Inset panels 2, cutting portholes

The centre section of the inset panel, now removed from the boat now has the porthole cut in its pre-calculated position as shown below:

I often wondered how boat builders cut neat holes for portholes in cabin sides.

This is how Dave does it using the oxy burner clamped to an old file with a sharpened tang pivotting around a hole punched at the  centre. Obviously the tang to burner tip distance defines the radius of the cut circle.

He does the hole in about a minute. Is that smart or what?