When it comes to choosing the right outboard motor for your boat, there are many factors to consider, such as horsepower, weight, and price, among others. Another important feature to consider is the shaft length, which determines how your motor will fit on your boat and ultimately perform. In this blog post, we will discuss the differences between 20 and 25-inch shaft outboards, including their advantages, disadvantages, and ideal applications. We will also provide a side-by-side comparison table to help you make an informed decision on the best shaft length for your needs. So, without further ado, let’s dive in.
Shaft Length Basics
The shaft length of an outboard is measured from the top of the mounting bracket to the anti-ventilation plate, which is located above the propeller. Choosing the correct shaft length is essential for the proper functioning of your outboard motor, as it ensures that the propeller is submerged at the correct depth. This in turn affects the efficiency, performance, and stability of the boat.
20-inch Shaft (Short Shaft) Outboards
The 20-inch shaft, also known as a short shaft, is a common length for outboard motors. This type of motor is often found on small to medium-sized boats with transoms that generally range from 15 to 17 inches. Short shaft outboards are also popular choices for auxiliary engines on sailboats and kicker motors for fishing boats.
25-inch Shaft (Long Shaft) Outboards
On the other hand, the 25-inch shaft, also known as a long shaft, is more suitable for boats with higher transoms and deeper hulls. These boats often include larger fishing boats, deck boats, and pontoon boats with transom heights that typically range from 20 to 23 inches. Long shaft outboards can handle rougher waters better and are often seen as more versatile, particularly in saltwater conditions.
Comparison of 20-Inch and 25-Inch Shaft Outboards
To help you better understand the advantages and disadvantages of 20 and 25-inch shaft outboards, we will now provide a comparison table that covers various aspects, such as performance, applications, and installation considerations.
| Aspect | 20-Inch Shaft (Short Shaft) | 25-Inch Shaft (Long Shaft) |
|---|---|---|
| Performance | Generally better on smaller, lighter boats. In optimal conditions, they can offer a smooth, efficient ride. | Offers improved performance on larger boats or in rougher conditions due to the increased depth of the propeller, resulting in better bite and less cavitation. |
| Applications | Ideal for small to medium-sized boats, auxiliary engines on sailboats, and kicker motors for fishing boats. | Better suited to larger fishing boats, deck boats, and pontoon boats with higher transoms and deeper hulls. |
| Installation | Easier to install, lighter, and more compact, making them a good choice for those looking for a portable motor option. | Installation may require additional support and reinforcement as these motors are heavier and may put more strain on the transom. |
| Price | Generally less expensive due to factors such as smaller size and lower weight. | Typically more expensive due to factors such as larger size, increased weight, and additional features that cater to specific boat types. |
How to Choose the Right Shaft Length
Now that we have provided an overview of the differences between 20 and 25-inch shaft outboards, it’s essential to discuss how to determine the right shaft length for your boat. First and foremost, you must measure your boat’s transom height accurately. Follow these steps to measure your transom:
- Place your boat on a level surface, making sure the keel is resting evenly on the ground.
- Measure vertically from the point where the keel meets the transom, up to the top of the transom. This measurement will indicate your transom height.
Once you know your transom height, refer to the following general guidelines to choose the appropriate shaft length for your boat:
- If your transom height is between 15 and 17 inches, a 20-inch shaft may be suitable.
- If your transom height is between 20 and 23 inches, a 25-inch shaft is likely a better fit.
However, it’s important to note that these are general guidelines, and specific boat and motor combinations or unique applications may require a different shaft length. It’s always best to consult with a marine mechanic or a boat manufacturer for tailored advice on choosing the right shaft length for your situation.
Wrapping Up
Selecting the right shaft length for your outboard motor is crucial for achieving optimal performance and stability on the water. Both the 20 and 25-inch shaft options have their advantages and drawbacks, depending on your specific boat type, transom height, and intended use.
Ultimately, taking the time to understand the differences between these two shaft lengths, measuring your transom height accurately, and seeking advice from a marine professional will ensure that you choose the right outboard motor for your boat and enjoy a safe, comfortable, and efficient boating experience.
Frequently Asked Questions
What shaft length should I use outboard?
When choosing a shaft length for your outboard motor, you should measure the distance from the top of the transom to the bottom of the hull or keel. Then select a shaft length that will position the engine’s anti-ventilation plate at or slightly below the waterline when the outboard is trimmed for running. For most boats, this will correspond to either a 20-inch or a 25-inch shaft length.
Can I put a 25-inch shaft outboard on a 20-inch transom?
It is not recommended to put a 25-inch shaft outboard on a 20-inch transom, as this will position the outboard motor too low in the water. This can lead to a reduction in performance, increased drag, and potential water ingestion in the engine. It is best to choose an outboard with the correct shaft length for your boat’s transom height.
Does shaft length matter on outboard motor?
Yes, shaft length is an essential factor for an outboard motor, as it determines the correct positioning of the motor in the water. The right shaft length ensures optimal performance, minimizes potential issues (such as ventilation and propeller cavitation), and prevents the engine from being submerged too deeply or too shallowly. A mismatch between shaft length and transom height can undermine the performance of your boat and damage your outboard motor.
What is the most common outboard shaft length?
The most common outboard shaft lengths are 20 inches and 25 inches, although 15-inch shafts can also be found for smaller boats. When choosing a shaft length, it is important to consider the specific transom height and requirements of your boat.
How do I measure my boat’s transom height?
To determine the ideal shaft length, measure the distance between the top of the transom (the vertical surface where the motor is mounted) and the bottom of the hull or keel. This measurement should be taken at the center of the transom for the most accurate results. Consult your boat’s owner’s manual for specific measurements and recommendations.
Can I use an extension kit to increase my outboard’s shaft length?
Yes, extension kits are available for certain outboard motor models, allowing you to extend the shaft length by 5 inches. However, using an extension kit can affect the overall performance and handling of your boat, so it is essential to consult your owner’s manual or contact the manufacturer to ensure compatibility and determine potential adjustments that may be necessary.
What happens if I use the wrong shaft length for my boat?
Using the wrong shaft length can lead to several issues, including inefficient and slow performance, decreased fuel economy, difficulty steering, and potential damage to the engine and hull. In extreme cases, the incorrect shaft length may lead to your outboard motor getting submerged in water, posing a safety risk.
What are the potential consequences of running an outboard motor too high or too low in the water?
When an outboard motor is too high in the water, it may encounter ventilation or cavitation issues, leading to erratic performance, difficulty steering, and eventual propeller damage. On the other hand, when the motor is too low in the water, it will experience increased drag and may ingest water, causing harm to the engine and negatively affecting performance.
Are certain boat types better suited for a 20-inch shaft, while others benefit more from a 25-inch shaft?
Yes, certain boat types are better suited for either a 20-inch or a 25-inch shaft length. Generally, smaller boats (such as small fishing boats, inflatables, and tenders) are best suited for shorter shaft lengths, while larger boats (like offshore fishing boats, pleasure cruisers, and sailboats) require longer shafts. However, it is essential to consider your specific boat model and owner’s manual when selecting the appropriate shaft length.
What factors should I consider when choosing between a 20-inch and a 25-inch shaft length?
When choosing between a 20-inch and a 25-inch shaft length, consider the following factors: boat type, transom height, boat usage, and performance requirements. Review your owner’s manual and make measurements to ensure the ideal shaft length is selected for your specific needs.
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