Navigation pre-accurate chronometer was pretty hit and miss. Lattitude was easy enough using the sun and the Polar Star. Longitude was always the problem and without being able to tell the time accurately, navigators could only guess how far East or West they were.
The actual speed calculation, in the sense of speed through the water, was made by dropping a weight attached to a string over the side. The string had knots ever fathom (6 feet) and by counting the knots as the line ran out, timed with a sandglass it was trivial to calculate speed.
Navigators knew that if they set off from Egypt and headed North, they would bump into Italy sooner or later. Later on, they knew that if they set off from Portugal and headed East they would bump into S America, probably Brazil, eventually.