Articles about Aerodynamics

Advances in sail aerodynamics
Sail aerodynamics - part two
Sail Dynamic Simulation
Streamlines & swirls
WindTunnel Movies
Sails shape & aerodynamics
The Quest for the Perfect Shape
Note on the effect of side bend
Anatomy of a Mini-Transat
Mini boat - Maxi challenge
Mini boat - Maxi challenge
470 Aerodynamics
Lifting bows with foresails
StressMapper >
Telling tales ...
The scientific Finn
Wind tunnel images


StressMapper genoa

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 of sailcloth.

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.

Principal stress & strain

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?««.

Warp & bias stress & strain

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.
Dacron vs. Dyneema

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 less.

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.

Dacron vs. Dyneema shape

Genoa Shape

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.
Cloth data

The cloth data base contains stretch & weight information on hundreds of sailcloth types from different manufacturers.

Copyright © 1995 WB-Sails Ltd. All rights reserved.

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