Repairing knots using a half-lap


Hull planks can contain areas of rot or knots. These have to be removed and repaired. That is what this skill involves.

First we look at what equipment the boat repairer uses for this and then in the second part the successive operations are analysed.

Tools, equipment and machines required for fitting a patch

Pencil and ruler
The circumference of the area of rot or the knot is marked out using the pencil and ruler.

The saw is used to saw out the patch that will be used to replace the area of rot or the knot.

Knife / knives
The knife and chisels are used to remove the area of rot or the knot.

The knot can be chiselled out using sharp chisels and a wooden mallet.

Wood glue
The glue is used to secure the patch or half-lap.

A smooth hull is achieved again by planing the half-lap.


In this example a knot is removed and replaced with a half-lap. The successive operations are described below, step-by-step.

  • Image 1 shows the knot that will be removed. If it only concerns a shake in the timber then it may be possible to fill this using wood glue mixed with sawdust if necessary. However, in this case the knot is too large and has to be removed. On top of the strake is the plank from which the half-lap will be cut. Note that this plank is half the thickness of the strake.

  • Marking out the knot. A polygon is drawn around the knot. In this case a diamond-shape is drawn because this is the best shape to fit around this knot. In other cases a pentagon can be used.

  • Transfer the circumference of the diamond onto the new plank. It is easier to transfer the lines onto the new plank if the lines around the knot are extended. The half-lap can now be sawn out.

  • The half-lap can be fitted after the knot has been carefully chiselled out. Chiselling out the knot is undertaken in the same way as woodcarving. The intentionally oversized half-lap is trimmed to size using the knife. Pay attention to the marks made on the plank and on the patch. This avoids making mistakes with equal-sided polygons.

  • A preserving agent and wood glue are applied before the half-lap is pushed into the aperture. The half-lap is pushed into the aperture. The patch is tapped home well using a mallet.

  • The surplus wood on the patch is planed off.

  • End result
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Tools Required

  • Chisels
  • Knife
  • Pencil
  • Plane
  • Ruler
  • Saw
  • Wood glue

Materials Required

None applicable