Ik wil dit Topic niet kapen met tecnisch gewauwel waar we het niet eens over eens zijn, vandaar!
Bij deze:
groups.google.com/group/rec.bo...ff2/3aadec5468e30711
I tried to stay out of this debate... but it seems I have an
overwhelming need to contribute / squash this thread once
and for all. If you don't like theoretical discussions you
had better quit reading now.
It appears to me that this thread suffers from serious
miscommunication due to the terms that are used. Let me be
explicit about the terms I will use:
Lift: a force vector parallel to the vertical axis (i.e.,
lift relative to gravity NOT necessarily aerodynamic
lift which is measured relative to the flow). Greater lift
means that less of the weight of the boat is supported by
the pressure of water on the hull. (Note: each Axis goes
thorough the center of mass (CM) of the boat.)
Drive: a force vector parallel to the longitudinal axis.
Pitch: a moment that determines bow-up or bow-down
attitude. I will define bow-down as a positive moment,
therefore: pitch = (drive * distance above center of
resistance) + (lift * distance behind CM)
I am simplifying the problem by ignoring forces parallel to
the lateral axis, as well as roll and yaw moments.
And now the tricky definition that I completely made up by
combining the definition of lift with a particular location
(other than the CM)...
Lift the bow: A decrease in the amount of weight supported
by the pressure of the water on the part of the hull ahead
of the CM (i.e., does it pull the front part of the boat out
of the water). This is NOT the same as a bow-up pitch.
Bow-up pitch means that the bow is being lifted MORE (or
depressed less) than the stern. However, Lifting the bow
can be measured without reference to what the stern of the
boat is doing; it is even possible to be lifting the bow AND
the stern. Concisely, the bow is lifted by a sail iff
there is mass behind the point where the sum of lift and
drive vectors intersect the longitudinal axis. (whew!) A
sail that produces drive with no lift can never "lift the
bow" because the vector will never intersect the
longitudinal axis, therefore no mass exists behind it.
Spinnakers and lift:
By these definitions, I think we can agree that ALL
spinnakers produce lift under most conditions. The sheets
and guys pull up and clearly generate lift. The halyard
pulls almost entirely perpendicular to the vertical axis and
therefore has no effect on the lift vector. I assume that
people who said that this was impossible were using a
different definition of lift.
Spinnakers and pitch moment:
Since the center of effort on a spinnaker is quite a
distance above the Center of resistance of the hull, drive
results in a bow-down pitch. On the other hand, the CE is
typically in front of the CM and therefore lift results in
bow up pitch. Because of this trade-off, I think it is
entirely *possible* to get a net bow-up pitch from a
spinnaker if the following conditions are met.
a) Low drive force. To get low drive force you need the
apparent wind quite far forward - shifting the sail's force
to be parallel to the lateral axis rather than the
longitudinal axis. When the apparent wind goes aft the
sails produce more drive, i.e., the halyard, sheet and guy
pull more forward and the bow goes down.
b) Spinnaker attached far forward relative to mass. This
maximizes bow-up pitch moment due to lift.
c) A short mast relative to spinnaker size. This minimizes
bow-down pitch due to drive.
Virtually no symmetrical spin. meets these criteria since
they are used only when the wind is aft of the beam. The
asymmetrical spinnakers on the 18' skiffs and I14 boats meet
all of these criteria. I think that it is certainly
possible that these sails produce a bow-up pitch. Is
difficult to know for certain because 1) pitch is also
affected by hydrodynamic forces 2) the CM on these boats is
very mobile - whether the spin. creates a bow-up or bow-down
pitch moment will depend on where the crew stands!
Theoretically there should be a crew position (far aft)
where putting up the chute causes the bow to rise relative
to the stern and another position (far forward) where
putting up the chute causes the stern to rise relative to
the bow. I don't know if it is actually possible to adjust
the mass enough to experience both of these conditions.
Spinnakers lifting the bow:
While I was fairly certain that only a few asymmetrical
spinnakers can give a bow-up pitch moment, it is possible
that a number of spinnakers actually "lift the bow" under
the right conditions (and not just in a waterspout). It's
just that they lift the stern so much more, that the effect
is insignificant. The effect is further obscured by
the fact that the main sail always depresses the bow going
downwind, since it produces no lift.
Gr Michel