Oz1 wrote:

Because it allows you to have a bigger mainsail for the
same
measured sailarea than if you used the halyard to the
same
purpose.
Does not Compute!

If you want to utilise the maximal luff-length as marked
between the black marks at the mast when the wind is light,
and the luff-tension is low you can't pull the halyard
further when the wind increases witch leaves you with a
saggy sail with the depth too far aft.
Or to say the same in other words:
If the sail is at the max luff-length, between the black
marks, when the wind is strong and the halyard is tight then
you will have to ease a bit on the halyard when the wind
decreases witch gives you a reduced sailarea.

If your boat has a boom-downhaul that could be used instead
of the halyard, but the argument would be exactly the same.

So the boat that uses the halyard or the boom downhaul to
trim the luff tension would have a reduced area in light
conditions compared to a boat that uses a cunningham.

Another advantage is that it is takes less power to
tighten
the cunningham than it does to tighten the halyard. This
makes it usable on cruisers as well.
Halyard doesn't have 8, 16,32:1 purchase.

Well sometimes they do. I have sailed on several boats where
the luff tension (of the foresail) were trimmed by a
"muscle-box" attached to the halyard I think with a 32:1
purchase. You could also easily use a purchase to the boom
downhaul. Many boats had these before the "cunningham" were
standard practise.
(In relation to power needed to increase luff-tension, using
the boom-downhaul would have almost the same effect as the
cunningham)

Besides that - the purchase is not the issue he the
power required to increase the luff-tension (maintaining the
pressure on the mainsail) when the boat is sailing is.

There are other factors but I don't think that matters
much.
To mention one thing, it also stresses the leach if you
use
the halyard, but that can be compensated by the tension
of
the boom vang or by adjusting the traveller.

Hmmm please explain (Red headed racist)

Sorry - now it is I who have to ask for an explanation.

--
Peter S/Y Anicula