Composite Sailings are sailings that use both principles of both GC and Parallel
sailings in order to cover the shortest distance between two points while avoiding an object at a given latitude. A Limiting
Latitude (in these equations referred to as Lv), is the closest to the pole that trackline is allowed
to go. Ice flows, land masses, or storms can cause a course to be limited to only certain latitudes to ensure safe navigation.
To Solve Composite Sailings:
Don't: Do not just draw your straight line on a gnomonic chart and sail the limiting latitude once you
reach it. This route will be much longer, and thus more costly than sailing as follows.
Graphically:
-First, find the appropriate Gnomonic chart for your ocean region
-Gnomonic charts are used because GC curves are represented as straight lines
-Second, find your limiting latitude
-If you know where an ice flow or land will be, plot it on the chart so that you know to avoid it.
-From your point of departure, draw a line that will be tangent (touching at only one point) to your limiting latitude
toward your point of arrival.
-This line will be the most efficient way to reach your limiting latitude, while still approaching your destination
-From your point of arrival, draw line that will be tangent to the limiting latitude
-Between the two points where your lines meet the limiting latitude, you will sail along the limiting latitude (this
will be a Parallel Sailing)
Mathematically:
-Use the equations at right to solve for your point of tangency; this will be the point at which
your GC track become a Parallel one.
-These equations can also be found on Bowditch page 350
-First, convert all of your positions into degrees and tenths rather than degrees and minutes
-As you know your limiting latitude already, solve for DLovx using one of the equations provided
-Either add or subtract DLovx to or from your initial or final longitude to find the longitude of
each of your points of tangency
-From here, use GC and Parallel equations to find course and distance information between the points.
