Sentence:  ... I had my Man "Friday" to teach as to what belong'd to the Navigation of my Boat; for though he knew very well how to paddle a Canoe, he knew nothing what belong'd to a Sail and a Rudder, and was the moſt amaz'd when he ſaw me work the Boat too and again in the Sea by the Rudder, and how the Sail <WSD>gyb'd</WSD>, and fill'd this Way or that Way, as the Courſe we ſail'd chang'd: ... 
Meaning of the word marked with <WSD> tags:  Of a fore-and-aft sail or its boom: to shift, often forcefully and suddenly, from one side of a sailing vessel to the other.