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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:

Actual Rudder During Build Phase

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.


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.

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WINGS 10 New Features and Development

February 2021 Update

WINGS 10 is the NEW version of WINGS for Keel and Rudder Design NOW BEING released. It will replace all previous versions of WINGS including the current WINGS 32. The new version will include major improvements in display quality by supporting high resolution screens up to 4K. An all new Parametric Design Dialog will include a highly revised dialog screen with new input capabilities and includes an image of the keel or rudder type that is being designed via the parametric design process. The next few images show the sorts of parametric design methods we are discussing that are specific to rudder design. Keel design parametric data will also include keel bolt locations, size and material strength. All calculations are per American Bureau of Shipping for Offshore Racing Yachts. We may add CE / IMO calculations as well for stock materials and dimensions for various classes of ocean applications.

Keel or Rudder design via “parameters” (parametric design) requires the entry of a few defining overall characteristics of the keel and rudder so that WINGS 10 can automatically create a preliminary design and draw it for the user instantly. This allows for the very rapid creation of a design without the tedious entry of individual curves, lines, corners etc that many CAD tools are based on. All of our Vacanti Yacht Design LLC software tools are based on the parametric design concept. Programs like WINGS then offer detailed editing tools that permit the detailed refinement of the original Parametric design to meet specific needs. This allows infinite design variation and optimization.

WINGS 10 now includes Keel Bolt Design Capability and provides required minimum bolt diameter per 2021 ABS Rules for Offshore Sailing Yachts. The image below shows a standard fin keel with bolts drawn per diameter and per location in the bolt table. Depth of the bolt is shown in the PROFILE view below left.

WINGS 10 computes the required minimum bolt diameter based on keel weight, center of gravity, bolt locations and choice of steel for the bolts. A list of 24 Steel bolt choices is provided that ranges from Aqualoy to Stainless 304 and beyond. User can enter Mass of the vessel for use in computing grounding stress per ABS rules, Pressing Calculate button updates all bolt locations in the drawings and computes new stresses and min bolt diameter at base of thread depth. A Text Data file for all bolt and material information is recorded for use in implementation of the keel.

Flanged Keel Design Bolt Placement

WINGS retains the same NURB (Non-Uniform Rational B-Spline) surface design that we made popular in PROLINES. The only imitation to the NURB surface is that we enforce the aerodynamic shape of the foils (NACA 00, 63, 64, 65, 66 Series and custom foils designed in FOIL 10). Other than that the overall shape and thickness distribution is entirely editable by the user.

WINGS 10 Keel Rendered with WIN 10 Built in Tools

WINGS 10 retains the ability to use any combination of foil shapes to achieve a desired volume distribution along the span of keels in particular. While it is not likely that a rudder might use more than one foil type a user many also modify the placement of any number of foil shapes as a function of span.

WINGS 10 will include the new STL (Stereo Lithography) file format that is now available in the PRO version of PROLINES 8. This STL file will permit 3D printing of scale models of keels and rudders, export to hull drag analysis packages such as HydroComp NavCad and permit 3D CNC machining of molds or finish milling for Keel and rudder manufacturing. STL files can also be used for photo realistic rendering of any keel or rudder design.

WINGS 10 Now Directly Calls WIN 10 Rendering Tools

We are also working on new Rudder Design features such as rudder stock thickness distribution and rudder torque and bending calculations to permit safe design of steering gear and rudder stock material selection.

We invite comments and suggestions for desired WINGS 10 features and reports of existing bugs or fixes that may be needed in WINGS 32 so that we can address those issues before WINGS 10 release.

Cheers and all the best in Keel and Rudder design!

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PROLINES 8 Improvements Under Development

We have received some requests for changes and improvements to PROLINES 8 (all versions) that include use of the mouse wheel for zoom control in any view (once a view is active) and ability to rotate the perspective view by holding the Left Mouse button down and dragging. Both of these concepts are under review for incorporation in a future release of PROLINES 8.

We are also currently working on changes to the initial Parametric Design for a new Hull Design. We have added a new Catamaran Hull type that allows for the instant creation of a dual hull catamaran for power or sail. This new hull type will require changes to Hydrostatics and Stability calculations before it can be released. This is a significant change and will require considerable work to validate all calculations before the new version is released.

We are also considering the addition of a body view image of the selected hull type during the parametric hull design (File, New…) design dialog. This will better indicate the basic shape of the hull type that is being designed.

We plan other changes to PROLINES existing fairing tools to offer easier to use and understand data plots of curvature or slope of hull shape in various planes. We have already heavily improved the Edit, Visual Vertex feature in PROLINES and may add capability there as well.

Please write and send us your concepts for PROLINES features or comment on any issue you have found so that we can correct any errors.

Cheers and all the best for great boat/ yacht / ship designs with PROLINES.

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PROLINES 98, PROLINES 7, LOFT 2011 and FOIL 5 Versions are Now Obsolete

As of October 31, 2020 Vacanti Yacht Design LLC will no longer honor requests for License Keys or technical support for any version of PROLINES, LOFT 2011, FOIL 5 or older other than PROLINES 8, WINGS 10 and FOIL 10. All previous versions of PROLINES 5 (DOS), PROLINES 6 (Windows 3 Thru Windows XP) and PROLINES 98 or PROLINES 7, FOIL 5 and older will be obsolete. Due to changes in Windows operating systems 7,8 and 10 we can no longer offer support for these older versions of PROLINES, LOFT and FOIL.

We do offer low cost upgrades for each version of PROLINES, LOFT and FOIL that have been designed for Windows 7, 8 10 and beyond. We know that there are thousands of users of the older versions of PROLINES, WINGS and FOIL so we make an unlimited offer for an upgrade to the new version of PROLINES , WINGS or FOIL that will not expire with time.

In addition to adopting new Windows 10 technology features, PROLINES 8 now includes many new features for editing, new 3D CAD file export, direct integration with HydroComp NavCad that is an excellent Propulsion analysis tool for all PROLINES hull designs. Many more features will continue to be added to PROLINES 8 at no cost to existing owners of PROLINES 8. We encourage you to consider upgrading soon. WINGS 10 will also include STL file export as well as many other new features.

November 2020 Update on WINGS 32 / WINGS 10.

WINGS 10 is currently still in development with many major new features being installed and tested. New features include ABS calculations for Rudder Stock and Keel Bolts, Improved Parametric design tools, Improved display and tools and support for very high resolution 4K screens. WINGS 10 will offer separate parametric design entries for rudders and keels and separate hydrostatics calculations.

We will support WINGS 32 owners until WINGS 10 is released. No further unlock codes or other technical support will be provided for WINGS 32 once WINGS 10 is released.

New PROLINES, WINGS, FOIL Features Solicited

We encourage you to contact us and send requests for new features that could potentially be added to our software. Please us a Contact Us form to send your suggestions.

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Cruising in San Juan Islands of Pacific Northwest

August in the San Juan Islands brings warm weather, light winds and opportunity for enjoying the best of boat types. Our style is a North Pacific 39 pilot house trawler. Speeds to 9.5 knots but cruise at 8 allows needing just an hour or two to reach most destinations and up to 4 hours for longer trips. Many enjoy high speed cruisers that run in the teens and near 20kts. But they can leave huge wakes and cause problems for shorelines and other boaters.

Anchorages are protected and beautiful in the San Juan’s. Plenty of room for lots of boats of every description. The beautiful area compliments some lovely boats as well.

In these COVID19 days people are friendly and mutually respectful wearing masks when gathered on trails or small stores for supplies.

Shaw Island General Store has a long history and is the center of traffic on and off the island with a Washington state ferry terminal adjacent to the store.

Local Dungeness crab is a delicacy alone or added to any meal. Crab is readily caught with traps and requires little cleaning before being prepared for dining.

Choosing the right anchorage is critical to a good night’s rest when winds in the nearby Strait of Juan de Fuca can reach 30 knots overnight usually WSW but can become southerly and spread among the islands. Steep chop spaced close together can challenge any boat design at rest or underway.

Having boated in the Pacific Northwest for over 30 years on 4 boats we owned and others we chartered has lead to an appreciation for many boat types and concerns about others.

Early mornings frequently are accompanied by ribbons of fog. As the sun rises it burns off and is replaced by warm sun and modest breezes.

Anchored in Parks Bay a research preserve on SW Shaw Island

The forecast is for rain and wind late tonight. So boats have gathered in Parks Bay to gain protection from expected strong Southerly and South westerly winds. The bay is open to the North but tall trees and rising hillside provide protection.

UW Research Dock

The south end of the bay shallows slowly after much of the bay is 45 feet deep. At the head of the bay a UW Research dock is graced by an older trawler that likely transports staff to and from nearby Friday Harbor where a large research facility is installed.

The head of the bay shallows to a more useful 20 to 25ft for anchoring.

High thin clouds and a steeply dropping barometer portends change is coming. In 72 hours the calm should return.

Sunset at Parks Bay

Sunset with gathering clouds was followed by a wonderful peaceful night with no wind in the bay despite a strong WSW wind blowing in the Strait of Juan de Fuca just a few miles south.

Early morning brought steady cool rain that ended just before departure at 9 AM for Island Marine Center in Fishermen’s Bay just 7 Nmi to the SE.

Our trip to Fisherman’s Bay on Lopez Island with a squall arriving was complicated by shoal draft of barely 5 ft at low tide in the serpentine entrance channel to the bay. Clearly we needed to plan for tide and wind conditions when choosing a departure time. We consulted our subscription to PredictWind weather service to note that winds would be lighter before 10 AM and that tides provided by Navionics charts confirmed a 10 AM arrival would be safest to navigate the entrance channel.

Entrance Channel

Planning was important because those 7 miles cross San Juan channel and are open to the wind from the adjacent Strait of Juan de Fuca.

Our planning proved accurate as winds were modest and depth in the approach channel was excellent. Shortly after our arrival winds increased dramatically and have been blowing well over 20 and gusting 28.

Wind in the harbor

Mostly clear blue skies accompanied the wind all day

Lopez Island looking West

Friday promises calm winds and a chance to explore nearby anchorages.

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February 2021 News Updates

Watch our Blog Section for information on Boating in the Pacific Northwest, Marine Radar and AIS and Progress on any of our software products

WINGS 10 Released this week!

WINGS 10 now includes Rudder Stock Design and Analysis per ABS Rules and Keel Bolt Design and Analysis Per 2021 ABS Rules as well. Full graphical display of the chosen rudder stock and rudder or keel bolt pattern and bolt locations is provided in both cases. Now designers can not only create CAD files and compute LIFT / Drag and hydrostatics analysis for Keels and Rudders they can also generate required installation hardware information for both Keels and Rudders. This makes WINGS 10 substantially more useful than any previous version and serves the needs of Professionals and Home Builders alike. WINGS 10 now also includes full 3D Realistic Rendering using WIN 10 tools directly within WINGS 10. With this release any user who has purchased WINGS 32 after November 11, 2020 is entitled to a free upgrade to WINGS 10.

We are also working on B-PLATES and ABS Construct versions for release and use with the new PROLINES 8 Rx.x editions (Basic, PRO and Student)

PROLINES 6, 98, 7, LOFT 2011 and older, FOIL 5 and older are now obsolete and are no longer supported. We encourage users of these programs that have been around for 10 to 20 years to consider upgrading to PROLINES 8, LOFT 2018 and FOIL 10 that offer many new features and will operate on WIN 10 and later.