Ever since early sailors used square sails to help add speed to their oars, the sailboat has become increasingly more complex. The type of construction varied with the availability of materials. Wood was a common material, but was not available in areas like Lake Titicaca where the boats and sails both were constructed of reeds from the shoreline. When iron and steel became available, wood was replaced for the most part. And finally, the invention of steam removed sailing, for the most part, from daily shipping. Today sailing is enjoyed as recreational, and racing has become a very publicized event.
To understand modern theory, you must first look at the development of sailing in the working boats of old. Early developers knew that the speed of the boat was proportional to the size of the sail. They put huge sails on boats in efforts to speed shipping. Yet, the size of these sails were so large they became unmanageable without mass amounts of crew. Where labor was cheep, the Far East, huge sails were fitted, but for the rest of the world, a knew solution needed to be found. Sails became split into smaller units and ships carried more masts. Some ships had as many as seven masts and forty nine sails set at once.
Bit by bit, the sails evolved into triangles with moving points and arms that could turn, enabling people to sail upwind. From here we approach today. With computer assisted design and programs that can sail the boats before they are even made, we enter the twentieth century. One must realize that a sailboat may not sail at the wind and certain angles to the wind. This is called the luff zone. The efficiency of a boat in upwind situations can be determined by the angle at which the boat no longer gains forward momentum. The first sailboats had fixed sails and heavy improperly balanced ships.
They were very inefficient and slow. Today modern technology and mathematics join together to create boats that fit the formulas. Everything about to be described effects another characteristic of the boat’s handling and capabilities. When talking of sailboats, buoyancy and stability are the first two areas of concern. With the force of high speeds, the boat must stand true in the water and move precisely. There are two “centers” of concern in a sailboat that should be described. These are areas where numerous forces exerted on the boat and rigging come together.
The center of effort, or CE, is the geometric center of the set sails. The center of effort is the balance point of the sails and is very important to the speed of the craft. Also, the center of lateral resistance (CLR) aids in proper sail trim and boat balance. This is the center of all underwater portions of the boat. Without these areas in balance, the drive, forward motion, of the boat would be lost. It was Archimedes who first saw that a boat would displace its own weight in water. Based on this, ratios developed based on various areas of underwater hull, and areas of cubes with the same dimensions.
The most important being the waterplane coefficient. This is the ration between the underwater plane of the boat and a rectangle having the same length and width. These along with various other, and more complicated, things help the boat remain afloat. Buoyancy is quite important, but of an even greater concern is the boat’s stability. The stability is what gets the boat to stay up while the wind pounds at the sails. Here forces tend to play with the boat. Shown below are some of the major forces and how they act on the boat. The arrow showing righting lever force shows that the force is upward.
If the righting lever should for some reason shift, the boat becomes unstable and subject to roll over. Now that basic hydro-dynamics have been covered, the wind and sails are still left to discuss. Air moves in parallel streams that are very difficult to deflect. Low pressure air is far less dense than a high pressure area. A sail will achieve a low pressure area on the convex side giving it maximum lift. A sail is the same as a sideways airplane wing. On diagram A below, a sail is shown in its airstream. The air moves faster over the convex portion making a low pressure area.
Diagram B shows how the sail starts to lift and push forward when the wind flows across. A sail works much like the wing of an airplane. The sail is the same shape, but is vertical instead of horizontal. They obtain lift when force is applied. For a sailboat, this lift is transferred into forward motion by the shape of the hull. If the sail was not attached to the hull, the sail would move sideways. For the actual sails, any shape will work, but many sails have certain shapes to perform in various situations and conditions. Many boats today for round the world races like B. O. C. use asymmetrical sails that are cut for use on just one tack.
Storm sails are much baggier and heavier with less height to maximize drag and reduce drive. Light wind sails are tighter and bigger to harness every possible amount of wind. The forces the sail must overcome are important to the motion of the boat. In the theory of sail, four forces are exerted at 900 to each other. Diagram C shows the relationship and placement of the forces and their interaction. When any force exceeds drive, the boat will not move forward. Early sailors started splitting their large ineffective sails into smaller, better sails. By chance, many created a system where maximum drive was achieved during windward sailing.
The air flowing over the sail acts like that over a larger airfoil with a greater velocity. The air that moves between the sails, speeds up and improves the flow over the rear sail. A lot of the skill involved in racing performance is gauging the inter-relations between various types of sails. The keel is most probably the most important part of a sailboats forward momentum. The keel prevents sideways motion, drift. The keel produces a drag and drive from the flowing water and the angle of the keel in the water from the heeling boat produces side force or resistance.
A successfully designed and sailed boat balances these forces in such a way that there is a surplus of forward motion. Playing with the shapes of the keel and hull, we can obtain boats that perform for specific speeds, weather or even areas of the globe. Many boats today in around the world competitions, have winged keels. These keels were so secret at the beginning of the 1987 America’s Cup, that the New Zealand team kept the boat covered during the weighting in. This keel is one of the three newest achievements in sailing technology. The fins slotting out allow the boat, when heeled to have a flat surface of keel in the water.
The second great invention in the sailing is the fixed wing sail. The testing had been on some smaller boats, but eventually ripped the course against the Kiwi’s in the 1991 America’s Cup. The fixed wing takes the “sideways airplane wing” analogy to the limit. Made of kevlar and other carbon compounds, this wing fired the American crew to a finish 17 minutes and 32 seconds before the New Zealand team. The wing along with the catamaran was so effective it beat its own predictions. It has long been assumed that no craft powered by the wind could exceed the speed of the wind that was powering it.
In mono-hull racing the fastest speeds were the Open 20 class who achieve around . 92 times the speed of the wind in 20 knot winds. The birth of a catamaran shattered these figures. With no deep, heavy keel and two narrow hulls, these boats can in effect “create wind. ” They use the eddies created in front of the boat to add extra speed. Essentially, the cat’s motion creates air disturbance that it then uses again along with the wind to move forward. The Super 18 Hobie has kevlar wings off of the hulls, enabling a crew to be far off the boat itself, and is the fastest boat available to the general public.
It achieves a surprising 1. 63 times the speed of the wind with a centered mast between the floats. Then a team from Stanford devised a new idea. They decided to fix sails on a boat with three hulls like diagram E. This boat is the current title holder as the fastest sailboat in the world. Technology effects everything from life and death procedures to the speed and handling of a sailboat. Science and sailing go hand in hand. With new alloys and carbon fiber compounds being found everyday sailing only has one way to go. The future holds new promises for lighter faster more exciting sailboats.
