What does the master of the ship going to Guinea teach Crusoe?

From the text:

This was the only voyage which I may say was successful in all my adventures, and which I owe to the integrity and honesty of my friend the captain; under whom also I got a competent knowledge of the mathematics and the rules of navigation, learned how to keep an account of the ship’s course, to take an observation, and, in short, to understand some things that were needful to be understood by a sailor.