Which profile is ideal for the rudder blade, different to the Swede 55 keel fin? What was common in advanced boat building in the 1970s? What is used today? Which shape helps to avoid the stalling effect? How you determine the ideal profile for your boat.
Following the first article on suitable bushings for the rudder bearing and the second one on repairing a rattling rudder blade, this article focuses on correcting the profile. As experts Peter Andrin Steiner, Albane Leclerc, and Kevin Bulpitt are working on the rudder anyway, they check the profile of the rudder blade.
Already in 2002, I had asked Prof. Sven Olof Ridder in Sarasota, Florida for the profile selected in 1975 for Swede 55. As a fluid dynamics specialist at KTH Stockholm, Ridder had contributed a specific NACA profile.
The rudder shape was improved with the help of glued-on templates- Photo Peter Andrin Steiner/Swedesail
Deviating from Knud Reimers’ original long-keel design, the Swede 55 obtained a contemporary underwater hull with a split lateral plan — consisting of a keel fin and a freestanding rudder without a guiding skeg. The details are described on the introducing page to Swede 55 and the page on the development of Swede 55. The wonderfully modest, helpful, and friendly Ridder explained the profile, how to best check it, and, if necessary, how to transfer the shape to the existing rudder blade in a handwritten two-page telefax.
Epoxy filler put on the rudder – Photo Peter Andrin Steiner/Swedesail
In 2013, the time had come to do it. I asked Jan Böhm, then head of the well reputed Lütje yard in Hamburg, how I should proceed. He recommended Jörn Kröger from the Technical University of Hamburg-Harburg. Kröger specializes in computational fluid dynamics (CFD), the computer simulation of flow conditions. Kröger examined the recommended profile, advised on suitable alternatives, and sent me a digital template that could be enlarged for templates with chord lengths of up to 76 centimeters.
The profile of the rudder blade is not ideal
The template test revealed that neither the nose radius at the front nor the flanks of the rudder blade were correct. The leading edge, where the foil is exposed to the flow, is particularly important for rudder effectiveness and the lowest possible drag. The so-called nose radius was significantly rounder than desirable. It needs to be more elliptical. Steiner had already noticed this straight away when I brought the rudder to his workshop. Further, the rudder blade was missing a centimeter of width in the upper third of each side.
Comparison of the then planned NACA profile and the new HSVA profile – Dipl.-Ing. Jörn Kröger/Swedesail
The difference between the keel profile and that of a rudder blade is that the rudder has to cope with significantly larger angles of attack. This is particularly critical on fast spinnaker courses.
Kevin Bulpitt, a long-time Steiner colleague, has faired and painted many large yachts, from 24 meter long Wallys like the “Aori” and “Magic Carpet” to the J-Class “Velsheda.” He also has worked on large motor yachts, where surfaces with the immaculate finish of car bodies are expected. Steiner has collaborated with Albane Leclerc and Kevin Bulpitt since the mid-1990s, when he was working in the South of France on the Maxis of the Grand Mistral fleet for Pierre Fehlmann.
The first layer of filler is added- Photo Peter Andrin Steiner/Swedesail
How the Swede 55 rudder was optimized
So, templates were made and glued on the rudder. Bulpitt then applied the missing material to the blade using approximately 7 liters of epoxy filler in ten layers. To achieve this, the blade was coated alternately with epoxy filler and sanded. This was followed by three coats of epoxy and a final coat of antifouling. The latter being enough to keep the underwater hull free of fouling for a sailing season in the Baltic Sea. The material cost at the time was €300, and it took 30 hours of work.
Background on rudder profiles
Water is 8,800 times denser than air. From a fluid dynamics perspective, they are more or less the same substance. Here’s an in-depth look at the topic with references, links, and assessments.
The so-called NACA profile is a collection of various profiles from the US government organization National Advisory Committee for Aeronautics. You can find out whether your boat has a NACA profile by asking the designer, the yard, or the boat’s class association.
Every yacht designer has the following standard work on his shelf: Ira H. Abbott and Albert E. von Doenhoff: “Theory of wing sections, including a summary of Airfoil data,” Dover Publications, 1949/59, ISBN 978-0-486-60586-9.
Prof. Ridder had specified the NACA profile 63(2)-015 for the Swede 55 rudder blade. This sequence of numbers contains the following information: 6 = profile series, 3 = minimum pressure at 30 percent of the profile length, 2 = exceptionally low drag within +/- 0.2 cL of the design point’s lift coefficient (low-drag range), 0 = lift coefficient cL (in tenths) for an angle of attack of 0 degrees (here 0, since there is no profile camber), 15 = maximum thickness at 15 percent of the profile length.
The almost completely filled Swede 55 rudder blade in the Kressbronn workshop – Photo Peter Andrin Steiner/Swedesail
Dipl.-Ing. Jörn Kröger from the Technical University of Hamburg-Harburg pointed out that although this profile is low-drag, it is a so-called “laminar profile.” As a sensitive profile, it is not ideal for varying angles of attack. It is only suitable for small rudder deflections. It reaches its maximum lift at rudder angles of around 15 degrees. At larger angles, the drag increases sharply, and there is a risk of the flow breaking off. According to Kröger, even a normal rudder deflection for bearing off and compensating for the usual upwind tendency would cause the narrow range of very low drag characteristic of laminar profiles to be left behind.
In the book “Principles of Yacht Design,” Lars Larsson and Rolf Eliasson clearly distinguish between keel and rudder profiles. While the 6-profiles are generally applicable for keels, this profile series is only recommended for rudders on light and fast boats. For heavy displacement boats, NACA 4-series profiles are better.
In their study “Marine Rudders and Control Surfaces: Principles, Data, Design, and Application” for large ships, Anthony Molland and Stephen Turnock also recommend 4-section NACA profiles for rudder blades.
According to Kröger, profiles that have slightly greater drag in the neutral position than the one recommended by Ridder are preferred for rudder blades in shipbuilding. However, they are better able to cope with large angles of attack. Kröger cited the HSVA-MP73 as an example of an ideal rudder blade based on current knowledge. The abbreviation “HSVA” stands for Hamburg Shipbuilding Research Institute. According to Kröger, it offers a clever compromise between moderate drag and good rudder effectiveness at large angles of attack. Sailors interested in optimizing their rudders should take this as a starting point.
Regarding the nose radius (the leading edge of the rudder), the rule of thumb is: the larger the nose radius, the less sensitive the profile is to adverse separation during large rudder deflections. The recommended NACA profile has a narrow nose radius, making it less suitable.
Gamle Swede launched with the new rudder – Photo Swedesail
After selecting the appropriate profile, a job to be planned with some lead time, the corresponding profile was digitized and transferred to templates. For the Swede 55 rudder described above, Ridder recommended three template levels.
Photo on top by Peter Andrin Steiner: Kevin Bulpitt shaping the Swede 55 rudder in the Kressbronn workshop. Thanks to Naval Architect Juliane Hempel (Kiel & Radolfzell), Dipl.-Ing. Jörn Kröger from the Technical University Hamburg-Harburg, Prof. Sven Olof Ridder († 2012), and Peter Andrin Steiner.
