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Course Pages
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US POWERBOATING Navigation
No aspect of the sailor's world is more mysterious
to the landsman than the practice of navigation. To find a precise
point in a trackless waste seems neither art nor science, but magic.
Yet in no other sphere of progress has the continuity of development been
so clearly based on the heritage of the past, nor has the accumulated
knowledge been so universally shared by men of all races, creeds and
nations. --
Carleton Mitchell
Longitude &
Latitude
Aids To Navigation
Planning a Landfall
Current
Speed-Time-Distance
A good knowledge of
navigation is a must for any cruising sailor. One must know
where one is and how to get to one's destination along with the expected
travel time it will take.
Knowing where one is can be determined by plotting a fix based upon two or
more bearings (three or more are preferable) and by the use of
navigational aids. Once you have located your position on the
chart you can now draw a straight line between your current position and your
final destination. This line can be used as your track, or as your rhumb
line if wind conditions do not permit you to sail directly toward you
destination.
On-line carts can be viewed and printed from the NOAA Office of Coast
Survey On-Line Viewer.
http://ocsdata.ncd.noaa.gov/OnLineViewer/
 Once
your position is known the location can be determined in degrees of
latitude and longitude. Lines of latitude are circular
bands running east and west parallel to the equator.
Lines of latitude are designated from zero to 90 degrees north and south
of the Equator. Lines of longitude are circular bands which run north
and south through the two poles. Lines of longitude are
designated from zero to 180 degrees east and west of the Greenwich
Observatory in London, England.
Degrees of latitude and longitude are divided into 60 minutes.
Depending upon the chart, each minute of latitude is either divided into
tenths or sixths (each division equal to 10 seconds). As measured on
the Earth, lines of
longitude converge on each other the further north one travels, as the
distance between the lines of latitude stays the same. Thus, a
minute of latitude (measured on the vertical scale of the chart) is used
as the definition of one nautical mile. It should be noted that a
flat chart will distort distances found on a round globe. Thus,
always determine the distance of a nautical mile on the latitude scale
which is closest to where you are charting.
 To
measure the distance of your planned track, place your dividers on the
vertical scale of the chart and measure the distance of one nautical mile
by measuring the distance of one minute.
Now walk you dividers down the track and measure its distance.
View Video on How to Measure Distance
 To
determine the direction of travel use the parallel ruler and the chart's
compass rose. Walk the ruler for the track to the compass rose and
read the direction in degrees True North or degrees Magnetic North.
View Video on How to Use a Parallel Ruler and
Compass Rose to Determine Direction
 In the
Northern Hemisphere,
the north star can be used to roughly determine latitude. Using a
sextant, the degrees the star is above the horizon will roughly equally
the ship's position in latitude. The north star can be found at the
tail of the Little Dipper or by following the line produced by the two
stars on the cup of the Big Dipper.
The sun can also be used to
determine direction. In the northern hemisphere at 12:00 noon
shadows will fall to the north, in the southern hemisphere the shadows
will fall to the south..
More about early navigation methods ( Mariner's Museum ).
http://www.mariner.org//educationalad/ageofex/navmethods.php
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Planning a
Landfall
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 If
you plan
a dawn landfall you can use both vision, sound to identify the aids to
navigation along with seeing the navigational
lights. The picture to the right shows a green lighted daymark. At
twilight you can see both the daymark's square green shape and green light.
You should also plan a landfall upwind to
your destination. This way you can sail downwind and find your
port. If you aim directly for the port and miss, you may have
difficulty telling which direction to head to your destination.
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Current
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Staying on course is not as
simple as pointing your boat towards your destination, you must
compensate for current. The easiest way is to select an object to
sail to, such as a buoy or daymark, and a point behind the object
on land. This forms a range. Now adjust your course so these
two points are always in line and do not worry about the direction your bow is
pointing. This will assure you that you will stay on course.
Thus, your "Course To Steer" may be very different from your "Track" or
"Course Made Good". To determine the effect of current on
your "Course To Steer" go to the
Course to Steer" Calculator
 The
picture on the right shows a strong current (dark area) surrounded by quiet water.
Currents may become much stronger.
Winds which blow against the current will produce higher waves and seas
since the speed of the wind works against the speed of the current.
 A
riptide is a strong current produced by the rapid inflow or outflow of tidal
water. Tidal currents can be quite strong, producing white caps
(picture to the right) and sometimes large whirlpools.
Go To: Information about the "Old Sow" whirlpool on the
Smithsonian web site.
http://easternmaineimages.com/whirlpool.htm
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Speed-Time-Distance
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Go To Top )
It is important to know your estimated time
of arrival at a destination especially if you need to check into a
marina or return a charter by a certain time. It also is important
in estimating the needed fuel if you are under power. Because of
current, the distance traveled is not as accurate as predicting fuel
consumption as the amount of time the engine is running at a certain
RPM. The time remaining is equal to the distance to be
traveled in nautical miles divided by the speed in knots.
Time = Distance / Speed
This equation can be used either for units of statue miles and miles per
hour or nautical miles and knots. The most common units used for
navigation are nautical miles and knots; however, there are
areas such at the Intracoastal Waterway where distance markers are in
statue miles. You must be sure what units your charts and aids to
navigation are using and be sure to set your knot meter or GPS
accordingly.
The other common use of the speed-time-distance formula is to determine a
position based upon distance traveled and direction. This is
called dead reckoning. The distance traveled is determined
by speed multiplied by the time traveled.
Distance = Speed X Time
In times passed, the stars
and sun could be used to determine one's position in latitude.
Longitude was a problem and the time plus speed traveled equation was used to
estimate miles traveled east and west. Thus, the development of an
accurate timepiece capable of keeping accurate time while being tilted
and jostled at sea was a major navigational milestone.
Go To Speed Time-Distance-Calculator
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