If you've been sailing for any length of time, you've probably come to understand that a sail is much more than a triangle of cloth. It's a complex three-dimensional structure with opportunities to adjust elements to dial in the exact performance for your needs. This is even more the case with reaching and running sails. Here are the key elements to think about and how they work together.

Mid-Girth, Depth, & The Balance Between Luff & Leech Area

Understandably, sail size is a critical design feature. The optimum foot length for any downwind sail is a function of aspect ratio or the height (luff) versus width (foot). When too tall and skinny, the sail won’t be stable. Too wide and short, and the sail will not twist properly to achieve the right flying shape. No matter what type of downwind sail you are building, you have to start with the correct aspect ratio for the boat’s rig proportions. This is what dictates size. The next consideration is the girth of the sail halfway up; this is known as the mid-girth. The smaller the mid-girth, the more genoa-like the sail becomes and more capable at closer apparent wind angles. Bigger mid-girths equal wider apparent wind angles. At the small end of the spectrum, a genoa will have a mid-girth of around 50%; at the top end, a running asymmetrical spinnaker will have a mid-girth of over 100%. Quantum’s AWA 40, 60, and 80 cover everything in between.

Overall depth is another important variable. A flatter sail means closer apparent wind angles and a fuller sail means broader angles. As a sail gets wider mid-girth, additional depth is necessary to support the area so that the sail will hold its shape and not just flap at the edges.

The final variable to consider is the amount of area outside the straight-line luff and leech. For close apparent wind angles, the luff will be straight and any additional girth will be in the back end. As you design for wider apparent wind angles, this balance gradually changes, with more of the sail’s width shifted to the front end. A running sail has a significant amount of area forward of the straight-line luff. When the sheet is eased, this area moves past the centerline and projects to windward. This is what allows the sail to perform in broad apparent wind angles.

Materials: Nylon Versus Composites

For more traditional asymmetrical spinnaker sails such as the A3 and A2, nylon continues to be the best material, but it is not necessarily the best choice for the new generation of specialized reaching sails. In order to handle higher loads at closer apparent wind angles, a stronger material is needed. The best material choice is based on boat size, righting moment, and target apparent wind angle. In some cases, the best option could be as simple as a heavier nylon or polyester spinnaker cloth, but often a composite material with more strength and durability is the better choice. Quantum’s proprietary Fusion M™ tri-radial construction process is used to create the ultimate custom structure for membrane reaching sails. Quantum collaborated with industry leaders to develop a range of composite materials specifically for high load reaching sail applications.

Boat Characteristics

 A boat’s design has a big impact on which downwind sail option is right for each sailor. For more traditional cruising boats with large foretriangles and overlapping headsails, reaching angles are well served by a large genoa. An A2 or A3 rounds out your sail inventory to cover the broader angles. Conversely, modern boat designs tend toward small foretriangles with non-overlapping jibs. These smaller headsails lack the punch of their larger genoa cousins when it comes to reaching. AWA 40, 60, or 80 reaching sails fill this gap. Some other questions to address when determining your boat’s best sail options: Is there a bow spirit? Is the boat a multihull? Where are you sailing? How fit and enthusiastic is your crew? Quantum’s experts can help you sort these and other questions to make sure you get the perfect downwind sail option for you and your goals.

Article Originally Published by Quantum Sails

To learn about Quantum's racing and cruising downwind sails or to request a quote, visit www.quantumsails.com. For more engaging articles and videos, check out our library of resources and expertise on the Quantum website.