TEACHING STUDENTS TO SAIL
UNDERWATER SURFACES The first half of how a boat is propelled forward was covered in Sail Theory.
The other half of the explanation for how a sailboat is propelled forward are the underwater foils (i.e. daggerboard, centerboard, leeboard, and rudder). Oftentimes the difference between being fast and slow is the shape and condition of these underwater foils.
Basic Foil Theory The concept of high and low pressures on either side of the centerboard is central to sailing efficiently. The centerboard creates “lift” in much the same way that a sail does. When a sailboat is sailing upwind, it is continually slipping slightly to leeward. Therefore the water is hitting the centerboard on the leeward side first.
Windward Side L Centerboard
The windward side becomes the low pressure side; the leeward side the high pressure side. High pressure (leeward side) wants to move toward low pressure (windward side). The result is a force to windward lifting the sailboat upwind.
Leeward Side
When the lift of the sails is combined with the lift of the underwater foils, the result is is the sailboat moves straight ahead. In actuality, a sailboat always goes just a tad sideways. When the underwater foils are not optimized or the water flow is turbulent, the underwater foils develop “drag” through the water. This “drag” reduces the amount of lift and the sailboat slips more to leeward.
Importance of Surface Condition If the foils develop any “drag” at all, they quickly reduce the amount of lift the foil generates. Therefore it is critical to keep the foils as smooth as possible. Be sure the foil is, kept clean and polished.
early on so they understand the importance of the underwater foil.
Centerboards A centerboard pivots on a central pin that allows it to be raised and lowered. There are a variety of different systems for raising and lowering the centerboard. It is important that the system works easily, and that the centerboard can be safely secured when raised. Some sailboats have preventer lines that, when cleated, keep the centerboard stable and prevent it from falling down into the centerboard trunk in the event of a capsize.
It is good to teach your sailors to care for the foils
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LIFT
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