Sailboat Part 3: The Physics of Sails

With a hull you have a boat, but it isn't a sailboat until you add sails. While a sail may look like it's just a sheet, sails are actually a type of purpose-built wing, and share many physical concepts with aircraft. However, because the usage is also different from an aircraft wing there are also considerations for sails that aircraft do not require.

basic sail forces

Lift as Propulsion

A sail that is just a sheet can only generate propulsion via drag, which would limit sailing to travelling away from the wind. However, real sails always have camber, or in other words are curved, which allows them to generate lift. While the way a sail generates lift is similar to an aircraft wing, an airplane generates lift using the relative speed of the craft to support its weight, whereas a sail uses the relative speed of the wind to move the boat forward.

While sailboats generally cannot sail directly against the wind, because sails generate lift they can sail perpendicular to the wind and even upwind to some degree. In general a sailboat that can sail closer to the wind is faster, and the distance to the wind (in degrees) that a boat can sail at is known as its pointing ability. Pointing ability is important because as a boat accelerates it creates its own relative wind, which causes the apparent wind to shift toward the direction of travel. This means that a sailboat that is moving quickly will also effectively be sailing closer to the wind.

Points of Sail

the points of sail

The orientation of the boat relative to the wind is known as the point of sail. There are three basic points of sail, sailing towards the wind (called close haul or beating), sailing perpendicular to the wind (called reaching), and sailing away from the wind (called running). Generally sailboats are fastest when reaching since the speed of the apparent wind is maximized at near right angles, and sailboats can even sail faster than the true wind speed on a reach.

The point of sail also includes a tack, which describes the side of the boat that the wind is coming from. If the apparent wind is angled from the left side of the boat then the boat has a port tack, and if the wind is angled from the right it has a starboard tack. More generally, side of the boat that the wind is coming from is called windward and the side facing away from the wind is called leeward.

When moving from one tack to another, the sails must be rearranged to the opposite side of the boat in order to continue to generate lift. Furthermore, because sailboats are unstable (or unable to move) when facing directly into or away from the wind, the skipper must ensure that the boat has sufficient momentum to turn all the way through the wind when changing tacks. Changing tacks by turning against the wind is called tacking, whereas changing tacks by turning away from the wind is called jibing (pronounced "jai-bing"). Jibes are particularly dangerous because the sails can sometimes violently swing themselves around, causing injury due to bludgeoning or even capsize due to excessive changes in the forces on the boat. 

Forces on the Boat

Much like a wing, a sail is subject to aerodynamic considerations such as lift-to-drag, aspect ratio and so on. However while wings are symmetrically paired on each side of an aircraft, sails produce considerable imbalanced forces which a boat's hull(s) must counteract. Sails produce two main forces on the boat, pitching nose down due to lift, and rolling away from the wind due to drag. In order to transmit the forces effectively to the boat, masts are built as tensegrity structures with the mast itself providing compressive resistance which is then tensioned by cables called stays that are attached to various points on the hull.

sail forces pushing a trimaran to the limit

Due to the height of the mast(s), sails actually gain significant leverage on a boat, tending to tilt it rather than slide. If the aerodynamic forces on the sails exceed the ability of the hulls to counteract, the sails will literally drag the boat upside down in a slow, dramatic motion. Other considerations aside, a taller sail will generate greater forces than a shorter one. Even though a taller sail might have a greater aspect ratio and thus better lift-to-drag, the extra leverage caused by the height favors shorter, less efficient but higher area sails instead.

The camber of the sails are also important with regards to the forces on the boat. If a soft sheet sail is aligned too closely to the wind, it loses its shape and "luffs", which negates the lift. A sail with a smaller camber can point closer to the wind, but generates correspondingly larger forces. Most sails and sailboats are designed to point within 30 to 40 degrees of the apparent wind, however some racing craft (like the BMW Oracle, pictured) can point as close as 20 degrees with soft sails. As opposed to sail height, pointing ability is much more important to performance and thus "thinner" sails with less camber are preferred. A boat with greater pointing ability can take a more direct path when beating and travel faster when reaching which are major advantages in a race.

Wing Sails vs Soft Sails

a soft wing sail, showing parts of the 'skeleton'

Normal cloth sails are not very aerodynamically efficient for various reasons. Because they are more or less loose they cannot perfectly hold an aerofoil shape and "luff" if pushed too close to the wind. The "wingtip" of a soft sail also tends to have a truncated shape, which leads to greater induced drag, and soft sails cannot generate lift downwind.

To counteract these disadvantages, several varieties of "wing sail" have been developed which can maintain a more complex shape more rigidly. The original wing sails were made of stiff composite materials which can be quite heavy, however later designs include sails that have a soft, stretchable covering over a rigid "skeleton" which can change its shape to tack. Wing sails in general can not only produce greater lift-to-drag when compared to soft sails, but they can also point closer to the wind, averaging 15 degrees as compared to 20 for the best soft sail. Wing sails can also be swiveled backwards to generate lift when the wind is coming from behind the boat.

a hard wing sail with a hinge for adjusting the camber

Wing sails do have several disadvantages, however. Wing sails are universally heavier and more complex than their more traditional counterparts, especially for rigid composite sails. Large wing sails require heavy machinery in order to adjust the camber of the sail, even more so with rigid wing sails which can grow to become small skyscrapers on top of the boat. The extra weight requires balancing but provides no advantages otherwise. The extra breakable parts also constitute a significant cost liability.

The greatest disadvantage, though, is that for most wing sails there is no way to reduce the sail area in cruise. Soft sails have what are called "reef points", which allow the sail to be partly folded to reduce the sail area if there is excessive wind or if you want to slow down without stopping. Without the ability to reef, strong winds force you to take your sail down completely (which is also difficult to do with a wing sail), or else the sail can actually rip the mast down or even flip the boat, both of which are quite dangerous. Soft wing sails can usually be reefed, but at the cost of extra complexity.

Finally, wing sails tend to be ugly, which along with their other disadvantages (cost, weight, complexity) makes them relatively unattractive in spite of their potential advantages. I note them here mainly as a potential possibility, not as something I would use for my own design.