| DESIGN AND CONSTRUCTION OF CENTERBOARDS AND RUDDERS By | Paul Zander (paulz@ieee.org) Version 1.5, August, 2001
| Here is a summary of information on design and construction of centerboards and rudders that I have compiled from postings as well as mail with other readers of rec.boats.building. 1. Basic Concepts 1. BASIC CONCEPTS "Foil" is the common term that applies to wings, rudders, keels and centerboards. To the aerodynamicist and hydrodynamicist they are all the same. A basic concept of fluid mechanics is Reynolds number.
If two different situations have the same Reynold's numbers the fluid flow will be the same. This allows one to take results for airplane wings and apply them for center boards and rudders provided the Reynold's number is the same. If the Reynold's Numbers for two different situations are not the same, one can not make valid predictions of the fluid flow. It may be the similar, or it may be very different. Later there will be different suggestions applicable to the size and typical speed of different boats. Angle of attack (AOA) is the small angle a boat has relative to its flow through the water. If you are sailing straight down wind, the AOA of the centerboard or keel will be 0. Pinching into the wind with a lot of leeway will result in a high AOA. The AOA of the rudder naturally depends on what the helmsman is doing. Drag is the force parallel to the oncoming flow. Lift is the force perpendicular to the flow. Generally speaking, drag tengs to slow the boat. Lift on the rudder is how it turns the boat. Lift on a centerboard or keel is what makes it possible to sail up wind. All of the above are interdependent. For a well designed symmetrical foil, when AOA is 0, lift is 0 and drag is small. As AOA increases, lift and drag both tend to increase. At a certain AOA, lift will reach a maximum, and drag goes up rapidly. This is called the stall angle. Note that the hull also generates lift and drag which will effect the total performance of the boat. In a sail boat the flow of air over the sails also stalls if the AOA of the sail gets too high. This is the fluid dynamics explanation behind why you lose speed when you pinch into the wind. Determining a "good" foil shape requires either experimental models or a rather large computer program to determine the lift and drag as a function of AOA for a variety of candidate shapes. Then repeat that process for a range of speeds. Final select a shape based on the expected conditions and your sailing style. The problem is just too complex to have a computer program that solves Newton's Laws of Motion and cranks out the "best" shape. 2. CROSS SECTION SHAPES 2.1 Shape for Most Boats Many sail boats have foils based on NACA 4 digit airfoil design, which was defined in 1933 and based on a lot of trial and error by many people in the early years of aviation. Because it was based on small slow airplanes, the range of Reynold's numbers is similar to that of interest to sailors. Amazingly this design is still very good for boat foils, although some special exceptions are listed below. This shape is published [9] in table form as NACA-00XX, where XX is the thickness expressed as a percentage of the chord. Harold Ginsberg[2] has a shareware program, NACA4GEN [8], that will compute the shape and also generate computer files readable by other CAD programs. Here is the formula, if you wish to do your own computations:
NACA continued its work in developing better foil sections. The NACA 6 series has a region of lower drag for AOA of 1 or 2 degrees. It has been successfully used for airplane wings. Pollock[6] compared the two shapes and concluded for the Reynold's Numbers that apply to most sail boats, the older NACA 00XX actually had less drag. If you are building one of the high performance ocean going trimarans, the 6-series might be a better choice. Heck, if are building a big high-performance boat, you can probably afford a consulting engineer to explore this issue. Pollock[6] also analyzed the effect of different thicknesses. Thinner foils have less drag, thicker foils have higher stall angle and greater maximum lift. He summarized his results as follows: "For running the section would be as thin as possible while for beating and probably reaching, around 8 per cent is a good thickness from a drag point of view. Taken overall the practical range of t/c values is 8 per cent to 12 per cent with the thicker sections probably tending to be better for slower boats." 2.2 Thin Foils Class rules also are a consideration. I have a dinghy that has the thickness of the centerboard limited to just under 6 percent. In personal mail, Tom Speer[5] shared his results from a Fortran program known as "the Eppler Code"[14] which he managed to fit onto his PC. Speer compared the performance of the NACA 00XX shape to the design that has been used on some acrobatic aircraft. The "Extra" section uses an elliptical shape for the leading portion of the foil and a straight wedge for the trailing portion. The formula for an ellipse is:
For 9 and 12 percent thicknesses, the NACA shape is clearly superior. For 6 percent, the Extra shape is predicted to have less drag. (I am considering building one of each shape.) 2.3 Parallel Sided Foils Some classes specify the foils must have parallel sides with some amount of fairing of the edges allowed. Pollock[6] first considered an elliptical shape for the leading edge. A fairing length about 2t gives good lift, but high drag. A fairing length of 4t gives low drag, but not much lift. He developed a new section which, for low speeds, is nearly as good as the NACA 0004 section.
The best would be Xle = 4 * t, but 2 * t wouldn't be all that bad if the class rules restrict it. The trailing edge fairing should be as long as possible, with the suggested shape:
2.4 Really Thin Foils When the foil is really thin, it acts as a flat plate. There isn't much you can do to affect its performance. Analysis of even 6 per cent thickness shows much less sensitivity to shape than thicker foils. A limiting situation is model boats with sheet metal keels. Just round off the edges enough that they are safe to handle. 2.5 The Trailing Edge All shapes ideally have an infinitely thin trailing edge. Practical considerations, like nicks and dings, require a minimum thickness. All of the people who have studied this agree that the best thing is to compute the shape as if the foil were to be slightly longer. Then cut it off at the desired thickness with the edges square. 3. PLAN FORM Plan form is the aerodynamicist's term for the side view of the foil. The preceding analysis of cross section only used a 2 dimensional view, but the real world is 3D. Near the bottom of a foil, the water may turn toward the bottom edge instead of going straight back and as a result produce less lift. This can be minimized by making the trailing edge vertical and not swept back. Pollack[7] noted that while a "shark fin shape" might theoretically have less drag, he would, "hesitate to recommend this shape for sailing applications because it would be a pig to manufacture accurately and would be prone to tip stalling. The fish overcame this latter problem by having fins with some flexibility which deflect appropriately under hydrodynamic loading - very clever of them!" The trailing corner should be kept square even if the class rules allow it to be rounded. Wings on the keels of some racing yachts and fins on the wing tips of new airliners help make the end portion of the foil more efficient. Longer (deeper) foils will have a better lift to drag ratio, but also cause increased heeling because the lift is created further down. The length of the foil will be limited by some combination of class rules, frequency of running aground, and ability to keep the boat upright. If the foil must be short, then use a rectangular plan form to get all the area you can. For longer foils there will be less drag for the same amount of lift if the leading edge is tapered back. Some designs have a straight leading edge with the length at the bottom from 40% to as little as 20% of the length at the top; others use an elliptical shape. Depending on the design criteria, either might be better. Obviously a straight edge will be easier to build. 4. CONSTRUCTION COMMENTS Whatever material and technique you choose, to achieve the theoretical results of a computer you must be extremely precise when building a foil. Construct accurate templates. It is especially important to avoid any waviness or irregularity in the shape along either axis. Remember that rec.boats.building stands for "recreation" so take your time and enjoy the process of making sawdust or spreading gelcoat. Here are some rather unusual techniques to consider: The following technique for wooden foils was suggested by Richard Engelbrecht- Wiggans [1]. Compute the chord positions vs. thickness in 1/32 inch steps. Laminate an approximate shape from plywood and /or verniers. Use a belt sander to shape the board. If you do it right, the glue lines between the layers will be straight, and at the pre-computed positions. Since wood is too fragile to make the trailing edge of a foil very thin, he [1] also suggested using factory-made fiberglass glazing. Cut two strips of fiberglass as long as the trailing edge and several inches wide. Lay the strips on top of each other, and tape them together along what will become the trailing edge. Make a fixture with the desired angle of the trailing edge to hold the strips. Mix up a batch of epoxy with thickener to glue the previously made wooden portion of the foil to the fiber glass. The standard method for working with fiberglass requires first making a precision female mold. Strojnick[17] suggests this technique that is sometimes used for making airplane wings. Lay up a skin of fiberglass on a piece of Plexiglas. When it has partially set, but not yet hard, peel it off the plastic and form it around a male mold for the final cure. This gives a very smooth finish without the difficulty of having to make your mold very smooth. His books go into much more detail. Parker[4] suggested:
Another technique is to make a female 1/2 mold (r&l), layer with gelcoat, glass, carbon or whatever, then 1" strips of styrofoam. Vacuum bag the whole thing. When it is cured, plane off the styrofoam flush and epoxy the 2 1/2's together. This requires a smooth female mold. Speer[5] wrote: I saw a really slick technique at the University of Sheffield in Scotland that they used to make constant chord models for airfoil testing. They had a large block of beeswax - about 2 feet wide, 3 - 4 feet long, and around 4 inches thick - chocked in place on a table. They used a numerically controlled hot wire to cut the airfoil shape in the wax to make a female mold. Then they laid up a fiberglass half skin directly on the wax mold. Both halves were joined and bonded to an aluminum spar, and foam was poured into the inside. The wax was then melted down and recast into a block, ready for the next airfoil shape. The beauty of it was its reuseability and the accurate shape produced by the hot wire. There was no need to allow for the thickness of the skin, etc., because it was a female mold. Didn't even need a release agent! I have a friend who's a rigid wing landyacht builder/sailor, and there's a company near him in Portland that has a numerically controlled hot wire that they use for cutting styrofoam for packaging. They cut the cores that he used for the fairings on his axles (airfoil shaped to produce downforce and reduce heeling/skidding). There may be other industries that could fabricate components we would find useful. 5. EXPERIMENTAL RESULTS I built and tested a wooden centerboard based on the above formulas for parallel sided foils. For the leading edge, I started with a piece of hardwood, and used a router to cut lengthwise grooves marking the thickness at several places. Finally I smoothed the wood to the final shape with a belt sander. The body of the board started as a sheet of thin plywood which American lumberyards call "door skin". Several thickness were clamped and epoxied to achieve the desired thickness less one more layer on each side. The trailing edge was shaped with a belt sander. Then the final layer of wood was glued to the outside. This achieved the desired shape, and also a nicer wood grain finish on the outside. The last 2 inches (5 cm) of the trailing edge were cut down and thin sheets of fiberglass sheets were set in for additional strength. With the trailing edge completely shaped, I trimmed the excess from the front of the body, and cut a tongue which matched a groove cut into the mating surface on the leading edge. After checking the total length, everything was epoxied together. The resulting center board not only looks very good, but functions well. Numerous comparisons were made with other foils. This board certainly works much much better than ones made by taking a piece of wood and sanding until, "It looks good." However, I reluctantly concluded that a commercially made board with a fully tapered shape would allow the boat to point a few degrees closer to the wind while maintaining the same speed. Obviously accurately fabricating a board to a more complex shape would be more difficult than a parallel sided board. For now, I decided to focus on sailing instead of sawdust. 6. ACKNOWLEDGMENTS I am an EE / computer programmer, not a "fluid mechanic". What I posted here is a distillation of a correspondence with the following readers of rec.boats.building. They deserve the credit. [1] Richard Engelbrect-Wiggans (eplus17@uiuc.edu ??) 7. FURTHER READING The following books have been suggested. Unfortunately only the first was stocked at any of my local book stores. [9] Abbott and von Doenhoff, Theory of Wing Sections, Dover Publications, $13.95 US [10] Bethwaite, Frank, High Performance Sailing, 1993, distributed by McGraw Hill. ISBN 0070057990 [11] Larsson and Eliasson, Principles of Yacht Design [12] Marchaj, Aero-Hydrodynamics of Sailing, Dodd, Mead & Co. [13] Hoerner, Fluid Dynamic Drag, Fluid Dynamic Lift [15] Strojnick, Laminar Aircraft Technologies, Laminar Aircraft Design, Laminar Aircraft Structures Two references for vacuum bagging which could be useful: [16] Merrick, Gordon, Vacuum-Bag Veneering, Fine Woodworking Vol. 84, October, 1990 [17] Square, David Shath, Basics of Vacuum-Bag Veneering, Fine_Woodworking Vol 109, December, 1994 | |||||||||||||||||||||||||||||||||||||||||||