"Wayne.B" wrote in message
...
On Fri, 21 Jan 2005 01:40:47 -0500, "Jack Painter"
wrote:

Did you mean "speed over the ground:"?

=====================================

No. When you are evaluating sailing performance you are usually
concerned with "Velocity", i.e., speed through the water. Obviously
SOG is important from a strategic and tactical viewpoint but sail
trim, sail selection and helming are first viewed with respect to
speed.


Understand wrt sail trim, and those are the known characteristics of a boat
that I referred to. When I raced in the Great Lakes, speed through the water
was a good evaluator of trim. Not enough information in the Chesapeake Bay
though, when currents are a major factor.

Velocity = speed + direction. To evaluate the effectiveness of a course to
mark, speed through the water is not relevant, and SOG from a GPS is still
only part of the equation, it requires an additional computer calculation to
determine effective VOG, that is; Course Made Good + SOG.

On a visual bearing to the next mark as a lay line, within a minute of
averaging time, most computers will settle at a reliable Course Made Good,
which is velocity over ground adjusted for the effect of currents and slip.
This also allows a calculation of time to mark. SOG however is instantly
available (a few seconds delay at most) and is more relevant to verifying
the known performance standards of sails, trim, degrees off the wind and
speeds that result from this. If current is dragging you to weather for
instance, then more heel (allowing more slip) could maintain best speed over
the best course to the mark. Historical GPS tracks around a course do not
tell the story it took to get there. Neither could speed through the water
by itself help trim the boat for best course to a mark that you won't
achieve without more information.

Jack, who was a Gary Jobson fan for about 25 years.