Kristian Ferrell
Putting screws into a fiberglass boat requires careful preparation and execution to avoid damaging the boat. The first step is to assess the load the fastener will need to handle and select an appropriate fastener. When drilling, it is important to use a sharp drill bit and to ensure the hole is slightly bigger than the thread. If the hull is cored, it may be necessary to epoxy small blocks of wood to the hull and secure the screws to those blocks to avoid water intrusion. When screwing, it is important to tighten gently by hand to avoid snapping the screw or cracking the gel coat.
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What You'll Learn
Choose the right screw type and size
When it comes to choosing the right screw type and size for a fibreglass boat, there are several factors to consider. Firstly, assess the load that the fastener will need to handle. Different fasteners are designed for different challenges. For example, if you are attaching something to an unusually thin panel or if more strength is required, stainless steel bolts, washers, and nylock nuts, along with a treated plywood backer, are recommended. It is crucial to select the proper drill size, which should be slightly larger than the shank of the screw but smaller than the outer diameter of the threads.
Another important consideration is the type of screw. Stainless steel sheet metal screws are commonly used for fibreglass boats. These screws should be used specifically for fibreglass and should not be used to fasten hardware to extruded aluminium spars, as they are designed to be held in place by a single thread and can distort the walls of the hole. If you are screwing into solid fibreglass, ensure that the pilot hole is large enough to prevent the screw from cracking the gel coat. A 9/64" pilot bit (0.140") is recommended for a #10 fastener, and a 5/32" bit (0.156") can be used if it is too tight. To prevent gel coat chipping, start with a smaller hole, such as a 1/16" bit, and then expand it with a countersink.
Additionally, it is important to note that wood screws are not suitable for fibreglass. If you are dealing with a cored hull, it is crucial to keep it watertight. While sealants can be used temporarily, they will eventually leak. A more permanent solution is to use an epoxy plug in the hull and then insert the screw into this plug. For a non-critical application, such as attaching a bilge pump, some boat owners have suggested using epoxy to attach small blocks of wood to the hull and then securing the wires or hoses to those blocks.
When selecting the appropriate screw size, it is recommended to drill a hole slightly smaller than the major diameter of the self-tapping screw thread. For instance, a #10 screw has a major thread diameter of approximately 0.195 inches, while a #8 screw measures around 0.165 inches. Based on these dimensions, a 9/64 (0.141) drill is suitable for a #8 screw, and an 11/64 (0.171) drill for a #10 screw, providing about 0.012" bite per side.
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Prepare the surface
When preparing to put screws in a fibreglass boat, it is important to take the necessary steps to ensure a strong and secure hold. Here are some detailed instructions on how to prepare the surface:
Before you begin, assess the load that the fastener will need to handle. Choose an appropriate fastener for your specific application. For example, if you are attaching something to the boat that may experience a load, you should consider using bolts or machine screws to secure it safely.
Next, gather the appropriate tools and materials. You will need a sharp drill bit that is suitable for drilling into fibreglass. Using a dull or low-quality drill bit may result in breakage. It is also recommended to use stainless steel sheet metal screws when attaching items to a fibreglass boat. These screws are less susceptible to corrosion and are designed to hold strongly in this material.
Now, you must determine if your boat has a solid fibreglass construction or a cored hull. Drilling into a cored hull can compromise its watertight integrity, so special care must be taken. If you have a cored hull, consider using an alternative method, such as epoxying small blocks of wood to the hull and securing items to those blocks.
If you are screwing into solid fibreglass, ensure that your pilot hole is the correct size. It should be slightly smaller than the major diameter of the self-tapping screw thread. This will provide a strong hold without causing the fibreglass to crack. Always use a clearance drill to counterbore the hole and prevent the screw from cracking the gelcoat.
Additionally, you can apply a small amount of lubricant to the screw before insertion. This will help reduce friction and make it easier to tighten the screw by hand. Remember to tighten gently and avoid using a cordless drill or screwdriver, as this can snap the screw or crack the gelcoat.
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Drill pilot holes
When drilling pilot holes into a fiberglass boat, it is important to use the correct drill size. The drill bit should be slightly smaller than the diameter of the screw. For example, if using a #10 screw with a major thread diameter of approximately 0.195 inches, a 9/64 (0.141) drill bit would be appropriate. This will ensure that the screw has enough grip without causing too much interference or cracking the gelcoat.
It is also important to consider the type of screw being used. Stainless steel sheet-metal screws are commonly used for fiberglass boats. However, it is important to note that sheet-metal screws should not be used to fasten hardware to extruded aluminum spars as they are designed to be held in place by a single thread and can distort the walls of the hole. If using a wood screw, the diameter of the drill bit should match the root diameter of the screw.
When drilling into a cored hull, it is important to seal the holes with epoxy or resin to prevent water intrusion and potential rot. One method is to fill the holes with Marine Tex, a putty-like substance designed for marine environments, before drilling the pilot holes. Another option is to epoxy small blocks of wood to the hull and secure the screws to those blocks, preventing water from entering the core.
Additionally, it is recommended to start with a small pilot hole and gradually increase the size to prevent chipping or cracking of the gelcoat. Using a piece of masking tape over the drilling spot can also help prevent gelcoat chipping. Always tighten screws gently by hand to avoid snapping the screw or cracking the gelcoat.
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Seal the holes
Sealing the holes in a fibreglass boat is an important step to prevent water intrusion and avoid rot. There are several ways to seal holes, depending on the materials available and the specific requirements of the boat.
One method is to use epoxy to fill the holes. This can be done by mixing an epoxy like West System with a filler to achieve the desired consistency, and then injecting the epoxy into the hole using a hypodermic needle. It is important to ensure the epoxy completely fills the hole. To prevent the epoxy from escaping, cover the hole with tape until it cures. Once cured, a new pilot hole can be drilled down the centre of the epoxy plug, and a screw can be installed.
Another option is to use a sealant like 3M 5200 or 4200, which can be applied to the holes to prevent water intrusion and seal the wood. This method is often used for transom wood to avoid rot. It is important to note that silicone should not be used as a sealant on fibreglass boats.
For a stronger seal, some people choose to epoxy small blocks of wood or pieces of aluminium to the hull before securing wires and hoses to these blocks. This method avoids the need to drill and screw directly into the hull, reducing the risk of water intrusion.
When sealing holes, it is important to use the proper drill size. The hole should be just bigger than the shank of the screw but smaller than the outer diameter of the threads. Using a drill bit that is too small or not correctly lined up with the pilot hole can cause screws to break. It is also recommended to use stainless steel sheet-metal screws when sealing holes in fibreglass boats.
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Screw by hand
When screwing into fibreglass, it is important to use the correct type of screw and to prepare the hole correctly to avoid damaging the fibreglass.
Firstly, you should select the appropriate screw for the job. Stainless steel sheet-metal screws are the best option for fibreglass. Wood screws are not suitable for fibreglass, and should never be used.
Next, you need to prepare the hole. Begin by drilling a pilot hole. The pilot hole should be slightly smaller than the diameter of the screw. For example, if you are using a #10 screw, which has a major thread diameter of approximately 0.195 inches, you should use a drill bit of 11/64 (0.171 inches). If you are using a #8 screw, which has a major thread diameter of around 0.165 inches, you should use a 9/64 (0.141 inches) drill bit. If you are screwing into solid fibreglass, make sure the pilot hole is big enough and counterbore the hole with a clearance drill to prevent the screw from cracking the gelcoat. To prevent chipping the gelcoat, you can start with a smaller bit (1/16 inch) and then open it up with a countersink. You can also lay masking tape over the spot you plan to drill.
Once the pilot hole is drilled, you can screw by hand. Place the screw into the hole and tighten gently by hand, rocking it back and forth as it goes in. If it gets too tight, you will crack the gelcoat or snap the screw. If this happens, go one drill bit size up and repeat the process.
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Frequently asked questions
Use stainless-steel sheet-metal screws.
You have to keep it watertight. You can use an epoxy plug or a sealant like 3M 5200 or 4200. However, sealants may not prevent water from eventually reaching and destroying the core.
The drill bit should be slightly smaller than the screw. For example, if you are using #10 screws, use a 9/64 (0.141) drill bit. If you are screwing into solid fiberglass, make sure your pilot hole is big enough so that the screw doesn't crack the gel coat.
Start with a small drill bit to make a hole, then open it up with a countersink. You can also lay a piece of masking tape over the spot you plan to drill.
Blind rivets, also called pop rivets, can be used if you only have access to one side of the fastener hole. You can also use bolts or machine screws for essential components.
