The Physics of Sailing Explained

Es posible consultar la obra de Byron D. Anderson a través de Google Books. Constituye una magnífica síntesis de principios básicos para entender alguno de los fenómenos básicos de los que depende la navegación de un velero. Es posible que forme parte de la poca literatura de divulgación que existe sobre esta materia. Desde el enlace del título se puede acceder a una versión limitada de la obra.

Byron D. Anderson (2003): The Physics of Sailing Explained. Sheridan House.

Authoritative yet accessible, The Physics of Sailing Explained is the perfect work for those sailors who want to enhance their understanding and enjoyment of life at sea. It will enable readers to better grasp how sails, keels, and hulls work together to keep seafarers afloat, and will sharpen their skills with a more subtle and thorough appreciation of why various boat design features are present and why certain tactics work in certain situations. Anderson, a professor of physics at Kent State University and an avid sailor, outlines the science behind seagoing in such a way that anyone can understand and benefit from without having to trudge through a physics text or become a naval architect. With the help of this invaluable book, sailors will be better prepared to handle any situations that might arise on the water.

Topics covered include:

• What limits the speed of a sailboat and what is “hull speed”?
• Can a sailboat ever go faster than its hull speed?
• What is the best shape for a sailboat?
• Can anything be done to reduce the friction of a sailboat moving through water?
• What is the effect of turbulence created by a sailboat on how it moves through water and what can be done to reduce turbulence?
• Why is a keel necessary on a sailboat?
• How does a keel work?
• How has keel design improved over the years?
• How do sails work?
• What is the fastest direction of sailing with respect to the wind direction?
• Is it true that some sailboats can sail faster upwind than downwind?
• Why are modern sails so tall and narrow for upwind sailing and much fuller for downwind sailing?
• What produces the tides?
• Why are there two high tides each day?
• Do the tides follow the Moon around each day?
• What produces the winds?
• What causes the global wind patterns?
• What is the Coriolis force and how does it affect global wind patterns?
• What causes the global current patterns?
• Why does the Gulf Stream exist?

En la editorial Sheridan House, hay un fragmento de uno de sus capítulos:

Induced Drag

Just as with the water flowing past the hulls and keels, vortices are generated by the wind flowing past the sails. As discussed above, the wind flow produces a higher pressure on the inside and a lower pressure on the outside of the sails. And just as with the water flowing along a keel, this produces a net flow of air over the top of the sail, from the high pressure side to the low pressure side. (See the discussion and Figure 2.7 illustrating this for a keel in Chapter 2.)

Because the wind is flowing primarily toward the stern, these upward and downward flows meet at the back of the sail at angles with respect to each other and produce a twisting effect on the flow of air off the sail that becomes more pronounced as you move up the sail.

Furthermore, if a sail has much of a gap between the deck of the boat and the bottom of the sail, a vortex can be generated there as well. These vortices are shown schematically in Figure 3.4. The production of these vortices requires energy, and this energy must come from the wind power in the sail. An example of these vortices produced at the top of a sail is shown in Figure 3.5, which shows boats from the 2001-2002 Volvo Ocean Race proceeding through a fog just after leaving Cape Town, South Africa. The induced vortices are seen clearly in the fog and indicate that a considerable amount of energy has been dissipated this way.

Just as for a keel, you can use wing theory in order to try to design the most efficient sail shape. As discussed for keels, the most efficient shape for minimizing vortices is a tall, narrow sail, although as we will see, such a shape is not the most optimum for downwind sailing and is often not practical. Ultimately, as was the case with keels, the best shape is one that provides an elliptical distribution of lift along the sail. However, this shape is generally not very practical for mainsails, in particular, since you need to take advantage of the space behind the mast and in front of the backstay, which is why you generally see triangular sails.

Recently, it has become common, especially on racing boats, to see mainsail shapes that are more elliptical at the top in order to try to reduce the effects of induced drag vortices behind the mainsail. But this shape requires changes in the geometry of the backstay rigging and/or a willingness of the crew to “help” the mainsail move from one side to the other on tacks and jibes. Otherwise, this extra curvature or “roach” on the trailing edge of the main tends to get hung up on a fixed backstay, especially in light air".