It's seldom realized but wind forces are not used in engineering
sailboat rigs. To design a rig properly, you have to know the
vessel's stability. You determine the maximum normal sailing angle
and the righting moment associated with it. That righting moment is
then divided by the sail area and the distance between the center of
effort and a point usually taken as half the draft. This give the
load on each square foot of sail which is then used to design the
spars and rigging.

The theory is that sail will be reduced or the wind loads eased by
sheet handling or course change when the vessel is heeled beyond
normal angles. This won't always be the case but that's what the
factors of safety are for.

For very conservative rigs, such as on a solo (non-racing) long
distance ocean cruisers, you might look at the righting moment at the
peak of the righting arm curve. No amount of wind can put more
pressure on the rig than at that point because the boat will simply
blow over farther to where there is less righting moment.

You also sometimes want to look at loads under reduced canvas because
they will be much higher per unit area and may locally overstress
components that would be fine heeled to the same angle under the full
sail plan.

--

Roger Long



"DSK" wrote in message
.. .
Mic wrote:
http://home.cfl.rr.com/irwin104/specs.html

Foresail SheetLoads
100% Foretriangle @ 30 Knots: 834.8 lbs
100% Foretriangle @ 35 Knots: 1136.3 lbs

That's just plain silly. An Irwin 10/4 is not going to be carrying a
100% jib in 35 knots of wind, nor a 150 in 20 knots.

Fresh Breezes- Doug King