Sails vs Wings - Brian
Today the BMW-Oracle syndicate defeated Alinghi in the Americas Cup. The race was unconventional from the start, featuring multi-hull boats rather than the traditional 12m single hull boats, not to mention the multi-year legal battle waged against Alinghi by BMW-Oracle to gain an advantage in location and boat design. It was also different in terms of sail technology, and ultimately that was what I believe was the reason for the victory of BMW-Oracle. The technologist in me is excited to see the improvements in sail technology, the traditionalist in me thinks the entire race was an abomination and not to the spirit of a gentlemans yacht race that is the Americas’ Cup; the race is partially about technology in hull and keel design, but also about real racing and tactics rather than courtroom battles and showcasing the latest in CAD design and wind tunnel testing; in short it is not to be the F1 of car racing where the only limitation on potential is effectively money.
BMW-Oracle used a wing sail, first used to a lesser degree in the 1988 Americas Cup, while Alinghi used a traditional sail. Who cares and what difference does it make? A lot it turns out, especially in the light wind conditions of Valencia, Spain.
To understand why, you need to understand how sails work in general. If you have an idea how airplanes fly you are ready to understand sailing; think of a sail as an airplane wing turned upright. The concave portion of a sail (the windward side) is the same as the bottom of an airplane wing; the convex portion (the leeward side) is the same as the top of an airplane wing. The pressure differential between the two sides creates lift for airplanes and drive for sailboats; the leeward side has lower pressure and thus is pulled away from the windward side. For airplanes this means going up, for sailboats this means going away from the wind. Sailboats use a keel to convert that lift from the sail into forward motion.
The wing sail is a complex variant of a sail, really a mixture of an airplane wing and a traditional sail. A traditional sail is a triangular piece of fabric, attached at 3 points to a sailboat – the bottom of the mast, the top of the mast and the end of the boom. The 2 attachments to the mast are relatively fixed, leaving the boom as the way to adjust (trim) the sail. The sail in the wind forms a curve in 3D, but is easy to imagine as a simple curve in 2D from the mast to the end of the boom. The sail is symmetric, its windward and leeward shapes are the same. To trim the sail, sailors adjust the angle of the boom relative to the wind to attempt to capture the maximum amount of wind. I realize this a simplification and doesn’t account for the vang, traveler and short-sheeting – they don’t really matter for this discussion.
A wing sail such as that used by BMW-Oracle is like 2 sails put together; the wing still attaches to the mast at the bottom and top, but it does not attach to a fixed boom like a normal sail. Rather, the wing closest to the mast is different; the mast itself is very wide, allowing for the wing to have 2 distinct sides at each end of the mast, essentially being 3D where a normal sail is not; this allows for 2 distinct sides with different angles to the wind and typically more lift, for the same reason airplane wings aren’t symmetric. It then tapers to a point midway and ends in a gap. After the gap the second part of the wing begins and is much like a traditional sail. What difference does this make? It has several advantages. The 2 wings can be trimmed separately, allowing for more efficient trimming. The gap between the wings allows high pressure air from the windward side to feed low pressure air to the leeward side, generating more lift. (Airplane wings do the same trick during take-off and landing, if you have sat in a wing seat and observed the ailerons extending it causes the same effect). The BMW-Oracle wing sections were also divided into at least 9 vertical segments, I believe they were independently trimmed but cannot verify that. The wing also has disadvantages that make it unsuitable for general-purpose sailing, mainly that it cannot rotate freely and the rigging necessary to trim the 2 wings is complex and difficult to manage without a crew and these days a computer.
So while BMW-Oracle proved triumphant in sail technology I don’t think that makes them better sailors. It also doesn’t make any of us better sailors since wings are basically out of the question for anyone without a multi-million dollar sailboat and a professional crew and engineering team to sail it, so don’t expect any of this to make your J-24 faster, ever.
