You cannot fit a square or octagonal mast in a round hole, but.....

The Jannie en Klaas makes her way to the quay at the end of a trip...
crabber to be ABS with the Intuition alongside
just what are we looking at here?......how to turn a square plank into a round mast, cut your octagon....

using an off-set circular saw....
and chamfer off the corners - easy!
always use a bar to roll the part finished mast....
and simple tools for the job.