s a developer of yacht design and analysis softwai-e, I have i long awaited a chance to iLss% comment on what I consider /;/ %to be an incoiTect keel design technique-the cuiTent practice of using a very short keel root chord length on fm keels. (For those of you who don't deal often with "foil" hngo, the "Iceel root chord" is the section of the keel closest to the hull. For definitions of this and other terms, please see the glossary.) X TjTDically, a short root chord is com­ bined with an "elliptical" keel shape, which allows the use of longer chord lengths lower, near the middle of the keel draft.--- - ..-. .^ - ;' The logie of the current elhpt)cal keel design methodology appears hard to arfrue with, because the short root chord is intended to reduce the keel/huli interference drag (see glossary), and the longer chord lengths near the middle of the keel span can hold large volumes of lead, thereby significantly lowering the vertical center of gi-avity of the keel. The lower center of gravity means improved stabihty, and increased speed. The only obvious design problem is the thm root section, which makes attaching the keel to the hull very difficult. Using the "ellip­ tical" planform shape to achieve a lower center of gi-avity is a good concept, but I don't beheve it is necessary to incur the difficult mechanical problems of a very small root chord to reduce drag. My chance to prove that the small root chord is an unnecessary evil came when I was asked to design a new keel for the J/35 My Fair Lady. The new keel was part of an overall plan to make her more competitive in the 19S8 SORC IMS class. In order to understand how the design of tliis J/35 keel evolved, let's ni-st examine the problems I believe are m- herent with the current ellipticaUieels. Besides the obvious atiachmeni problems the short root chord creates, there are two main areas where perfor­ mance may suffer: 1. reduction of the end-plate effect (see glossal^) of the keel against the hull; 2. the placement of the longer chord lengths near mid-draft of the keel causes the center of effort to move towards the tip of the keel, especially when the elliptical shape is combined with a large leading-edge sweep-back angle. David Vacant"! explains the innovative keel Now let's explore these concepts ir he designed for the SQRC-winning J/35 David Vacanti is in his lUh year as a My Fair Lady, and makes a case for a long Pj-incipal Engineer at Boeing, anc keel mot chord. works exclusively on advanced researcl projects in the Boeing High Technolog] Center. He also owns Vacanti Yach Design, a small softioare clovelopnien fvnn that provides advanced yach design and analysis software for IB1\ PCs. performance. and showed clearly that large sweep- more detail. In order to understand my back angles on high-aspect ratio keels fu-st point about a reduction in end-plate Another factor that exacerbates the design problems of the short root chord are detrimental to performance.^ effect, let's take an extreme case where It is important to note that signifi­ we shorten the root chord until it no is the sweep angle of the leading edge. Towing tank di-ag tests, conducted at cant amounts of drag are generated by longer attaches to the hull, and we have keels with large sweep-back angles some magical way to carry this keel (gi-eater than 25 degi-ees). Minimum along under the hull as we sail. As we Typical Elliptical Keel, vortex drag for any given keel sweep shorten the root chord, we diminish the angle is achieved by selecting the proper end-plate effect to the point where the taper ratio. (See Taper Ratio gi-aph.) keel is removed from the hull, and we However, the taper ratios for large have an open end generating additional sweep angles requii-e an extremely short vortex drag and reduced lift. Now, in­ keel tip that approaches a point, while stead of having only the keel tip open smaller sweep-back angles allow longer and generating vortex di-ag (remember keel tip lengths. Since the extreme taper that wings were added to Australia IFa ratio required of large sweep-back keel to reduce vortex drag), the open angles is never achieved in practice, root chord is also creating vortex drag. these keels exhibit high levels of vortex When the keel is attached to the hull, drag. Even if the proper taper ratio is more lift is generated because there is achieved in highly swept keels, the only one open end of the keel for the minimum vortex drag levels of these water flow to escape around. However, keels is always higher than that of a there is some undesirable interference minimally swept keel. di-ag at the keel/hull joint. Not only does a large sweep-back My second concern with the cm'rent angle cripple the performance of high- eUiptical keel, designs and their short Delft in the Netheriands, showed that aspect ratios keels; it also helps move root chords is with the keel's center of the keel center of effort still lower on the effort. If you have sailed an unballasted sweeping back a keel's leading edge 45 degi-ees or more is beneficial in reduc­ keel span by encoui-aging water to flow racing dinghy with a centerboard, you down the span of the keel instead of are keenly aware that heel angle can be ing the drag of a low-aspect ratio keel along the desired root-chord direction. considerably reduced when sailing up­ with long chord lengths, while a much Obviously, sweep-back angle simply wind in heavy air by pulling up the smaller sweep-back angle is required for compounds the problem of reduced centerboard part way. A sailboat hter- high-aspect ratio keels with shorter stability described above. ally trips herself on her keel when chord lengths.'These experimental find­ • Wliile the mathematics and scientific resisting the sideforces of the sails when ings are also supported by research papers are impressive, there is nothing hard on the wind. This occurs because work done for the keel of Stars & Stripes more convincing than a winning record the keel side force or lift acts at a center '87, which stated in summary, "The in a major saihng event. The SORC- of effort location below the center of primary planform parameter which can winning keels carried by Abracadabra buoyancy, opposite to the direction of affect the level of viscous drag is the in 1986 and Splint in 1987 sported the sail forces, which act through the leading-edge sweep angle. Large sweeps longish root chords, 15-degi-ee sweep- sails' center of effort located high above can have an adverse effect on the back angles, and a special high cross- the center of buoyancy. The result is a laminar boundaiy layer...Optimization section NASA foU shape after they were large rolling moment to leeward. Mov­ of foil shape and planform geometry for optimized by Bernard Nivelt with my ing the keel area such that it concen­ low viscous di-ag can produce an efficient analysis software. trates lifting forces farther down thé keel capable of overall sailing perfor­ keel span, like that shown in elliptical mance gains." ^ In 1985 I pubhshed a At this point we can summarize a keels, tends to lower the keel center of reference work on keel performance formula for low performance by speci­ effort and is detrimental to stabihty and parameters using computer analysis. fying an elliptical keel planform, com- Glossary winglets on 12-Meters and shoal-draft tip, the water leaving the tip is forced Chord: All lengths mecjGurod from the cruising keels. It's a means to force to spin. The spinning motion is called leading edge to the trailing edge of the water flow to remain over the a vortex, much like the swirl of water a keel, rudder or v/ing are referred to keel surface end prevent its escape seen in a bathtub drain. The spinning as chord lengths or chords. around an open lip. waterflow represents energy that was Interference Drag: Drag occurs Root Chord: The end of a keel or dissipated by the keel without gener­ whenever the energy present in a rudder that attaches to the hull Is ating lift. Any time we expend energy fluid flow is dissipated In some referred to as the "root" end. When without developing useful work, like wasted form without creating lift. we refer to the length of this end, we lift, we account for it as drag-In this Whenever two objects of different call it a "root chord." case vortex drag, sizes or orientations are placed in End-Plate Effect: Even the Wright Planform: The shape of the keel close proximit/ to one another, the Brothers understood that if an open when it is viewed from abeam of the fluid flow between them must adopt wingtip could be sealed with an "end hull, allowing the viewer to see the a speed and direction approximately plate" such as a flat piece of light leading and trailing edge shapes the average of the two Independent metal, they could prevent the natural flows. This cannot be done without Span: The height of the keel, escape of air from the wing bottom the slower flow speeding up (absorb­ measured from the root to the tip to the top side, and thus prevent a ing energy) and the faster flow loss of lift and generated drag.