I think that I have erred in my explanation: apologies. For a 50m ship, one would take the square root of 15 (3.87) and the square root of 50 (7.07), and divide the latter by the former to get 1.83, and multiply the length ruler on the *y* axis by 1.83. One would then duplicate the length ruler and re-scale it as a width ruler. Suppose, for example, that this 50m long ship was 4m wide: one would take the re-scaled ruler, rotate it by 90 degrees, and scale it by 0.08 (4 / 50) on the *x* axis.

One would then use freeform scaling to get the ship's bow and stern just touching the red vertical bars on the length ruler, and use *x* axis constrained scaling to get the ship's sides touching the vertical bars on the width ruler.