A spinnaker flying forward of the bow is applying a load that could (in
theory) turn the boat around so it was lying with its stern pointing into
the wind. Imagine a vessel with some enormous fitting sited so far forward
of the mooring point that when the breeze blew it simply turned around so
that it was effectively downwind of the mooring.

But a boat and spinnaker in this configuration would be unstable, because
the load is being applied through sheets and braces (and a mast) that are
positioned well aft of the mooring point. The tiniest bit of force
off-centre would cause the load to be applied in a way that continued the
turning moment until the boat was either trying to sail the mooring out
sideways, or had the spinnaker wrapped around the forestay.

I think the way to answer the question is to imagine setting the spinnaker
with the boat moored by the stern, then imagine dragging the mooring line
towards the bow - how far can you get before the arrangement becomes
unstable?

We have hoisted the spinnaker while anchored by the stern. I have also
sailed off the mooring under spinnaker alone (after moving the mooring to
the stern) but my spinnaker is too expensive to put your question to a
practical test!
--

"Blorgad" wrote in message
...
My friend and I are having a disagreement regarding the behaviour of
sailboats on a mooring. We both have limited sailing experience, although
he
has a fair amount more than I. We were discussing running up a spinnaker
at
mooring in calm air, just to practice putting it up. We eventually
concluded that this wasn't practical, but the argument came in as to our
our
different conclusions as to why it would not be. I felt that there would
be
no point to put it up, as it would just get blown into the rigging and not
really tell us anything and possibly get torn. He was of the opinion that
somehow the spinnaker could push the boat to one side, fill and turn the
boat around. Initially he said it would go right upwind of the mooring,
then reduced his claim to say that it would turn around at the point where
it was attached to the line at the bow.

I am of the opinion that this is physically impossible and that under no
combination of sail at a mooring could the boat turn around so that the
aft
was upwind. I am of the opinion that you could get the boat to swing to
one
side or the other a bit by backing the jib, but you could still never turn
the boat around.

I would gratefully accept as many informed opinions on this from a sailing
or physics point of view.

Fair winds,
Miles