The underwater control surfaces on an FD are asked to perform more than one task. Both the rudder and centerboard provide lift upwind, and to much different extents provide directional energy. Both the rudder and centerboard shape, range of adjustment and weight are governed by the class rules.
Centerboard
The centerboard pivots vertically on an adjustable carriage pin fore and aft in the centerboard well, so there is opportunity to adjust the wetted surface of this blade, the amount of side force and with that movement an important component in the balance of the boat. The ability to change the position of the centerboard dynamically allows the sailors to keep the balance of the boat as close to optimal as possible through a wide range of conditions. Control systems are usually set up with purchase to ensure the centerboard can in fact be pivoted properly under load. You want a low friction interaction of the board with the inner contact surfaces of the centerboard well. One of the by products of achieving a low friction state with the board in the well is the occasional unintentional lifting of the board downwind in heavy air. I counteract that by attaching a bungee to the aft head of the board. The tension is just enough to keep it where you want it in heavy air, but still allowing easy movement.
Because of the particulars of the class rules, the shape of the centerboard is restricted to a certain perimeter profile and thickness. Also based on these restrictions the aspect ratio and chord thickness cannot be what would be considered optimal today. By ensuring that the board is at the class maximum of 23 mm and that it can be immersed to the maximum amount possible (1060 mm) below the hull surface, the efficiency can be maximized.
Different builders have their own takes on what constitutes the right leading edge radius, generally the design balance is providing a rounder nose for an easier groove to keep flow attached, vs. a narrower entry for less form drag. These differences are subtle. All of the good builders, Waterat and Mader being two good examples, do a fine job both in shape and engineering.
While some have experimented with some bend in the board, it is best to build a board as stiff as possible, transmitting as much energy from side force as possible to the hull. If the surface is smooth to the touch, it is considered to be smooth in terms of the drag of the water.
Different techniques have been employed in the construction of centerboards in an effort to achieve the desired strength and stiffness. The most common takes a composite approach, incorporating a vertical timber piece with foam as the core with carbon and fiberglass / epoxy skins.
Rudders
Rudders are naturally thought of as the piece of equipment steering the boat, but with the centerboard it performs other functions as well. When going upwind in the FD, if the boat is properly balanced the rudder acts as a trim tab, aiding the boat in creating more lift. Not only through the inherent shape of the blade but by it’s angle in relation to the centerline of the boat. Usually anything more that a few degrees off centerline creates significantly more drag than the amount of lift provided.
On paper a vertically aligned rudder blade would be considered more optimal in a hydrodynamic sense, but that vertical alignment requires far more attention to the rudder angle that other angles. As a result most FD rudders have a sweep aft of 5 degrees and more.
In future articles we will explore how best to manipulate the foils for maximum performance.
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Description of the centerboard and rudder designs, differences in manufacturers, construction, rigging options, etc.