 knots instead of miles.

By marking off the number of knots sailed in the proper direction the
sailor might easily tell where he was, provided there were no currents
or tides and the vessel moved at a uniform speed and made no leeway. As
a matter of fact the ocean is full of currents, streams and tides and
moreover a vessel, when sailing, or steaming for that matter, is carried
to one side or the other and forwards and backwards by these as well as
by the wind.

Besides a sailing vessel moves more slowly or faster according to the
strength of the wind and is often obliged to tack or to alter its course
to suit the winds.

All of these matters must be considered in working out a position by
dead reckoning and the course of a sailing vessel when thus _pricked
out_ on a chart often looks like the track made by a drunken man, as it
zigzags hither and thither, swings about, and varies widely from one
side or the other of its true course.

In order to come anywhere near accuracy by means of dead reckoning a
mariner must be thoroughly familiar with the tides and currents through
which he is sailing. He must be able to judge the strength of the wind;
he must know just what his vessel will do under varying conditions; he
must be able to guess very closely the leeway she makes, and he must
bear in mind all the changes of course, all the tacks and all the
shifting of sails which have been made. Only by such knowledge and by
long practice can a sailor determine where he is by dead reckoning and
even then he can only locate his position approximately. It seems
remarkable that any man can come anywhere near the truth by such means
but many sea captains have become so expert that they can figure out
their position by dead reckoning and come within a very few miles of the
right result every time.

Very few amateur sailors will ever need to go into the details of dead
reckoning, but it is often convenient to be able to determine roughly
where you are and you should strive to become so accustomed to your
boat’s speed under vari