Ocean swells are a crucial guide in sailing. Navigators recognize up to eight different swells, one from each octant of the compass. Dominant and most reliable are those from the north, northeast, and east, associated with the tradewind season, winter in the Northern Hemisphere. 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. The navigator can steer by the alignment of these peaks or "wave nodes," as they are called.
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.
When setting out, the navigator lines his vessel up with landmarks on the island of departure according to whatever alignment is indicative of his particular star course. He then trims his sail to hold that course and keeps track of the angle of progress in relation to swells or "wave nodes."
When he reaches the point of "one tooth," where the island of departure is visible as only a single point on the horizon, he sights back to see how the island lies in relation to the course he is maintaining. If it is in direct alignment, no compensation for current is necessary, if it is out of alignment, he uses the degree of shift to estimate the strength of the current.
There are working rules for how to adjust one's course. For example, a navigator may adjust his course by half a compass position for each full compass position the island has shifted out of alignment.
Application of navigational lore also requires fine tuning the systems of knowledge and adapting them to local conditions.
The course directions given in "Island Looking" and other training exercises are not in fact the ones a navigator actually follows, unless he has never made the particular voyage before. They indicate where one island lies in relation to another but do not show how best to travel between them.
In practice, a navigator may begin with one star course and change to another course at some "drag" point along the way. Adjustments must also be made for currents and changing conditions as they are met. Sailing against the wind is likely to require planning a series of tacks from drag to drag.
The navigator must learn how to make all these adjustments of course for the voyages he actually expects to make. Years of sailing experience are necessary to develop skill as a navigator.
