Scout wrote:

Thanks for the input Walt,
At first glance, I thought the author was ignoring the force of the wind on
the boat itself for simplicity's sake, and making a point about forces
developed by the sail. But then I realized that while the wind is
approaching the sail at the same angle in both diagrams, the sideways force
stayed relative to the boat's centerline and not the vertical plane of the
sail.
But from a practical standpoint, does total force = net force?

Um, yeah. The terms are more or less interchangeable.

And if so, shouldn't the net force vector show the direction of the
actual path, or movement of the boat?

Yes, if you include all the forces. But this diagram leaves out an
important force - the resistance force of the keel.

Simple physics analysis usually starts out with a free body diagram
where you draw all the forces on the object. Let's take a really simple
example: an apple sitting on a table. There's the gravitational force
pointing straight downward. So why doesn't the apple fall downward?
Because the gravitational force is not the full story; there's an equal
but opposite resistance force from the table pointing up that exactly
cancels the gravitational force. The net force is zero, and the apple
doesn't move.

In the sailboat example, there's a lateral resistance force that opposes
the sideways force and nearly cancels it out. They don't show it in the
diagram, but it's there. It comes from the centerboard or keel, due to
the fact that it's hard to push the boat sideways through the water.
Add it in and the net force vector will be aligned with the direction of
the actual path of the boat.*

If your boat has good lateral resistance (a big keel or centerboard)
this force will be nearly equal to the sideways force. If your boat has
little lateral resistance (i.e. you've pulled up the centerboard or
you've let the boat heel) this force will be less than the sideways
force and you'll crab sideways slipping to leeward.

--
//-Walt
//
// http://cagle.slate.msn.com/working/040514/matson.gif


* or more properly, the instantaneous direction of acceleration. If the
boat's on a steady course, this is the same direction as the boat's
path. Let's not split hairs where they don't need splitin.