Mounting An Outboard: Aluminum Boat Engine Installation Guide

Installing an outboard motor on an aluminium boat is a complex process that requires careful consideration and planning. The process involves several steps, including measuring the boat transom's height and thickness, selecting the appropriate shaft length, and ensuring the transom can support the motor. Additionally, it is crucial to correctly position and secure the outboard motor to the transom, taking into account factors such as steering and engine vibration. While some people choose to perform the installation themselves, others prefer to seek professional assistance to ensure a safe and proper installation.

Selecting the right shaft length

The shaft length of your outboard motor is critical to ensuring optimal performance, speed, and energy efficiency. The height of your boat's transom will determine the shaft length required. Shaft length is important as it determines how high or low your outboard motor sits in the water.

If your outboard motor is positioned too high, cavitation may occur, leading to speed loss, energy wastage, and propeller damage. On the other hand, if the motor sits too low, water resistance will increase, reducing speed and energy efficiency. Therefore, finding the right shaft length is crucial.

To determine the correct shaft length, refer to a shaft length chart. This chart will provide you with the suitable shaft length range based on the height of your outboard transom. Additionally, consider using an outboard motor bracket if you need to further lower your outboard motor to reach the water.

It is also important to test the outboard motor at different heights to find the optimal mounting height. This height will be influenced by the specific characteristics of your boat. Consulting a marine engine dealer is recommended if you need further assistance in determining the correct shaft length and mounting height.

Positioning the outboard engine

Firstly, gauge the boat by measuring the height and thickness of the boat transom. The mounting height of the outboard engine is critical to its performance. If it is too high, cavitation may occur, leading to speed loss and propeller damage. On the other hand, if it is too low, water resistance will increase, reducing speed and energy efficiency. The optimal height should be determined through testing at different heights or by consulting a marine engine dealer.

Secondly, choose the right shaft length for your outboard motor. Different transom heights require different shaft lengths. Refer to a shaft length chart to make the appropriate selection. If the outboard motor still cannot reach the water, consider installing an outboard motor bracket.

Thirdly, ensure that the thickness of the transom is suitable for mounting the outboard engine. It should typically fall within the range of 30 to 70mm. If you are re-powering an older boat, carefully examine the transom for any deterioration, especially if it is made of laminated plywood. Consult a dealer to assess the structural integrity of the transom before proceeding.

Now, you are ready to position the outboard engine. Centre the outboard engine on the transom by aligning it with the centre line (keel line) of your boat. This step is crucial for maintaining the boat's balance and ease of steering. Tighten the transom clamp bolts securely and evenly. Use stainless steel bolts to prevent rust, and remember to check them occasionally for tightness as engine vibrations can loosen them over time.

Finally, fix the outboard engine to the boat using screws. This step ensures that the engine is firmly secured and reduces the risk of it falling into the water or sustaining damage due to loose clamp screws. Check the screws and clamps before each use, as mechanical vibrations may cause them to loosen over time. For added security, consider using a cable to avoid complete loss of the outboard motor in the event of it falling off the transom.

Tightening the transom clamp bolts

Firstly, place the outboard motor on the transom, ensuring it is positioned as close to the centre as possible. This is important for the balance of the boat and ease of steering. The outboard should be placed on the centre line (keel line) of the boat. If the boat has an asymmetric shape, consult a professional for advice.

Next, you can begin to tighten the clamp screws. It is important to do this evenly and securely. Stainless steel bolts are recommended as these will not rust. Ensure that the clamp screws are tightened securely to prevent the outboard motor from falling off or moving on the transom, which could cause a loss of control and serious injury.

Once the outboard is in place and the clamp screws are tightened, it is important to occasionally check them for tightness during the operation of the outboard motor. Engine vibration may cause the clamp bolts to loosen over time, so regular checks and maintenance are essential for safety.

Converting from inboard to outboard

Converting an inboard to an outboard engine involves several meticulous steps and considerations to ensure the structural integrity of the boat. It is a complex process that requires a good investment and can impact the boat's performance if not done correctly. Here is a detailed guide on converting from inboard to outboard:

Step 1: Consult an Expert

Firstly, consult an experienced boat dealer or marine professional to understand the process and its outcome comprehensively. They can guide you through the specific steps required for your boat and help you choose the right outboard motor.

Step 2: Examine the Transom

The conversion process starts with a thorough examination of the transom, which is the vertical section at the rear of the boat. The transom will need modifications to accommodate the outboard engine.

Step 3: Create Space for the Outboard Engine

You will need to create space for the outboard engine by glassing an I/O plug in the transom. This involves filling in the existing hole in the transom and mounting a hydraulic steering system. The old jet pump or inboard pump steering will not work with an outboard engine, so it needs to be replaced.

Step 4: Adjust the Bunk Brackets

The service team will also need to adjust the bunk brackets of the boat trailer to ensure proper alignment with the new engine setup.

Step 5: Choose the Right Outboard Motor

Once the modifications are complete, you can explore different outboard motor options available in the market. Consult with the dealership to find the most suitable model for your boat.

Step 6: Mount the Outboard Motor

Finally, marine professionals will help you mount the chosen outboard motor onto the transom. Ensure that the outboard engine is mounted on the centre line (keel line) of your boat for better balance and easier steering.

Additional Considerations:

• Weight Redistribution: Converting from inboard to outboard will affect the weight balance of the boat. You may need to add weight to the bow to counterbalance and eliminate handling issues.
• Performance Impact: The installation of the outboard at the transom adds weight to the stern, which can impact the boat's handling and performance.
• Maintenance and Efficiency: Outboard engines generally require simpler, faster, and less costly maintenance than inboard engines. They are also more fuel-efficient, resulting in lower operating costs over time.
• Cost of Repowering: The cost of repowering from inboard to outboard can vary depending on several factors, but it typically ranges from $25,000 to $40,000.

Using a jack plate

When installing a jack plate, it's important to select the correct setback. There is no set rule for this, and the optimal position will depend on your boat's specific characteristics. Performance can be affected by the flotation at the back of the boat, as more setback means moving the centre of gravity farther back. It's also important to consider the location of trim tabs, as they may affect the need for greater setback to get the propeller into cleaner water.

There are two main types of jack plates: manual and electric. Manual jack plates are simple and effective, requiring no batteries or wiring. They are also usually more lightweight and affordable. On the other hand, electric jack plates offer added convenience with motor height adjustments at the push of a button.

When installing a jack plate, it is crucial to follow the manufacturer's instructions carefully. The process typically involves measuring your boat's transom, cutting and assembling the jack plate components, and then attaching it to the transom and the outboard motor. It is important to use high-quality materials and tools, such as marine-grade aluminium and stainless-steel hardware, to ensure durability and corrosion resistance.

Overall, a jack plate can be a valuable addition to your aluminium boat, providing enhanced performance, improved fuel efficiency, and greater versatility in navigating shallow waters.

Frequently asked questions