repeat the measurement process several times and calculate the average of your readings.
Selecting the Right Outboard Motor Shaft Length for your Boat
Using the measurements: Match your measured height to the closest shaft length available (15 inches, 20 inches, or 25 inches). For example, if your transom height is 17 inches, you would choose a long shaft (20 inches).
Consider other factors: Don’t forget to consider other factors such as horsepower, weight, fuel efficiency, boat type, and intended usage. These will help you narrow your choices.
Consult a dealer or professional marine engineer: If you’re unsure or need guidance, it’s always a good idea to consult a boat dealer or marine mechanic.
Jack Plates and Their Role in Adjusting Outboard Engine Height
Jack plates are versatile devices mounted between the boat’s transom and the outboard motor. These plates allow for vertical adjustment of the motor’s height, enabling you to fine-tune your motor’s position for optimal performance.
A properly adjusted jack plate can improve your boat’s handling, reduce cavitation, and increase fuel efficiency. It is particularly useful for shallow water navigation, where the ability to lift the motor can help prevent damage to the propeller and the motor’s lower unit.
When selecting a jack plate, consult the manufacturer’s guidelines for compatibility with your specific boat and motor. Remember that adding a jack plate might affect the required shaft length, so it’s essential to consider this when selecting.
Manufacturer Charts for Selecting the Right Shaft Length
Many motor manufacturers provide charts to help boaters select the appropriate shaft length for their boat size and type. These charts consider factors such as boat size, hull design, and transom height, offering guidance on the optimal type to ensure peak performance and handling.
Consider that the suggested shaft length may vary between different brands and models when using a manufacturer’s chart. It’s crucial to double-check the recommendations for your particular outboard to ensure accurate results. In cases where you are uncertain or need further assistance, consult a professional for personalized guidance.
Auxiliary Motors, Propellers, and Anti-Ventilation/Cavitation Plates
When selecting the right shaft length, consider your primary outboard and any auxiliary or kicker motors you may have. Auxiliary engines, often used for trolling or as a backup in case of failure, require the same attention as the main one.
Propellers play a role in your boat’s performance, and choosing the right shaft length helps maintain the correct propeller depth in the water. Too shallow, and you risk ventilation or cavitation, which can reduce performance and efficiency and even cause damage to your motor.
Anti-ventilation and cavitation plates, located just above the propeller, help prevent air from being drawn into the propeller’s flow. Correct selection is vital in maintaining the correct positioning of these plates.
Selecting the correct shaft length ensures optimal water performance, safety, and efficiency. By accurately measuring your boat’s transom height and considering factors such as horsepower, weight, fuel efficiency, and intended usage, you can decide on the right type for your specific needs. Don’t hesitate to consult a professional if you’re unsure.
FAQs About Outboard Shaft Length
Q: What are the outboard shaft lengths?
A: The three outboard shaft lengths are 15 inches (short shaft), 20 inches (long shaft), and 25 inches (extra-long shaft).
Q: Why is it important to select the correct outboard shaft length?
A: The right length is crucial for optimal propulsion, safety, and handling. The wrong size can lead to cavitation, poor steering, reduced control, and potential damage to your boat and motor.
Q: How do I measure my boat’s transom height?
A: Measure the distance from the top of the transom to the waterline (or estimated waterline if the boat is not in the water). Use a tape measure and a spirit level to get accurate results.