Keels and rudders are hydrodynamic foils - when they are moved through water they cause both lift and drag forces to develop. Lift is the positive force that allows a boat to move to windward - drag is the negative, resisting force.
A good sailboat keel design has a high lift-to-drag ratio. Meaning, a far greater amount of lift is developed compared to the drag created. Since a keel develops most of its lift from the nose section, any premature separation of water flow at the nose is detrimental to the effectiveness of it as a lifting device.
NACA foils are designed to produce more lift than drag while maintaining a longer laminar flow resulting in reduced drag. NACA foil sections have a parabolic nose shape as illustrated in Figure #1, middle drawing. This shape causes the local flow of water at the nose of the keel to change direction gradually and remain attached to the surface of the sailboat keel.
Shapes, other than parabolic (NACA) for their leading edge, such as keels with round bar or pipe, (Figure #1 bottom drawing), will result in angular shapes at the leading edge. This causes a premature separation of water flow forming eddies and turbulence. This increases drag in the area where it develops the most lift. Figure #1 top drawing shows an overlay of the two types of leading edges. Easy to see, there is a notable difference between them.
It is counterproductive to build a boat with the keel designed with round bar or pipe for the leading edge rather than a formed NACA section. Although the leading edge is easier to construct, it would not produce the lift of a NACA type keel at its nose. Why? Because it creates turbulence at the nose. Turbulence causes separation of the water flow, creating drag. The lift to drag ratio drops. A better performing sailboat keel has a high lift to drag ratio.
The most common NACA foil sections used in keel design are of the 00-Series foil family. While this family of foils is conservative, they provide good lift-to-drag ratios over a wide range of leeway angles. They are also less affected by surface roughness.
Other foil families used in sailboat keel design are the 63-, 64-, and 65 Series foils. These types of foil sections are be more valuable in high performance type boats designed for specific conditions. Other more advanced keel sections are possible but require extensive tank testing.
COMPUTER DEVELOPED KEELS AND RUDDERS
NACA type foils were designed and in use well before the onset of the computers. Now, marine design computer software has made it relatively easy to develop NACA foil keel and rudder patterns or offsets for the "one of" builder. This makes it cost effective and, more importantly, accurate.
To achieve maximum lift along with laminar flow around the nose, the keel needs to be fabricated to exact form specifications. Reviewing Figure #2, you can see the NACA sections are easily fabricated for only the aft 3/4 chord length of the keel - curvature is low in this area. The more difficult portion to shape from steel or aluminum sheet material is the forward 1/4 chord length or the nose section. In this area, the curve to be formed needs to be bent in a press break to exact specifications.
After a keel has been optimized, with respect to the foil type and planform using NACA sections (or other documented foils), the shape can then be developed to a flat pattern. This pattern is then transferred to computer numerical controlled cutting equipment (CNC) for cutting, or numerical offsets can be used for hand transfer directly to the plating.
Figure #3 shows a keel developed to a flat pattern, along with bend lines. In this development, the bend lines identify the location of the bends along each line at 3-degree angles. This is accomplished in a press break, available at any local sheet metal or fabrication shop.
The entire keel shell can be formed by this method. I recommend forming only the nose section and fit it onto the keel building jig separately because the size and weight of the material is awkward to handle .
The remaining aft 3/4 chord of the keel between the nose and the trailing edge is easily formed. It only requires a few hits in the press break. Keels formed with this method are true NACA type foils. For the extra fabrication time, you end up with a keel that provides maximum lift and minimum drag, increasing the overall performance of the boat.
KEEL BUILDING POSITIONS
There are two building positions used to construct any metal hull or keel. Upright or upside down. In my experience constructing the keel and hull independent of each other and upside down is easier than the upright method. See Figure #2.
The upside down method allows the builder to work at ground level most of the time. The building strongbacks for the keel and hull require far less material in their structures and simple in design.
KEEL TO HULL CONNECTION
The hull is plated from the boat sheer line towards the keel. This includes all hull plating except the opening required for the placement of the keel located between the transverse frames of the (also upside down)hull.
Since the sailboat hull is also built upside down, the keel is lifted into place on the hull using the same gantry used to build and plate the hull. The floor sections of the prefabricated keel align with the frames of the hull and tack welded in place. For more information click the below Link.
With the keel secured to the hull, the final section of the hull can be plated. After the hull is fully welded and ground smooth it can be rolled over. Ballast is then placed into the open top of the keel per specifications.
A FINAL NOTE ...
Builders can easily fabricate a sailboat keel or rudder using NACA foil shapes that provide optimum lift and minimal drag.