Meaning of navigable semicircle

nav'igable sem'icircle

Pronunciation: [key]
— Naut. Naut.
  1. the less violent half of a cyclone; the half blowing in the direction opposite to that in which the cyclone is moving and in which a vessel can run before the wind.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.