Sails Aerodynamics - A Seatrip around sail shapes

Abstract

The Aerodynamics of sails and sails interaction problem are of high interest for many decades of racing yachts. Soft sails have a long history for sailing yachts. Rigs have changed over time and regions of the world. The square rig has been used for a long time but was mainly a downwind rig without positive interaction between sails. The Latin rig use triangular sails and has better performances than the square rig in upwind conditions and is more manoeuvrable. The Bermuda or Marconi rig with a mainsail fixed on a mast and a headsail or jib have higher performances in upwind conditions and allows more easy tackle. It is largely used today on racing yachts and is well adapted to a large range of boat size. It is the most known rig configuration for sails interaction. It will be one focus point of this paper. For simplicity, we will not consider the high length boats using multi masts system, as it is a variation on the same theme. Knowing this restriction, it is a complex subject if we think about the slow evolution and the many variations used in a given period of time. This may seems a paradox knowing that racing boats are driven by a unique objective which is speed on sea water in given wind conditions. But as every sailor know, the way to sail fast on water are numerous depending on sea state, wind conditions and boat architecture and is a subject of many contradictory stories. It will be seen how the sail and rig evolution may be related to aerodynamic research and tools evolution over years to help designing advanced sails and rigs for racing sailing yachts. The fact that aeronautic research has frequently played a role of precursor in maritime new findings will be emphasized. Interacting sails (mainsail + jib) is a solution, which is the result of a long evolution over years of practice, and trial and errors on various boats. It has been used for its ability to generate a high driving force without too much heeling force and its consequences leeway and heeling moment. It has been described and explained in a long list of publications. We will go through selected ones to focus on main parameters affecting sail interaction before to go further toward recent evolutions of this classic and largely used rig. It will be shown how tools evolution has play a key role in the understanding of the sail interaction problem with Gentry, Marchaj and Milgram publications. Later, a well-known evolution has been proposed on Star & Stripes in 1988 with a semi-rigid mainsail. Then, more recently a transition occurs by replacing softs or semi-rigid mainsails by fully rigid ones, as it has been popularized by the famous multi-elements wingsail of the giant BOR 90, and later, by the two last editions of the America’s cup yachts (AC72, AC45, AC48, AC50). All these evolutions are based on a symmetric rigid mainsail and a classic soft jib or gennaker. The aerodynamic basics and performances of these new configurations will be described from recent publications. Explaining the rational behind this evolution, and anticipate possible future evolution will be proposed. Putting this sail design evolution story in perspective, we will show how design tools for sails and rigs have largely contributed to the understanding of sails aerodynamics and to these evolutions from soft to rigid sails. Few figures illustrating these elements are given below and will be completed.