The boat we were refitting recently had to have some port lights with tight rounded corners cut into the newly replaced marine ply cabin. I know some people have trouble doing this job – so here is how I achieved it.

Having traced the outline of the pattern we’d cut for the actual opening size, I selected the circular cutters, which would cut the correct radius of hole required to make the corner. So how do you position the guide-drill so that the edge of the cut hole coincides exactly with the traced curve of the opening? Anyone who has tried doing this merely by eye will know that nine times out of 10 you’ll get it wrong.

The answer is to make a paper or card pattern of the hole saw’s outside edge and mark its exact centre. The corners of the mounting hole of this cutter can be joined with straight lines to mark the centre of the circle. You could also fold the pattern in half at right angles – where the folds meet is the centre of the circle. You now have a circular pattern of the hole size the tool will cut, with its centre accurately marked.

The spigot for my hole saw passes through a flat-sided hole in the cutter’s base – drawing a line through opposite corners lets you mark the centre

Place your pattern on the spot where its diameter matches the outline of the corner to be cut, and carefully mark the position of the centre hole

When you are happy with the position of the mark, drill a small pilot hole for the cutter’s guide drill.
I find a manual hand drill is best as it offers more control.

Ease the cutter onto the surface to ensure alignment. Accurate work with the jigsaw will mean the straight edges of the port light run seamlessly into the round corners.