Rudders are critical to sailing performance. Undersized lack control while oversized induceexcessive drag. Unlike aircraft wings, rudders mustoperate equally well at extreme angles-of-attack. Old-school aerodynamic theories are still applied tothe design of underwater wing sections. The commoncross-section profiles are known as NACA sections. Here's a table defining 2 popular NACA profilesapplicable to yacht rudder design.

Special thanks to Lars Larsson and Rolf Eliasson fortheir comprehensive Principles of Yacht Design book. Several of the charts and illustrations presented herewere scanned from Chapter 6.

SEDATION 4 - Rudder Design http://www.sedation4.com/rudder.html

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I adopted the 63- series based on drag and liftcriteria. SEDATION 4 is large and not veryaerodynamic so rudder loads are anticipated to bemuch higher than ordinary "swoopy" catamarans. Athigher angles-of-attack (more turns of the helm'swheel) the slimmer NACA profiles exhibit severalundesirable characteristics. The fatter 63- series ismore forgiving in this performance envelope.

I designed the rudders based on a sail-plan area of768 ft² - 20% bigger than a typical 37' cruisingcatamaran. The rudders can always be retracted(raised) to reduce drag, but they don't magically growif more surface area is required. Total rudder area is12.29 ft² so each rudder has 6.145 ft² (884.7 in²) ofplanform area. The NACA 63-015 profile is consistantalong the travel length, to enable sliding up-and-downwithin the mount for shallow water operation, thentaperes to the tip with a 50% scaled 63-015 profile.

SEDATION 4 - Rudder Design http://www.sedation4.com/rudder.html

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The leading-edge geometry is critical in reducing dragat high attack angles. The nose radii of 3 popularNACA profiles relative to variances in thickness ratioare shown at right. SEDATION 4 rudders use athickness ratio of 15% so the basic nose radius is~1.7% of the chord length.

The influences of nose geometry on lift and drag, atvarious angles-of-attack, are substantial.

Flutter (oscillating vibration induced by cavitationalvortices as they break from the trailing edge) is a veryundesirable characteristic in rudder performance. Comparisons of trailing-edge geometry's' influence onflutter made the choice simple. The asymmetricaltrailing-edge shape (#8) is least vulnerable to impactdamage, so that decision was a no-brainer.

SEDATION 4 - Rudder Design http://www.sedation4.com/rudder.html

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Calculating the 63-015 profile geometry was trivial. Aspreadsheet was created with basic entries from theNACA Section data above, then appropriate constantsand scale factors were integrated in the formulae. Using my trusty Ashlar Vellum 3D software, I createdthe profile shown here. Click the image for a detailedpage of profile information.

Exporting the splines in DXF format to Pro/DESKTOPwas easy. The travel distance was extruded first. Thetapered skeg was then lofted between it and a 50%scaled 63-015 profile at the bottom. Training edgedetails were added and I changed the model's color tomy favorite color. Time to make some molds!

SEDATION 4 - Rudder Design http://www.sedation4.com/rudder.html

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SEDATION 4 - Rudder Design http://www.sedation4.com/rudder.html

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