Don't know if this helps or not, but if all else
fails, think of the last time you flew a kite.
Remember how much harder it pulled on the string when
it was nearly overhead compared to when you first
launched it (straight downwind)? It's something like
that.
A slightly more technical attempt at an explanation
follows.
The thing that makes sailing anything other than
straight downwind is the dramatic shear, both in terms
of velocity and viscousity, of the two fluids involved
(air and water).
There are a couple of other useful things to
remember:
1) in other than a dead run, the centerboard produces
useful lift orthoganal to the direction of the boat's
travel (lift in other directions may be produced, but
it is un-useful and should be counted as drag).
2) the dynamics of a sail while reaching or pointing
are incredibly different than that of one running. It
is the difference between a gyrocopter and a
helicopter, or between a parachute and a hang glider.
with those thoughts in mind, we go on:
When running straight downwind, the theoretical
maximum speed is equal to wind speed. period.
When bearing off slightly, we could, in theory, retain
the downwind vector, but add the new, slightly
off-wind vector. Depending on the amount of wind, the
efficiency of the sail, and the amount of lift
generated by the daggerboard, there is some
theoretically optimal angle at which to sail. Bear in
mind, that as one speeds up on a beat or reach one
introduces yet a third vector which may in fact
*increase* the amount of lift the sail produces. At
some point the lift vectors generated by the sail and
board (note: these vectors will differ in both
magnitude and direction- the difference their lateral
components is your leeway) will achieve some stasis
with the drag vectors of the system (both hydro and
aero dynamic)and the theoretical max speed is
acheived.
Howard
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