StressMapper is based on full scale measurements,
performed with electronic strain gauges glued to the
surface of the sail. The sail stretch was measured out
on the sea, in different winds and at different courses
from close hauled to dead downwind.
The test boat, appropriately called ÓSail-LabÓ, was
a 32-footer equipped with a 21 channel datalogger. In
a previous project, Sail-Lab was used to measure slamming
stresses in the hull.
A mathematical model for the sail stretch was developed
on the basis of the measurements. The model forms the
calculation unit of StressMapper. It allows for the
wind speed and direction as well as the cut of the sail
and the sailcloth properties. The calculation unit is
linked to a material database which contains test data on hundreds of different types
With the help of StressMapper, the designer picks
up a strong enough cloth for the leech, body and luff
areas of the sail. If no suitable cloth is found, the
cut of the sail can be changed into a more sophisticated
one. The program handily predicts the max. apparent
wind that the sail can safely take without overstretching.
There«s even an overload warning if allowed loads are
surpassed in some point of the sail.
A larger stretch can be permitted for Dacron than laminates,
since woven fabric recovers better than film based laminates.
For someone who is racing, stretch becomes a problem long
before the yield point of the material is reached. From
performance point of view, a stretch of 0,3 to 0,4% at
most can be allowed, while for cruising purposes a 1%+
stretch is acceptable. Remember that 1,5% elongation in
a 15 m leech is 15 cm - a lot by any standards.
To compensate for the stretch, you sheet in, the leech
closes, the sail gets fuller and flow moves aft. StressMapper
shows in the shape window the deformation of the sail as wind increases in a very
tangible way. As a by-product, the program tells you
the sail weight and sheet & halyard loads.
In another window, bias and warp/fill
stretch can be studied separately.
StressMapper has proved to be an excellent help for
the designer. Demonstrating how the cut of the sail
affects the usable wind range of the sail is easy. The
stretch is directly reflected in the sail shape and
the customer can see how important correct the sailcloth
and cut is. With the help of StressMapper, it is a little
easier to answer the classical question ``Why should
I pay more for a more sophisticated design?««.
The bias stretch is indicated by the figure to the left
of each point. Sometimes the bias stretch is more important
than the warp/fill stretch. Then a laminate with a thicker
film must be chosen, or the cut has to be changed. For
a given cloth, the warp or fill and bias overload limits
can be very different.
The genoa of a 36-footer in 20 kn apparent. It is obvious
how much more the traditionally cross-cut Dacron sail
stretches. The Dyneema (Spectra) sail also weighs considerably
StressMapper indicates the relative stretch in percent,
in five strategic points of the sail. Above each point,
you have the actual tension in the sailcloth in [lbs],
as felt on a 2 inches wide strip of cloth. This is to
conform with the standard method of sailcloth testing.
This gives you a practical idea of the loadings a sail
must withstand: Near the top of the genoa, the tension
in the cloth would be 61 lbs/2 inches, or approximately
30 lbs/inch - quite a lot for the less than 1 mm thin
membrane. The stretch is directly reflected in the shape
(below). Sail shape is important even for the cruising
sailor. Although he may not be in such a hurry to get
there, a sail that holds its shape better means less
heel, a better balanced helm, less sail changes or reefing
- all adding to the pleasure of sailing.
A simple graphic relates stretch to the shape of the sail
in a way that is easy to understand. The small icon on
the left displays the radial-vertical cut of this sail.
The cloth data base contains stretch & weight information
on hundreds of sailcloth types from different manufacturers.
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