SEDATION 4  Rudder Design

Rudders are critical to sailing performance.  Undersized lack control while oversized induce excessive drag.  Unlike aircraft wings, rudders must operate equally well at extreme angles-of-attack.  Old-school aerodynamic theories are still applied to the design of underwater wing sections.  The common cross-section profiles are known as NACA sections.  Here's a table defining 2 popular NACA profiles applicable to yacht rudder design. Special thanks to Lars Larsson and Rolf Eliasson for their comprehensive Principles of Yacht Design book.  Several of the charts and illustrations presented here were scanned from Chapter 6.
I adopted the 63- series based on drag and lift criteria.  SEDATION 4 is large and not very aerodynamic so rudder loads are anticipated to be much higher than ordinary "swoopy" catamarans.  At higher angles-of-attack (more turns of the helm's wheel) the slimmer NACA profiles exhibit several undesirable characteristics.  The fatter 63- series is more forgiving in this performance envelope.
I designed the rudders based on a sail-plan area of 768 ft² - 20% bigger than a typical 37' cruising catamaran.  The rudders can always be retracted (raised) to reduce drag, but they don't magically grow if more surface area is required.  Total rudder area is 12.29 ft² so each rudder has  6.145 ft² (884.7 in²) of planform area.  The NACA 63-015 profile is consistant along the travel length, to enable sliding up-and-down within the mount for shallow water operation, then taperes to the tip with a 50% scaled 63-015 profile.
The leading-edge geometry is critical in reducing drag at high attack angles.  The nose radii of 3 popular NACA profiles relative to variances in thickness ratio are shown at right.  SEDATION 4 rudders use a thickness ratio of 15% so the basic nose radius is ~1.7% of the chord length.
The influences of nose geometry on lift and drag, at various angles-of-attack, are substantial.
Flutter (oscillating vibration induced by cavitational vortices as they break from the trailing edge) is a very undesirable characteristic in rudder performance.  Comparisons of trailing-edge geometry's' influence on flutter made the choice simple.  The asymmetrical trailing-edge shape (#8) is least  vulnerable to impact damage, so that decision was a no-brainer.
Calculating the 63-015 profile geometry was trivial.  A spreadsheet was created with basic entries from the NACA Section data above, then appropriate constants and scale factors were integrated in the formulae.  Using my trusty Ashlar Vellum 3D software, I created the profile shown here. Click the image for a detailed page of profile information.
Exporting the splines in DXF format to Pro/DESKTOP was easy.  The travel distance was extruded first.  The tapered skeg was then lofted between it and a 50% scaled 63-015 profile at the bottom.  Training edge details were added and I changed the model's color to my favorite color.  Time to make some molds!