Andrew Pace
Boat stringers are the wooden or fibreglass frame components that extend along the hull's length, providing essential support for the deck, hull, and other boat components. They are critical to the structure and quality of any boat, especially fibreglass boats, and undergo significant stress, so they require careful maintenance and replacement when damaged. This article will provide a comprehensive guide on how to replace wood stringers in a fibreglass boat, covering topics such as damage assessment, material selection, removal of old stringers, surface preparation, core replacement, bonding, and finishing touches. Replacing wood stringers can be challenging, but with the right tools, knowledge, and attention to detail, it can be a rewarding project that ensures the safety and longevity of your boat.
How to replace wood stringers in a fiberglass boat
| Characteristics | Values |
|---|---|
| Step 1 | Prepare the surfaces – Clean, dry and sand all surfaces. |
| Step 2 | Remove the damaged stringer – Cut the tabbing at the core/hull joint with a grinder but avoid cutting the hull laminate. Measure the thickness of the fibreglass skin. |
| Step 3 | Make the core – Cut a piece of timber to fit the size and shape of the removed stringer's core using the same type of wood. |
| Step 4 | Sand the surfaces – Abrade the bonding surfaces by sanding with 50-grit paper and brush away dust or loose material. |
| Step 5 | Bond the core – Wet out all contact surfaces of the hull and core with a resin/hardener mix. Apply a liberal amount of thickened epoxy to one side of the contact area. |
| Step 6 | Finishing – Replace the fibreglass skin. |
| Other tips | Impact damage – Look for fractures in the stringer, delamination of tabbing and core away from the impact point. Inspect the tabbing where the stringer attaches to a bulkhead or transom. Rot damage – Tap the suspected area with a hammer to detect rot. Inject epoxy for small areas of rot. Complete replacement – Mark the location of the outside surfaces of the stringer and remove the old stringer. Use reference points to locate the new stringer. Save large pieces of the core to use as patterns for the new core. Use the same species of wood as the existing core and trim to fit. Partial replacement – Cut out the damaged section of the stringer. Remove the skin and trim the exposed core ends to an 8-to-1 scarf angle. Grind the edges of the skin to 12-to-1 scarf angles. Trim a new piece of core material to fit the void and cut a matching scarf angle on each end. Equipment – New stringers made of plywood or fibreglass sheets, marine-grade plywood, marine-grade bonding agent, screws or epoxy glue. Safety – If your boat has a built-in fuel tank, be careful not to cut into it or its lines. Delicately drill a hole in the deck and measure its thickness before cutting through it. |
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What You'll Learn
Preparing the surfaces
Firstly, identify the location of the damage. This may involve cutting "access holes" to inspect the boat stringers for rot or impact damage. Impact damage will usually be visible as fractures in the wood. For rot damage, tap the suspected area with a small hammer; a "dead" sound indicates that the core is not firmly attached to the fibreglass. Small areas of rot can be treated by drying the area and injecting epoxy, but replacing the damaged area with wood is a more effective solution.
Once the damage has been located, mark the outside surfaces of the stringer. Removing the old stringer will require reference points to locate and position the new one. These reference marks should be far enough away from the repair area so they remain undisturbed during the preparation process.
Next, remove the damaged stringer and core. Use a grinder to cut the tabbing at the core/hull joint, ensuring you do not cut into the hull laminate. Save any large pieces of the core that you remove, as they can be useful patterns for fitting the new core. Measure the thickness of the fibreglass skin so that you can duplicate it with the new stringer.
Now it's time to start preparing the surfaces for bonding. Abrade the bonding surfaces by sanding with 50-grit paper and brush the area to remove any dust or loose material. All surfaces should be clean, dry, and sanded to create a "new" surface free of contamination and obstructions. This step is crucial for achieving a strong secondary bond between the new stringers and the hull.
Finally, use a grinder to clean the glass and remove any remaining old wood. This step is essential to prevent rot in the new stringers. Allow the channels and transom glass to dry for several days before proceeding with the next steps of the replacement process.
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Removing the damaged stringer
Mark the Location: Before removing the stringer, it is essential to mark the location of the outside surfaces of the stringer. This step serves as a reference point for placing the new stringer. Ensure that the reference marks are made away from the repair area to avoid accidental erasure during the preparation process.
Prepare the Work Area: Put on the necessary safety gear, such as protective eyewear and gloves, to safeguard yourself from debris and dust. Gather the required tools, including a grinder, cutting tools, and a small hammer.
Inspect for Damage: Use a mirror and flashlight to inspect the stringer for impact damage and rot. Look for fractures, delamination, and tap the suspected areas with a small hammer to detect rot. The "dead" sound produced by the hammer indicates that the core is not firmly attached to the fibreglass.
Remove the Stringer and Core: Using a grinder, carefully cut the tabbing at the core/hull joint, ensuring you do not cut into the hull laminate. Remove the stringer and core, saving any large pieces of the core for future reference or patterns.
Clean and Measure: Clean out all the original wood and grind the surfaces to ensure there is no remaining residue. Allow the ground areas to dry thoroughly. Measure the thickness of the fibreglass skin to ensure an accurate duplication during the replacement process.
By following these steps, you will effectively remove the damaged stringer, setting the stage for a successful replacement that will restore the structural integrity of your fibreglass boat. Remember to work carefully and seek professional advice if you have any doubts or concerns during the process.
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Making the core
Firstly, identify the type of wood used in the existing core. It is essential to use the same type of wood for the new core to maintain the balance of the boat's construction. Common types of wood used include marine-grade plywood, D. Fir lumber, and cypress.
Once you have the appropriate wood, cut it to size, ensuring it matches the dimensions of the removed stringer. This step requires precision to achieve an exact fit. Use power tools such as a grinder or hand tools like chisels and hand saws to cut the wood to the required shape. If you are repairing a section of plywood stringer, use an 8-to-1 scarf bevel for a 3/4" thick piece of plywood, resulting in a 6" long bevel. For hardwood or highly loaded core areas, opt for a longer scarf angle, such as a 12-to-1 scarf.
After cutting the wood to size, perform a dry fit to ensure a proper fit. Trim the wood as needed until you achieve the desired fit. This step ensures that the new core aligns perfectly with the existing structure.
Next, abrade the bonding surfaces by sanding them with 50-grit sandpaper. This step creates a rough surface that will enhance the adhesion of the epoxy during the bonding process. Make sure to brush away any dust or loose material after sanding to ensure a clean surface.
Finally, apply a resin/hardener mix or epoxy to all contact surfaces of the hull and core. You can use different hardeners with epoxies to change the cure time and achieve the desired adhesion and penetration. Apply a liberal amount of thickened epoxy or a colloidal silica mixture to one side of the contact area to ensure a strong bond.
By following these steps, you will create a solid core that forms the foundation of the stringer, providing the necessary support and stability for your fibreglass boat.
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Sanding the surfaces
Firstly, it is important to prepare the surfaces by cleaning and drying them. Remove any remaining debris or residue from the previous stringers. This step is crucial to ensure that the new stringers will adhere properly. Once the surfaces are clean, allow them to dry thoroughly. This step may take several days, so be patient before proceeding to the next step.
Next, you will need to abrade or roughen the bonding surfaces. Use 50-grit sandpaper to sand the areas where the new stringers will be bonded to the hull. This process creates a rough texture, which will help the epoxy or adhesive to form a stronger bond. Pay particular attention to hardwood or epoxy-coated stringer surfaces, as these require thorough abrasion for effective bonding. Remember to brush away any dust or loose material after sanding to ensure a clean surface.
After abrading the bonding surfaces, it's time to focus on the hull itself. Sand the inside of the hull to create a "new" surface, free of any contamination or obstructions. This step is crucial to achieving a strong secondary bond between the new stringers and the hull. Take your time with this process, and don't be afraid to sand, sand, and sand some more until you achieve the desired result.
Finally, once the new stringers are in place, don't forget to sand any jagged edges or fiberglass hairs that may be sticking out. These imperfections can be a safety hazard, especially in areas where you may need to reach or work. Use sandpaper or a grinding tool to smooth out these edges, creating a safe and comfortable working environment.
Remember, when it comes to sanding, it is always better to be thorough and take your time. A well-sanded surface will not only ensure a stronger bond but will also contribute to the overall aesthetics of the finished repair.
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Bonding the core
First, it is crucial to prepare the bonding surfaces. Sand the surfaces with 50-grit sandpaper to create a rough texture that will facilitate better adhesion. Ensure that the surfaces are clean, dry, and free of any dust or loose material. This step is essential to ensure a strong bond between the new core and the hull.
Next, select an appropriate adhesive for the job. A resin/hardener mix or epoxy is commonly used for bonding fibreglass and wood. You can use a thickened epoxy/404 High-Density or 406 Colloidal Silica mixture for a strong bond. Apply a liberal amount of the chosen adhesive to all contact surfaces of the hull and core. Ensure that the surfaces are thoroughly wetted out by the adhesive.
When applying the adhesive, pay attention to the bevelled edges of the core. The scarf bevels, which are typically cut at an 8-to-1 angle for plywood or a longer 12-to-1 angle for hardwood, provide a larger surface area for the adhesive to bond. Ensure that the bevelled surfaces are well-coated with epoxy, as this will enhance the strength of the joint.
After applying the adhesive, carefully fit the new core into place, ensuring proper alignment and a good fit. Use clamps or other suitable tools to hold the core securely in position until the adhesive has fully cured. The curing time may vary depending on the type of epoxy and hardener used, so refer to the manufacturer's instructions.
Once the epoxy has cured, inspect the bonded area for any gaps or imperfections. Use additional epoxy to fill and seal any gaps, ensuring a watertight seal. Finally, sand or grind any jagged edges or protrusions to create a smooth finish. This step is important for safety and aesthetics, especially in areas where you may need to reach or work during future maintenance.
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Frequently asked questions
Boat stringers are the wooden frame components that extend along the hull's length and provide support for the deck, hull, and other boat components. Over time, wooden stringers will rot and need to be replaced. You can check for rot by drilling a small hole through the fiberglass into the wood core and checking if the wood shavings are wet or rotten. If they are, you'll need to replace the stringer.
Replacing the stringers on a boat is not a simple job, and it's important to get it right, as stringers affect how the boat floats and moves through the water. You can replace just the damaged portion of the stringer, but it's often easier to replace the entire thing. To replace the stringer, you'll need to remove the damaged stringer and core, measure the thickness of the fiberglass skin, and create a new core using the same type of wood as the existing core. You'll then need to sand the bonding surfaces, bond the core with a resin/hardener mix, and replace the fiberglass skin.
The cost of replacing a boat stringer depends on the size of the boat and the construction of the stringer. On average, boaters can expect to pay around $1,000 to replace a stringer, but this price can increase significantly for larger boats or stringers made of composite materials.
