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?


Walt wrote:
Um, yeah. The terms are more or less interchangeable.

Unless you're trying to work out structural details


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.

Agreed- the diagram ignored the 2nd set of forces on the foils & hull,
and tried to oversimplify to the point of being misleading IMHO.


Simple physics analysis usually starts out with a free body diagram
where you draw all the forces on the object.

AAAARRRRGH no! Now you've summoned the dreaded Navspritigator, who will
soundly denounce anyone who dares utter the dread incantation "free body
diagram"...


In the sailboat example, there's a lateral resistance force that opposes
the sideways force and nearly cancels it out.

Right. And the net forward force accelerates the boat until it is
cancelled out by the drag (which increases as speed builds up).

... 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.*

It's also important to keep in mind the circumstances under which the
underwater foils generate this leeway-countering force. The boat must
have some forward speed. It is quite possible for the boat to move sideways.

Donal's observation that you have to bear away and give up some distance
to leeward to gain speed for the daggerboard (or other underwater foil)
to get a grip is quite accurate. There are a few boats that will
accelerate from a standstill close-hauled, but not many... and the ones
that will, they'll make notably more leeway at first, too.

Fresh Breezes- Doug King