There have been several correct answers.
The only reason to calculate a Course to Steer is to get efficiently across a moving body of water
. As was discussed in extreme detail earlier in the year on another thread, the fastest way from Point A to Point B across a moving body of water
is a straight line -- through the water. A straight line through the water is a constant heading, NOT constant COG.
Therefore, when solving this task you are working with Heading and Speed Through the Water, definitely not COG or SOG. COG and SOG will vary greatly during a successful passage
across moving water. The Heading will not vary -- because you are sailing a straight line through the water, which is the most efficient path.
If you look in the archives
, you will find an exhaustive discussion of this interesting problem. You will see mathematical proofs that the most efficient path across the English Channel
, for example -- an excellent example of the kind of passage for which you must have a Course To Steer calculation -- is an "S" curve as seen from the ground perspective -- an "S" curve over ground, but a straight line through the moving water.
If you attempt to sail a straight line over the ground in the English Channel
-- that is, hold a constant COG -- you might never get there. You will be fighting the tide first one way, then the other way, and you will add lots of miles to your passage. Your boat sails
through the water, after all.