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Rudder Design for C&C 35

WINGS 10 Designed C&C 35 Rudder

Many older sailboats are still being actively raced and cruised by families that love their vessels but wish they had some performance improvements that would possibly make the sailing experience a bit more exciting, safer or perhaps more competitive. I have had many requests for changes to vessels over the years and each one is a unique case.

It is very important however to understand that making structural changes to the keel or rudder is not a low cost project and it can be time consuming to complete. Making small changes such as accurately fairing a keel or rudder so that the foil shapes (running chordwise along the length of the keel or rudder) accurately reflect a well known NACA shape or at least so that the foil shape is symmetric on both sides can have some benefit and be a much lower cost project. Making small changes like fairing can be very valuable if the basic planform (outline shape of the keel or rudder) is fairly efficient design as built. Many times, rudders or keels that are very heavily swept back, have very odd tapers (root chord versus tip chord lengths), heavily curved edges and other deformations cannot be sufficiently improved by just making the foil shapes more accurate or symmetric. These keels and rudders can’t be improved without replacement.

Recently I was asked to redesign a rudder for a well loved C&C 35 that was still being raced on the Great Lakes. Because the owner had access to CNC machinery (Computer Numerically Controlled milling machines) an all new planform and foil shape was possible. The existing C&C 35 rudder was a heavily stylized 70’s design that had some serious defects such that no amount of fairing changes would be useful. Given these two factors a new design was deemed reasonable.

The owner required that the existing swept back rudder stock be re-used for the new rudder. This posed a serious challenge for trying to improve the Lift / Drag performance by more than just changing out the foil shapes.

Original Design

The original rudder design shown above was also installed at a 25 degree sweep back angle built into the rudder stock. It would require major structural changes to the boat to revise that aspect of the rudder design, so it was necessary to look for an alternative that would potentially reduce the impact of the sweep back angle.

The design above actually only has a very small central region where the the leading or trailing edges are not massively swept or tapered. The upper end of the rudder also has a very large aperture that made a large gap between the hull and the rudder top, causing loss of efficiency. The sweep back of the rudder stock that parallels the leading edge of the rudder caused the sweep of the leading edge in the lower third of the rudder to take an even more severe sweep angle relative to the water flow. Heavy sweep back angles result in loss in “Lift Curve Slope” (LCS) and increase in induced vortex drag. It can also result in the rudder stalling (loss of lift) at just modest angles of attack. This can be a serious safety concern when sailing off the wind, especially reaching under spinnaker or large Genoa headsail. “Round Ups” caused by heavy wind gusts can result in a broach if the rudder does not have sufficient control authority to keep the keel under the vessel.

Reduction in LCS means the rudder has to be steered to larger angles of attack to achieve the same lift or steering force that could have been achieved by a more efficient planform. This can mean the rudder stalls or generates high levels of drag under normal steering conditions.

After some consideration I arrived at the rudder design shown above. The drawing shows a heavy red line that represents the “Quarter Chord” of the planform. The sweep back angle of the Quarter Chord determines the LCS (Lift Curve Slope) of the planform. You can see that despite the required 25 degree sweep back angle of the leading edge, I was able to cause the sweep of the lower third of the rudder quarter chord to be reduced by at least 5 degrees by tapering the trailing edge of the rudder in this area.

The trailing edge taper has the effect of increasing the aspect ratio (ratio of rudder Span to Average Chord Length), reduce wetted surface, reduce lifting forces at the tip of the rudder and as a result reduce vortex drag.

New Rudder Design Rendered Using WINGS 10 STL file

The new planform design shown above will be created by CNC machining a new female mold and laying up glass mat and high density, closed cell foam. An interesting video that shows this process can be seen here: https://www.youtube.com/watch?v=HqBXDuI5NzY

In addition to the planform changes, it was very important to choose a foil shape. Let me rant for a moment regarding the tendency in sailing circles to misuse the the term “foil”. “Foil” does not refer to the outline shape despite the new AC 75 boats being called “Foilers” or when they sail to be called “Foiling” or being “on foil” etc!! An airplane does not “foil”, it flies on a WING. Not a foil! A FOIL shape is the specific NACA or Custom design shape the rudder or keel in the chord (fore and aft) direction. This shape is critical to lift, drag and stall characteristics of the overall planform – the outline shape of the wing or rudder or keel.

Simple examples of popular rudder FOIL shapes are NACA -0010 or 0012 as are cataloged in various books and online. Normally this is a good conservative foil shape to choose when compared with the more typical “low drag” laminar flow sections known as 63-010 or the higher 64, 65 or even 66 series foil shapes. These “60” series foil shapes have a definite place in keel design but are rarely good choices for rudders. Please look at my other papers to see explanations for why this is the case.

Lift Coefficient Vs Angle of Attack (Rudder Angle) for NACA 0010
NACA 0010 Foil Shape

The plot above shows the lift characteristics of the standard NACA 0010 rudder foil shape as a function of rudder angle. It is very clear that at angles above 9 degrees that the foil shape suffers a huge 60% reduction in lift that never recovered, even at angles as high as 20 degrees. Drag is also very high under these high steering angles so that the rudder becomes a brake rather than a steering function.

Custom Foil shape
Custom Foil Design

The new customized foil shape shown above takes its concept from a class of rudder foils known as “Fishtail” shapes. This type of foil and rudders that use “fishtail” foil shapes can be found in internet searches.

This particular variant of the “fishtail shape” was chosen for strength of the rudder and ease of manufacture. It was also chosen for its very graceful degradation of lift as can be seen in the plot above. Now it can be seen that at rudder angles above 10 degrees the rudder will continue to develop lift and will not have the dramatic loss of lift experienced with the NACA 0010 series at angles above 10 degrees. This will provide the owner with a substantial margin of safety when sailing on the Great Lakes in heavy conditions.

True “fishtail” foil shapes have been shown to achieve exceptionally high levels of lift coefficient without stalling and exhibit a very high foil shape “Lift Curve Slope”. But these shapes can have some serious mechanical structure issues at the trailing edge and are challenging to actually implement. Never the less they are used extensively on ships, tug boats and work vessels that require exceptional maneuvering capabilities with very large steering angles and must have very responsive helms at even low steering angles.

Despite these high lift features the new custom foil shape shown above has an exceptionally wide Drag bucket” or region of low drag coefficients that do not change rapidly with steering angle.

Summary

Despite all appearances, the new rudder design has virtually identical surface area to the original design. The more consistent chord lengths of the new design will keep the vast majority of the rudder in the same Reynolds number range (operating conditions) versus the original design with a very large variation in chord lengths that resulted in a huge range of Reynolds numbers. Small chords operate at low Reynolds numbers and result in highly degraded performance relative to the longer chords. Also the very short chords are typically never properly implemented and are achieved by simplistic fairing and smoothing “by eye”. This rarely results in good behaviors.

We expect that the CNC machined mold for the new rudder will result in high accuracy implementation of the foil shape and the new rudder planform will dramatically improve steering response and offer better “feel” in heavy round up conditions that are far less likely to result in a broach caused by loss of control (lift) at the rudder due to the old planform shape and the new foil shape. Drag will be reduced because the rudder will deliver the needed turning forces at small rudder angles. Smaller and less frequent movements of the rudder will result with an overall reduction in drag due to dynamic conditions.