Sail aerodynamics owe their heritage to airplanes, in good and bad. Much of the earlier work on sail aerodynamics is based on knowledge derived from the aircraft industry. The sailboat is so different from the airplane, however, that many assumptions turn out to be if not erroneous, at least misleading. Airplanes have long and smooth, only slightly cambered, thick wings. They are designed for specific speeds and operational conditions. Sails, on the other hand, are often of lower aspect ratio, highly cambered, thin and twisted, and have to operate in a variety of conditions and wind speeds in a turbulent layer of air above the sea surface. For the underwater hull & fins, lessons learned from airplanes are more valid than for sails.
In the last few years, advances in CFD (Computer Fluid Dynamics) and FSI (Fluid Structure Interaction) have changed the way we perceive sail aerodynamics. Old beliefs are proven wrong and new features found. This treatise is divided in two parts, in the first we look at the sails from the windward and in the second from the leeward side. Click on ”hotspots” below for more on the subject:
The mast is not a drag device
The mainsail behind the mast prevents the typical drag creating ”vortex street” from forming. Instead, flow is accelerated rapidly in front of the mast, resulting in a suction force towards the bow. This suction can create a positive drive up to 5% of the total drive of the sails. Traditionally, the mast has been considered as a nuisance only, creating air drag. It would seem to be more appropriate to think of the mast as part of the mainsail profile instead.
Most of the positive drive comes from the topmast above the hounds. In this part of the rig, wind is more twisted to the side, partly because there is no jib or genoa interfering, and partly because of the head vortices shed by the sails. Thus, for a 9/10-rigged, modern boat equipped with a jib rather than a genoa, the mast drive can be negligible or it can be even negative, real drag.
In case of a genoa equipped boat, the lower part of the mast lies in a more beneficial airflow than for a jib equipped boat. Aft rake contributes to the intensity of the vortex behind the mast. This could be one reason for the benefits of raking the mast aft in fractionally rigged boats.
Note that in aerodynamic terms, the mast does produce drag, as the aerodynamic drag is defined in the direction of the apparent wind. But in hydrodynamic terms, where the drag is oriented against the direction of the motion of the boat, the drag is often negative.
Above: The mainsail prevents the usual drag creating ”vortex street” from forming behind the mast. The zone of dead air is much less severe than in the case of an isolated pole. As a consequence, the mast can add to the total drive of the boat, instead of dragging back like an empty pole would.
Below: Close-up on the Star-mast reveals a strongly accelerated flow on the front-leeward side (in red), especially in the upper part of the mast.
Luff vortices on the windward side
When you are pinching high into the wind, flow separation occurs on the inside (windward side) of the sails. The separation mechanism is different from what we have learnt from long, slender and thick airplane wings, where leading edge separation tends to form confined ”bubbles”. Especially at the luff of the jib, which is raked back at an angle to the horizontal and the airflow, the separating flow has a tendency to swirl into a vortex or ”mini-tornado” sucking the air up towards the top of the sail.
The ”tornado” initially starts closer to the top of the jib, wandering gradually down the luff of the jib as the boat is steered closer to the wind. This is clearly witnessed by tell-tales placed closed to the luff: Initially, all tell-tales stream horizontally, then the yarns close the head start to stream upwards, followed by those in mid-luff and finally even the yarns above the tack flip vertical. This all happens before the luff of the jib starts noticeably to backwind, so tell-tales are useful indicators indeed.
A similar vortex can be seen on the windward side of the mainsail, although the mast interferes with the flow. The sharp luff of the jib promotes the forming of the vortex, while in the case of the main the roundness of the mast tends to dampen it. The main is also more prone to backwinding before the vortex is fully formed.
Above: The luff vortex formed on the inside of a Star jib. Below: Luff vortex on a Finn-dinghy sail.
Wind gradient effects
Wind speed is slowed down by friction closer to the sea level. This effect is often called the wind gradient, or also the atmospheric surface layer. When this change in wind speed with height is combined to the speed of the boat, we get what is called the wind shear. The apparent wind that the sails experience is on one hand weakened close to the foot of the sails, and on the other hand it is twisted so that the upper part of the sails experience a ”lift”.
While sailing upwind, the twist effect of the apparent wind is in the order of 3-5 degrees. Downwind it can be significantly more, over 20 degrees in some cases. The frictional effect is rather more important for smaller boats, reducing the power of the sails up to 25% compared to a uniform flow. From the sail design point of view, as well as velocity predictions (VPP), the wind gradient/wind shear phenomenon is important.
Above: In beam wind, the flow can be bent over 20 degrees from deck level to the mast top.. At the deck level, the inflow is clearly in front of abeam, while towards the top of the mast the air comes abaft the beam.
Below: Closer to the sea surface, wind speed is reduced (blue tones), while higher up it is accelerated (green-yellow-red tones).
Natural wind turbulence
Turbulence in the atmospheric surface layer closer to the sea varies, depending on the weather, wind direction and other factors. Turbulence intensity in the natural wind over the sea is in the order of 1% of wind speed, and more in rough conditions.
The wind turbulence has an influence on sails: it seems that flow is less prone to separate when turbulence increases. This could be the reason that some sails work better on one day than another, and also in different places & conditions. Unfortunately, little data is available about natural wind turbulence and more research is needed there.
The hull influences air flow on the sails, and the sails exert pressure on the hull – there is an interaction that sometimes benefits both. The deck acts as an endplate for the jib, carrying its load all the way down to the deck. This is in practice equivalent to increasing the aspect ratio of the sailplan.
The air flowing over the edge of the deck rolls into a vortex, which blocks some of the air under the mainsail foot. On the leeward side, the hull tangles the jib foot vortex together with the one shed under the mainsail foot, which leads in a reduction in the suction of the vortex core, beneficial for performance.
Above: Air rolling into a vortex over the deck of a Star-boat sailing upwind. The deck edge vortex deflects air upwards on the mainsail, preventing some of it from slipping under the boom.
Below: pressures on the Star-boat hull & crew at AWA 27 degrees: blue shades are pressure higher than atmospheric, red colors indicate lower pressures. Green is neutral, or near atmospheric.