Reinforcing Your Aluminum Boat Hull: Strategies For Strengthening

Boat hulls are prone to damage from ice, rocks, and general wear and tear. Owners of aluminium boats often seek ways to reinforce their vessels to prevent cracks and leaks. Common methods of reinforcing aluminium boat hulls include welding, adding a second aluminium plate, using plastic, UHMW, fiberglass, or Kevlar, and sealing with Flex Seal. Some owners opt for a combination of these methods, such as welding a second aluminium plate and then coating the hull in Flex Seal. While reinforcing an aluminium boat hull can provide added protection, it is important to carefully consider the advantages and disadvantages of each method, as well as the potential impact on performance and cost.

Using Kevlar or fibreglass to cover the hull

Kevlar is a strong, flexible, and resilient material that is often used in bulletproof vests. It is also used in boat construction, though to a more limited extent. Kevlar hulls are lighter and stronger than fibreglass hulls, but Kevlar is difficult to work with, requiring specialised tools and skills. It is also expensive, with only one manufacturer (DuPont) owning the patent.

Kevlar is very strong in tension but weak in compression. Therefore, to make a hull panel that is "bulletproof" (metaphorically), it is best to put the Kevlar on the inside of the hull. This is because an impact on the outside of the hull will bend the panel, putting the inside in tension and the outside in compression. Kevlar is also very abrasion-resistant, making it a good choice for the outside of kayaks or canoes, where hull strength is governed by stiffness and even thin laminates are strong enough.

Kevlar also has the advantage of being lightweight, which can enhance a boat's speed and mobility. However, Kevlar hulls can be more brittle than fibreglass hulls, and older Kevlar hulls may become more brittle with age. Additionally, Kevlar does not adhere well to resin, requiring more time and money to achieve a good wet-out.

Fibreglass, on the other hand, is a composite material made from a plastic base reinforced by glass fibre. It is lightweight, reasonably durable, and non-corrosive. Fibreglass boats are also relatively easy to repair, although repairs may be difficult to perform "in the field". The repair process is generally straightforward if the workshop has all the necessary materials and equipment.

When deciding whether to use Kevlar or fibreglass to cover an aluminium boat hull, it is important to consider the advantages and disadvantages of each material, as well as the specific needs and applications of the boat.

Installing an aluminium keel or a second aluminium plate

Step 1: Sourcing the Aluminium

Firstly, you will need to source the aluminium for the keel or second plate. It is recommended to get this from a junkyard, as they often have a good selection of aluminium parts that can be used for this purpose.

Step 2: Preparing the Aluminium

Before installing the aluminium keel or second plate, it is important to clean and prepare the aluminium surface. This includes removing any paint, grease, or other contaminants that may be present on the aluminium. This step is crucial to ensure a strong and durable bond between the new aluminium piece and the existing hull.

Step 3: Fitting the Keel or Second Plate

Now, you will need to fit the aluminium keel or second plate to the hull. This typically involves welding the new piece to the existing hull. It is recommended to seek the help of a professional welder or boat repair specialist for this step, as welding aluminium can be tricky and requires specialised knowledge and equipment.

Step 4: Sealing the Joints

Once the keel or second plate is welded in place, it is important to seal the joints to ensure watertight integrity. This can be done using a marine-grade sealant or epoxy specifically designed for use with aluminium. Apply the sealant generously to all seams and joints between the new and existing aluminium pieces.

Step 5: Final Inspection and Testing

After the sealant has cured, it is important to inspect the work thoroughly for any signs of leaks or weak spots. Use a bright light to check for any pinhole leaks or areas where the sealant may not have adhered properly. If any issues are found, they should be addressed immediately by re-applying sealant or making any necessary adjustments to the welds.

Finally, once you are satisfied with the reinforcement work, it is recommended to test the boat in a controlled environment before taking it out on open water. This will allow you to identify any potential issues that may have been missed during the reinforcement process and ensure the boat's hull is strong and secure.

Applying spray-on bedliner or Flex Seal

Spray-on bedliners or Flex Seal can be used to reinforce an aluminium boat hull, but it's important to note that this method is not recommended by some boat enthusiasts. While it may be tempting to use a spray-on bedliner for its convenience and ease of application, it can make future repairs more difficult. If you choose to go ahead with this method, here is a step-by-step guide on how to apply it:

Preparation:

First, ensure that your aluminium boat is new or has been thoroughly cleaned and dried. Degrease the surface and sand it with 120-grit sandpaper to create a rough texture. Remove any dust with a sticky cloth. It is crucial to prepare the aluminium properly to ensure the bedliner or Flex Seal adheres correctly.

Priming:

After the surface is prepared, apply an all-etch primer to the aluminium hull. This step is essential to ensure the bedliner or Flex Seal bonds effectively with the aluminium surface. Follow the manufacturer's instructions for the best results.

Applying the Bedliner or Flex Seal:

Once the primer is dry, you can start applying your chosen product. For spray-on bedliners, use a spray gun and follow the manufacturer's instructions for the correct application technique and thickness. Ensure you are in a well-ventilated area and wear the appropriate protective gear, including a respirator. You can adjust the texture of the bedliner by controlling the air pressure and nozzle size.

For Flex Seal, follow the instructions on the product. Ensure the area is well-ventilated, and wear gloves and eye protection. Apply the Flex Seal in thin, even coats, ensuring you cover the entire surface. Allow each coat to dry before applying the next, and follow the instructions for the recommended number of coats.

Curing:

After you have finished applying the bedliner or Flex Seal, allow it to cure for several days, following the manufacturer's instructions. Do not use the boat during this period. The curing process is critical to ensuring the coating fully adheres and hardens, providing the desired reinforcement to your aluminium boat hull.

Maintenance and Repairs:

To maintain the coating, regularly clean your boat and inspect it for any signs of wear or tear. If you notice any chips or cracks, repair them promptly to prevent further damage. Depending on the product you choose, you may need to reapply the coating periodically to ensure maximum protection.

While spray-on bedliners and Flex Seal can provide a temporary solution for reinforcing an aluminium boat hull, it is important to remember that they may not be the best long-term solution. For more durable results, consider professional-grade products specifically designed for boat hull repairs and protection. Always do your research and consult with boating experts before making any decisions.

Reinforcing with UHMW plastic

Reinforcing the hull of an aluminium boat with UHMW plastic is a great way to protect the boat from damage and reduce friction when sliding over gravel bars. UHMW stands for Ultra High Molecular Weight, and this plastic is a favourite among river and jet boaters.

UHMW plastic is a good choice for shallow rivers with lots of gravel and debris, and for boaters who take their vessels into remote locations for hunting, camping, or fishing. The plastic does not strengthen the hull but is attached with flat-head (countersunk) grade 8 bolts and sealed with rubber back washers and a nylon lock nut. While bolting through the hull may seem counterintuitive, other methods such as gluing tend to peel away over time.

UHMW plastic is lightweight, self-lubricating, and highly durable. It disperses sudden impacts and reduces wear on the hull, protecting your investment. It won't create pitting on your aluminium hull like a steel plate might, and it won't rust. It is also easier to replace and repair than other hull protection methods.

When installing UHMW plastic, it is important to consider the coverage area. For boats that frequently encounter obstacles, wider coverage on both sides of the hull may be necessary. Mechanical fasteners such as nuts and bolts are recommended for a secure hold. It is also important to consider the type of sealant used. While epoxy is not recommended as it won't stick to UHMW plastic, 3M 5200 or polyurethane sealants are better options.

Overall, reinforcing an aluminium boat hull with UHMW plastic is a great option for boaters who frequently navigate shallow, rocky rivers and need the ability to slide over obstacles with minimal damage.

Adding rivets or braces

Rivets are an essential component of aluminum boats, helping to hold the metal sheets together and maintain a watertight seal. Over time, however, rivets can loosen or fail due to repeated flexing and springing back of the aluminum. This can lead to leaks, which, if left unattended, can cause further damage to the boat's structure.

To reinforce an aluminum boat hull with rivets, it is important to first identify any leaking rivets. This can be done by beaching the boat and gradually sliding it into the water, marking the leaking rivets as you go. Alternatively, you can fill the boat with water to the waterline and mark the leaking rivets from the inside. Once identified, leaking rivets can be repaired or replaced.

To repair a leaking rivet, you can "rebuck" or tighten it. This process involves holding a buck, or an anvil-like piece of steel, against the flat side of the rivet while hammering the crimped end. It is important to be cautious and avoid excessive hammering, as old aluminum rivets can split or break under heavy force. If a rivet cannot be repaired, it should be replaced. When replacing rivets, it is crucial to use solid aluminum rivets of the same size as the original. Drilling a hole just larger than the rivet's diameter in a hardwood dowel or steel bar can help achieve a tighter fit.

In addition to rivets, braces can also be added to reinforce an aluminum boat hull. Braces can be made from aluminum or other suitable materials and are typically placed at a 90-degree angle to the area of flexing. For example, if the front floor of a boat is flexing and cracking due to wave action, adding aluminum braces underneath the floor can help distribute the load and reduce flexing. These braces can be attached using rivets or welding, depending on the specific boat and the expertise available.

When adding rivets or braces to reinforce an aluminum boat hull, it is important to consider the location and angle of the reinforcements. By placing them strategically, you can effectively strengthen the hull and improve its performance in various conditions. Additionally, regular maintenance and inspection of the rivets and braces can help identify any potential issues before they become more serious.

Frequently asked questions