Racing downwind sails from Sailtech
Sailtech Downwind Racing
Engineering downwind sails that will perform, not only in straight line speed, but also be durable and forgiving to trim in changing wind angles, is a challenge and art. The design tools are amongst the most advanced 3D modeling and airflow. Specifics such as projected wind range, crossover charts and specifics of an individual sail can be modeled and predicted prior to the sail even being fitted to the boat - making the racing inventory more than just an estimation or guess at best as most sailmakers have done previously.
Selecting the Proper Spinnaker Material
Materials have a big effect on downwind sails and due to the relatively low apparent wind speeds that the sails see compared to working sails can have a big effect on the life of the sail. Most spinnakers, asymmetricals included, are built from nylon-based fabrics. These materials are light and range from .5 oz through to 2.2 oz. While nylon-based materials are relatively stretchy, which allows them to flap and be taken in relatively high wind speeds, overloading these materials does cause them to become more porous and reduces their strength considerably. Most people will admit to being caught over range with a spinnaker up or in a big broach with the spinnaker flapping and the boat on its side. Most people will also be surprised that the sail didn't break at that time, and then some time later while sailing along in nice conditions the sail breaks for what seems to be no reason at all.
A good guide for wind strengths for different materials that will keep the sails out of the dangerous overloading range is below. Note that these are apparent wind speeds, not true wind speeds. Also, these apparent wind speeds should be lowered if the sail is old or has been overloaded in the past.
- .5oz cloth 10 knots AWS
- .75oz cloth 14 knots AWS
- 1.5oz cloth 18 knots AWS
- 2.2 oz cloth 21 knots AWS
While nylon is a good material for asymmetrical spinnakers, other fabrics have more recently been developed for closer reaching designs that overcome the problem of the stretch and overloading of nylon. As the sails become smaller and flatter for sailing at closer angles, the yacht's stability has typically been the limiting factor and nylon sails have historically been strong enough. Now, with many of the more modern racing yachts with either water ballast or canting keels that have high stability and can carry reaching asymmetrical sails in much higher winds and at much higher boat speeds, alternative higher strength materials have been developed. Fabrics utilizing polyester and Spectra, Kevlar, and carbon fiber are common for high performance race yachts that carry only asymmetrical spinnakers. While these fabrics are considerably more expensive than nylon materials, they are much stronger and lower stretch.