Sail design – An overview

Sailors often underestimate the importance of having a sail inventory that matches perfectly with their boat and the type of sailing they intend to do. And this is true for all sailors: for the keen racer, looking for the fastest sail possible, for the day cruiser, who is looking for easiness of sailing, for the offshore cruiser, who’s after durability and safety.

Having a good set of sails is crucial for enhancing a sailboat’s performance on the water. But what do we know about the science behind sail design?

Sail design is a fascinating and complex subject. Surprisingly, there is not a lot of available material to properly understand all the challenges and decisions that a sail designer needs to face to deliver the best possible sail for the boat.

In this article, we aim to introduce the various aspects of sail design and manufacturing to get a clearer picture of what is behind the finished sails.

Rig and sail types

As we know, there are many types of sailboats out there in the water, and they display all sorts of rigs and sail shape types.

As to rigs, we are all familiar with the Bermuda rig, but there are plenty of other rigs that a sail designer can encounter on its way: ketch, yawl, schooner, cutter rig, schooner, and other less common types. Each rig has its own peculiarity, advantages, and disadvantages, and when designing a sail, it’s important to know what they are, and the purpose of each sail rigged on the boat. For example:

  • Having more masts (like in the case of a ketch, a yawl, and a schooner) gives extra flexibility with the sail plan: for instance, it is not uncommon to sail a ketch with its mizzen sail only or a cutter with the inner headsail only in a high breeze.
  • Sail interaction effects are significant on a cutter rig when sailing with both its headsails.
  • A gaff rig is very unique with its quadrilateral sail, which doesn’t necessarily follow the same trimming guidelines as the triangular sails, but still, there are many classic boat races where gaff sails are used.
  • Bermuda rig is by far the most popular rig type, and it’s the one used by all high-performance yachts today because it delivers the best performance for upwind sailing compared to other rig types.

Regarding sails, there is a wide range of types within each rig type that can be used. Sometimes the choice is simple: for instance, when a client needs to be replacing its mainsail. But other times, the sail designer has an open field to choose the right sail inventory, as is the case on racing programs.

Let’s look at a list of sails to see all the options:

  • Mainsail: pinhead or square top.
  • Headsails: GenoaJibStaysail. A typical racing yacht has several headsails, for instance, a light jib, a medium jib, and a heavy jib, plus the staysail.
  • Windseeker: used for very light upwind sailing.
  • Spinnaker / Gennaker: depending on the yacht and the type of racing, the boat may be equipped with symmetric or asymmetric downwind sails or both.
  • Code 0: used for reaching, both on racing and cruising yachts.
  • Spinnaker staysail: used, together with an asymmetric spinnaker, for reaching and downwind sailing.
  • Storm Sails: trysail and storm jib. Used for heavy weather conditions. Compulsory in all racing yachts, and very recommended for any cruising yachts in case of failures of other sails or equipment.
Each rig has its own peculiarity, advantages, and disadvantages, and when designing a sail, it’s important to know what they are, and the purpose of each sail rigged on the boat.

Sail construction and materials

Another important aspect of sail design is the choice of material, as this influences the design and the final flying shape of the sail. If you have ever been in the process of buying a new sail, you must know how confusing it may get, and often the choice is made only based on budget and on the sailmaker indications.

There is a great variety of sail constructions and materials to choose from, each with its own characteristics that make it more or less suitable for the application. Sails can be classified based on:

  • The cut of the panels:
    • Cross-cut.
    • Tri-radial.
    • Paneled membranes.
    • Single piece membranes.
  • The type of construction:
    • Woven materials.
    • Laminates.

Of course, these two groups have some crossovers. For instance, we can have a woven cross-cut or a woven tri-radial sail.

Regarding materials, historically, all sails were made of natural woven materials, natural polymers. Woven sail material is made by weaving (interlacing) threads over and under each other at right angles. Later on, the discovery of synthetic fibers has brought a huge step forward in sail development. Nowadays, some of the most common fibers used in sails are:

  • Nylon, used mostly for spinnakers and gennakers.
  • Polyester, mostly known as Dacron, which is the brand name given by the manufacturer.
  • Aramid: Twaron / Technora / Kevlar.
  • Vectran.
  • Ultra PE, commonly known as Dyneema or Spectra.
  • Carbon.

Each fiber has its unique set of properties, that will in turn determine the performance of the sail. Some of the most important parameters that need to be considered are:

  • Modulus: the ability of a fiber to resist stretch.
  • Tensile strength: the measure of the maximum force attained in breaking a fabric or fiber.
  • Flex strength: the ability of a fiber to retain its strength after being folded back and forth.
  • UV resistance: the measure of the resistance of the fiber to the effects of sunlight.
  • Air permeability: the measure of air flow that passes through the fabric.

The type of fiber used, together with the cut of the sail and the material construction, will determine the characteristics of the finished sail. The ideal sailcloth would:

  • last forever;
  • have a high modulus (so minimal stretch under load and good shape holding);
  • resist UV and flex; and
  • be cheap.

Of course, the reality is different, and we need to find the best fiber or fiber mix for the application based on their properties.

Sail aerodynamics

Sail aerodynamics is a great deal in sail design. Sails are thin airfoils, and they share many aerodynamic principles with airplane wings. When designing a sail, we must consider concepts like lift and drag generation, flow around sails (laminar vs. turbulent flow), differential pressures around the sails, and sail interactions.

Sails are thin airfoils, and they share many aerodynamic principles with airplane wings.

The added complication when dealing with sails is that they are flexible membranes. Moreover, the flow around the sails is not static but very dynamic: each boat’s motion implies a change in the apparent wind direction and angle “seen” by the sail, and these changes occur at very high frequencies.

The ability to predict the sail’s flying shape has always been a driving factor in sail aerodynamic research. To that extent, wind tunnel testing, full-scale testing, and computer modeling have been the main tools used to develop sail profiles.

However, there is a lot to learn still today about the behavior of sails, both in static and dynamic situations. Just to give an example, despite the technological advances and the computing capabilities of modern computers, it is really difficult to measure numerically and accurately the flow around downwind sails, which is a turbulent flow that separates from the sail. It is even more difficult to reproduce their dynamic behavior, which, as all sailors know, is very dynamic, even in light wind days (think, for instance, of how spinnakers are trimmed, allowing the luff to partially fold whilst sailing). This means that, for downwind sails, the designers often rely on their experience.

Sail manufacturing and finishing details

The finishing and detailing of sails is very related to the sail design itself. We could design, aerodynamically, the perfect sail, but if we make a mistake, for instance, on the connection of the sail corners to the rig, we will end up with a bad flying shape. It’s surprising how even a little flaw in the corner of the sail, or in the sewing of panels can affect the entire sail shape.

Some of the things that need to be addressed when designing a sail are:

  • Corners finishing and connection to the boat: use of rings, soft finishing, loops. Is there any particular requirement from the client?
  • Luff finishing: boltrope vs. luff cars or slides.
  • Extra accessories for the sail: furling unit to be coupled with the sail, spinnaker socks, etc.

Conclusion

It is fair to conclude by saying that sail design has more into it than what meets the eye. It spans from scientific studies to very practical considerations, all of them equally important in defining a good sail shape and final product.

Sail design online course

Navalapp has prepared a dedicated sail design online course where all the topics mentioned in this article are discussed much more in-depth. In addition, the course also discusses topics like sail design spiral, sail testing, sails inventories, and sail design software.

Looking forward to meeting you there!

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