Application requires using what one can actually see, and it requires adjusting the sailing directions in the light of actual experience. Ocean swells are a crucial guide in sailing. Navigators recognize up to eight different swells, one from each octant of the compass. During the summer, swells come from the southeast and south. The different swells have characteristic intervals.
Navigators take advantage of opportunities to check the direction of swells against the stars.
When two swell systems are moving across each other, like the converging wakes of motorboats, they make peaks where they come together. Currents reveal themselves by the shape of the waves. A current flowing against the wind produces steeper waves, one flowing with the wind flatter waves. The direction and strength of a current may also be revealed by the pattern of ripples on the surface of the water. Currents make a significant difference for how a navigator adjusts his course in actual voyaging.