![]() This could be one reason for the benefits of raking the mast aft in fractionally rigged boats. Aft rake contributes to the intensity of the vortex behind the mast. 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. 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 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. Most of the positive drive comes from the topmast above the hounds. It would seem to be more appropriate to think of the mast as part of the mainsail profile instead. Traditionally, the mast has been considered as a nuisance only, creating air drag. This suction can create a positive drive up to 5% of the total drive of the sails. Instead, flow is accelerated rapidly in front of the mast, resulting in a suction force towards the bow. The mainsail behind the mast prevents the typical drag creating “vortex street” from forming. Click on “hotspots” below for more on the subject: 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. Old beliefs are proven wrong and new features found. In the last few years, advances in CFD (Computer Fluid Dynamics) and FSI (Fluid Structure Interaction) have changed the way we perceive sail aerodynamics. For the underwater hull & fins, lessons learned from airplanes are more valid than for sails. 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. They are designed for specific speeds and operational conditions. Airplanes have long and smooth, only slightly cambered, thick wings. The sailboat is so different from the airplane, however, that many assumptions turn out to be if not erroneous, at least misleading. Much of the earlier work on sail aerodynamics is based on knowledge derived from the aircraft industry. Sail aerodynamics owe their heritage to airplanes, in good and bad.
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