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.
Design Result Update
We are really pleased to report that the C&C 35 was raced in the 2021 Trans Superior Yacht Race and took 1st Place in Class and placed 10th Overall on corrected time. The owner supplied an excellent presentation that shows the manufacturing methods he used and the results of the race, including crew comments on how the boat behaved with the new rudder. There was significant experience with this boat on similar races with the old rudder so the crew were able to comment with good knowledge of past and present performance.
So far as Vacanti Yacht Design LLC is concerned the most telling comments relate to what the crew described as the elimination of “weather helm” and not needing to “chase the sail” downwind. The specific “Fish Tail” foil shape we designed was specifically created to provide exceptionally large “stall angles”, that angle where lift coefficient suddenly drops to an intermediate value from near max lift and cuts overall lift (steering force) in half or more. The “fishtail” design does not stall until well beyond 20 degrees. This means the rudder will continue to provide control authority even at extreme angles of attack without adding a large step in drag as would otherwise commonly occur.
The planform shape was also designed to reduce the apparent sweep back angle of the mid chord line, especially near the lower 1/3 of the rudder span. The very highly swept and rounded leading edge of the original rudder design caused some serious problems with performance. The highly swept tip of the original led to the weather helm and loss of speed (drag increase as the rudder acted as a brake not a lifting surface. What really happens in a planform like the original is that the very short chord lengths result in very low Reynolds numbers for that region of the rudder. It is also likely that the very short chord lengths in that area are not true foil shapes but are simply cut off sections of the main foil shape so that there is no proper leading edge and transition shape present. This leads to a lot of problems with high drag flow over the planform.
The high sweep back angle of the leading edge makes it worse because it induces span-wise flow down the rudder from the root to tip. Transport Aircraft use sweep back to reduce apparent airflow speed via span-wise flow to avoid developing super sonic shock waves and exceptionally high drag. Sweep in keel and rudder design is useless really and is only used in moderation to achieve a desired taper from root to chord. Rarely is more than10 – 15 degrees of leading edge sweep back ever justified in keel or rudder design.
Vacanti Yacht Design LLC was prevented from completely changing the leading edge sweep back angle because it would have required massive structural hull design changes to implement a vertical rudder stock. So the design was done in a manner to reduce the impact of the sweep back to the largest extent possible. The near vertical trailing edge at the bottom of the rudder causes the location of the mid chord line to become more nearly vertical in this region.
The owner, Dan Larson commented on the addition of 6″ near the root chord – he is referring to the region between the hull and the top of the rudder aft of the rudder post. The water flow this far aft on the hull is not laminar and will be turbulent especially at hull speed (1.34*Sqrt(LWL in feet)). For the section of the rudder above the rudder post and up to the hull there is no leading edge section of the foil shape. Thus it is not clear how the turbulent flow along the hull and the rudder will interact. Apparently this resulted in cavitation (vibration reported by the crew) – which is the formation of air bubbles as flow separates from the hull or the rudder. The crew comments suggest the bubbles formed just against the hull. This problem has occurred because the huge leading edge sweep back angle created the gap in the first place. Modern hull designs with more nearly flat aft sections and nearly vertical rudder stocks will not have this issue.
Here now continues the original design methodology narrative….
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.
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.
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.
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.
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.
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