Replacing Rotted Boat Planks: A Step-By-Step Guide For Woodworkers

Owning a wooden boat is a labour of love, and it's likely that at some point you'll need to replace rotted planks. The first step is to remove the rotted wood, cutting out not just the rot but also a buffer of good wood around it. You'll then need to treat the surrounding timber to kill off any remaining spores. To join the new plank to the old, you have two options: using butt blocks, or scarfing a new plank in. The former is quicker but results in shorter planking on the hull, while the latter is more time-consuming but creates a stronger join. Once you've cut out the old plank, you'll need to sand, fit, and screw in the new one, before caulking and painting.

Identify the type of rot and treat the surrounding timber

Before you replace rotted wood on your boat, it is important to identify the type of rot and treat the surrounding timber to prevent further damage. There are two main types of rot that commonly affect wooden boats: wet rot and dry rot. Both are caused by fungi growing within the wood tissue when specific conditions are met.

Wet rot occurs when unprotected timber is in constant contact with moisture. This type of rot is favoured by a damp environment, and salty water can slow it down. To prevent wet rot, ensure that your boat is well-ventilated, regularly inspected, and that any condensation is dried, and mildew is removed.

Dry rot happens when unprotected timber has been repeatedly wetted and then dried out. While salty water can slow it down, dry rot is more likely to occur in boats that are tightly covered with canvas tarps or stored outdoors in open slips. To prevent dry rot, avoid shrink-wrapping your boat, and ensure all exposed timber is treated with an appropriate product, such as coal tar creosote, primer, and paint.

To identify rot, inspect the exterior and interior of your boat, including the flooring, transom, fixtures, and deck, for signs of decay or discolouration. Use a hard object to tap on surfaces that are susceptible to water and listen for a hollow sound, which could indicate rot below the surface. If you find a soft spot, probe it with a knife or ice pick for further inspection.

Once you've identified the type of rot, it's important to treat the surrounding timber to kill off any remaining spores. The infection can spread beyond the obviously rotten area, so use a fungicide to kill any remaining fungi in the wood. You can use ready-made products or an inexpensive remedy such as ethylene glycol (found in car antifreeze) or a borate solution like Sodium Octaborate. Remember to ventilate the area and wear a mask when working with fungicides and other chemicals to avoid breathing in bacteria.

Remove the plank you intend to replace

Removing the plank you intend to replace is the first step in replacing a plank on a wooden boat. This process can be time-consuming, depending on the condition of the fasteners, and can take anywhere from 15 minutes to a couple of hours.

Start by sanding the paint off the old plank to locate all the fasteners that secure the plank to the boat's frames. In the case of an old boat, the planks may have been refastened multiple times, resulting in multiple generations of fasteners. Once the paint is removed, it will be easier to find the wooden plugs and putty covering the fasteners.

The next step is to remove the plugs to expose the fasteners. This can be done by driving a small screwdriver into the plug and prying it out of the plank. After exposing the fasteners, use an appropriate screwdriver to back out the various screws. Some of the older screws may be corroded and challenging to remove, while others may have been stripped and cannot be unscrewed. For stubborn screws, use a wood chisel to cut away the wood around the screw until you can grab it with pliers or vice grips and twist it out.

Once all the fasteners are removed from the plank, use a saw to cut through the plank and then pry it off the boat. Some planks may be stubborn and require additional force to remove. It is important to note that removing the plank may result in splitting it into multiple pieces.

After the old plank is removed, the next step is to prepare the surface for the new plank. This may involve cutting scarf bevels into the ends of the old planks or taking other necessary steps to ensure a smooth transition between the new and old wood.

Prepare the new plank

To prepare the new plank for your boat, follow these steps:

Firstly, decide whether you will use butt blocks or scarf joints to join the new plank to the old wood. Butt blocks are pieces of wood screwed to the backs of the existing planks, with the new plank butted up against the old over the extension of the butt block. This method is simpler but results in shorter pieces of planking. Scarfing a new plank in, on the other hand, involves cutting a bevel into the end of the old plank, and a matching bevel into the new plank, before gluing the two together with epoxy. This method creates a stronger joint but takes much longer.

If you opt for butt blocks, simply screw the new plank to the butt block, ensuring the end of the new plank is butted up against the end of the old plank.

If you choose to scarf the new plank in, start by cutting a bevel into the end of the old plank on the boat, and a matching bevel into the new plank. The bevels should be at an 8:1 ratio or shallower. You can use a suitable jig and a router for this, or cut the bevel by hand using a hand plane. Once you have cut the bevels, measure the gap for length and width at 6-inch intervals, and cut off an appropriate length of planking stock. Mark the widths on the new plank and draw a line where the new plank edge should be, then rough cut the new plank to width with a sabre saw.

Now it's time to fit the new plank to the boat. Use a hand plane to plane a bit off the plank, checking the fit on the boat, and repeating this process until the new plank fits into the gap. Once the plank fits, cut the scarf bevels into the ends of the new plank using the same jig you used for the old plank to ensure matching bevels. Test fit the new plank again and make any necessary adjustments.

The next step is to drill and countersink holes for the screws that will be used to fasten the plank to the boat's frames. Mix up a batch of epoxy and paint the surfaces of the scarf bevels on both the new and old planks with it. Add thickener to the epoxy to make it into a paste, and put a layer of this thickened epoxy on the scarf bevels of the old planks. Put the new plank in place and screw it into the first frame at one end, then work your way down the plank, screwing it to the frames. To hold the scarf joint together, screw a piece of wood across the joint, putting a piece of wax paper underneath to prevent it from sticking to the plank.

Finally, clean up any epoxy that has squeezed out of the scarf joints, and remove the temporary blocking once the epoxy has set. Trim any bungs flush with the plank, then sand the entire plank so it is smooth and level with the adjoining planks.

Install the new plank

Now that you have cut out the rot and treated the surrounding timber, it is time to install the new plank.

Firstly, decide on your method of joining the new wood to the old. The traditional method is to use butt blocks, which are simply pieces of wood screwed to the backs of the existing planks between frames. Butt blocks are a bit wider than the plank and extend several inches past the end of the plank. The new plank is then butted up against the old plank and screwed to the butt block. This method is effective but results in shorter pieces of planking on the hull, which can make the hull less stiff.

The alternative method is to scarf the new plank in. This involves cutting a bevel into the end of the old plank and the new plank, and then gluing the two together with epoxy to form a strong joint. This method takes longer but results in longer planks, which can make the hull stiffer.

Once you have decided on your method, it is time to prepare the new plank. If using butt blocks, cut the new plank to the appropriate length, allowing for some overlap with the old plank. If scarfing, use a jig and a router to cut a bevel into the end of the new plank. Test the fit of the new plank and make any necessary adjustments.

Next, drill and countersink holes for the screws that will be used to fasten the plank to the frames. You may also need to cut a caulking bevel into the outside top edge of the plank.

Now you are ready to install the new plank. Mix up a batch of epoxy and paint the surfaces of the bevels or butt blocks. If using butt blocks, screw the new plank to the blocks. If scarfing, apply thickened epoxy to the bevels, fit the new plank into place, and screw it to the frames.

Finally, clean up any excess epoxy that has squeezed out of the joints. Once the epoxy has cured, remove any temporary blocking, fill any irregularities with epoxy putty, and trim and sand the plank to make it fair with the adjoining planks. The final steps are to caulk the plank seams and paint the new plank.

Finish the new plank

Once you have cut your new plank to size and ensured it fits, you will need to prepare the wood before fixing it in place. Paint the inside of the plank with a preservative, such as creosote, and apply linseed oil to the edges as a lubricant if you are using an oil-based caulking compound.

When you are ready to install the plank, mix a batch of epoxy and paint the surfaces of the scarf bevels on the new and old planks. Next, add a thickener to the epoxy to make it into a paste and apply a layer of thickened epoxy to the bevels on the old planks. Put the new plank in place and screw it into the first frame at one end. To hold the scarf joint together, screw a piece of wood across the joint. Put a piece of wax paper underneath this wood to prevent it from sticking to the plank. Work your way down the plank, screwing it to the frames. When you reach the other end, screw another piece of wood across the scarf joint to compress it.

Once the epoxy has set, remove the temporary blocking and fill any irregularities with epoxy putty. Trim any bungs flush with the plank and sand the entire plank so that it is level with the adjoining planks. Finally, caulk the plank seams above and below the new plank and then paint it.

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