Which rudder?

Discussion in 'Sailboats' started by Fidippide, Sep 4, 2009.

  1. Fidippide
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    Fidippide New Member

    Hi everybody, I studied 6 naca profile with xfoil: 0008, 0010, 0012, 0014, 0016, 0018 at various Re (from 100000 at 1000000). What do you think should be best choice for a fast sailboat? Thanks.
     
  2. daiquiri
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    daiquiri Engineering and Design

    What studies did you preform with Xfoil, in particular and what are the results? And then, the main question - what are the goals you need to obtain? ;)
    Generally speaking, I wouldn't go neither over 12-14% nor below 10% thickness for several reasons:
    - thicker foils have a higher drag at high speeds
    - lift-curve slope decreases as thickness increases above 10-12%
    - maximum lift coefficient decrease both below 12% and above 14% thickness
    - stall angle is maximum in 12%-14% range
    - thin airfoils are prone to the formation of a leading-edge separation bubble
    - thin airfoils are prone to the leading-edge stall.
    So, as a first hint, a NACA 0012 could be a good compromise between all of the above.
    Bear in mind that choosing the airfoil is just a part of the job. The size and the shape of the planform has a huge impact on the final hydrodynamic characteristics of lifting surfaces like rudders and keels.
     
  3. Fidippide
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    Fidippide New Member

    I totally agree with you. Thanks.
     
  4. jehardiman
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    jehardiman Senior Member

    While an approximate 0012 2D shape is going to show an optimum at speed (actually its about 0011), a 0018 shape is a much better choice as the whole reason for a high lift rudder is to control at LOW speed. Almost any shape with a thickness under 20% (including a rectangular block with 5% of the corners knocked off, go check the data) will work at high speed. But a rudder needs to control across the speed range and therefor should have as high an angle of stall as possible. That leads to thicker sections with lower aspects.

    FWIW, I wouldn't use a NACA 00XX airfoil shape for a hydrofoil such as a rudder. It is too fine aft. I would recommend a DTMB EPH or at least a hyperbolic trailing edge. Also, sweep and taper effects on spanwise AOA will greatly overshaddow any difference between a 0012 or 0018 2D data.
     
  5. Paul B

    Paul B Previous Member


    With a comment like this I doubt you have any experience with fast sailboats.
     
  6. jehardiman
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    jehardiman Senior Member

    All sailboats are slow compared to the moving foils I usualy design for. But correct, I really haven't built any rudders for sailboats over 20 knots. High speed (20+ knots) sailboats just don't interest me, too limited in their ability to be useful...shrug...

    FWIW, I don't think the modern trend to high aspect airfoil type rudders is correct but just a fashion, they look like they were designed by someone using some of the aero CFD code out there, not hydrodynamists. Then again, the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over. That is a luxury I do not have with operational foils.

    Honestly though, there is more than enough CFD, test, and full scale data to support my position (see chapter 9, PNA V3). Sail or power it makes no difference. It must be remembered that rudders are not keels nor wings, and therefor are selected for different criteria. They should not be selected for L/D at a low fixed AOA, but rather the force necessary to maintain command control against the course stability index across the speed range, vessel motion, and environmental conditions.
     
  7. Paul B

    Paul B Previous Member

    I hope you are kidding here, otherwise...


    You don't have any idea what you are talking about.
     
  8. jehardiman
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    jehardiman Senior Member

    No, Classic "paper water" problem. The analysis was caused by control problems in chop after delivery. In the analysis the outer third of the foil was basicly non-functional due to wave orbital and pitch effects even though the foil was "high speed", and noisy to boot. The fix was to shorten the foil, increase root cord, increase the TE, increase taper, and decrease sweep. This resulted in a lower aspect foil around 4-5 with less area and drag than the original.

    LOL, not even going to ask why I think that? :D
     
  9. MikeJohns
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    MikeJohns Senior Member


    And both insurance and efficient rescue services play their part in non-robust design. A typical rudder failure rate for offshore racing events is illustrative. Also the jury rudder systems people put so much faith in after a smooth water coastal test are usually inadequate in a boisterous seaway with a bit of a blow.

    Classic ship theory usually points to a Naca 0015 foil, but I think many racing boats will be using something like a Eppler generated shape and Dave Vacanti seems to have a good niche in the foil design software market.

    I'm never sure about those nice lift drag plots once the vessel hits a seaway it's probably all a bit superfluous. I have often see frustrated helmsmen with stalled rudders and wondered if they might have faired better with more of a barn door.

    But the planform can definately be a fashion statement :)
     
    Last edited: Sep 7, 2009
  10. Paul B

    Paul B Previous Member

    I guess guys like you and Johns will always believe that those who earn a paycheck related to workboats know much more than those who design pleasure craft. Just like those who work on diesel rigs know more than the folks that work on the F1 teams.

    The people at the top of "fast sailboat" design earn millions of US dollars. With the expertise you have, maybe you and Johns should team up and dominate that field and become rich. Can't be too hard seeing as how you two have so much more on the ball than the folks who dominate there now.
     
  11. Paul Kotzebue

    Paul Kotzebue Previous Member

    I've had the opportunity to work for and with some of the top sailboat designers during my career. The guys at the top have degrees from places like the University of Michigan and Webb Institute. They are in a different league than the self taught correspondence course educated cruising boat designers. The competence gap in sailboat design is huge .... much more than the general public is aware of.

    The top sailboat designers are serious about winning and don't care much for fashion statements. The "performance cruiser" designers will copy what they do without any meaningful engineering analysis. The public has no way to really tell the difference.
     
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  12. MikeJohns
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    MikeJohns Senior Member

    Yes definitely we have seen that throughout history and a lot of problems both structural and performance related have resulted from this .

    Racing boats are not the exclusive domain of low drag high lift foils of course and many naval architects around the globe can produce a very good foil just how good will depend on the depth of the pockets of the person who wants the foil.

    Racing boat technology definitely has it’s fashions and fads and I don’t think foils are immune from this, also if you have worked in the field you’d be well aware of just what conditions a foil is optimized for in the testing phase. Sometimes it takes some guts for a designer to drop a current fad and they can then start a sensible trend. If that has the added advantage of increased reliability in an often inherently poor structural form then everyone might benefit.

    A naval architect will look at the benefit performance tradeoff but a racing designer wants anything that he imagines can shave another second off a racing course. In reality how much real world lift drag data exists for those optimized foils in a seaway? Sometimes chasing minimal wetted surface and going for that last 1% foil drag in ideal flow has it’s detriments and structure is definitely one of them.

    Parameters which in the tank or in the CFD package see a foil operating nicely in the low drag bucket can vaporize once the boats is in a seaway. Much of the real flow is anyone’s guess and it’s then that the helmsman finds that foil lacking no matter how much time and money was spent developing it.

    Suggesting that 3d flow fields are an exact science that can be learnt at university is probably best summed up by Leo Lazauskas

    CFD is, of course, an enormous advance
    because we can now display our almost complete ignorance in full colour!


    There is no superior knowledge where you can sit down solve a system of pde’s and out pops the perfect foil and planform. In the real world there’s observation, tank testing and CFD and then there’s always a current trend.
     
  13. Paul B

    Paul B Previous Member

    Well said. I know some of the folks you've been involved with and I have no doubt their abilities far exceed some of the loud talkers here.

    This guy Johns is always spouting off about things like "frustrated helmsmen with stalled rudders", but he is always too cowardly to name the boats/designers that are so lacking. It is a ploy he and his ilk use all too often.

    Of course he is pretty much a failed "designer" whose agenda is to pontificate to local authorities until he can hopefully push through some sort of law that will bring him some income as a required "review" of design/builds in his country.

    Based on his earlier comments here it is obvious he knows little about how modern "fast" sailboats are designed and built.


    Then you have this comment from JEHardiman above: "Then again, the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over."

    This is just a horrible thing for someone to say. The insinuation is disgusting.
     
  14. jehardiman
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    jehardiman Senior Member

    Well, as a king once said "Honi soit a qui mal y pense", and my statement is not an insinuation on anyone’s character though if you wish to make a shoe out of it and place it on your foot while stating that it fits....so be it. It is a fact that the failure of a racing boat just means that the sponsor will likely throw more money at it and try again, an option that I do not have in my job.

    If modern high speed sailboat design was as good as you imply, then all the boats of a given design brief would look the same and would not fail underway. A quick survey of bleeding edge "high speed" sailing will show that designs are not standardized and that over the course of an ocean race 10-30% of the racers (depending on hull type) will suffer some form of failure (go read this thread from 3 years ago for a discussion and some data on high speed sailing failures analysis from the Route du Rhum http://www.boatdesign.net/forums/multihulls/canting-keel-monos-vs-multihulls-13511.html). We are left with only two options after accepting those truths, and I will assign the sins of vanity and omission over deliberate evil. If you feel I have insulted anyone professionally, you have options available in this state as I do not hide who I am.

    In the 25 years I've been doing hydrodynamic design there are some truths that I have learned.

    1) You spend 90% of the hydrodynamics money to get the last 10% of the answer. And in the overall scheme of things, that 10% performance can easily be eaten up by environmentals. Like someone suggesting a "laminar flow" hydrofoil for open water use with a span greater than 4 inches, crossflows are just too great for that to ever meet full performance.

    2) Most analysis fails to consider environmental effects properly. Generally it is a schedule vs time vs money issue because many codes do not support oscillatory cross-flow or rigid-body motion interaction with flow. But sometimes a project just makes a poor decision on how to manage that part of the analysis. When a NG Ocean Systems project could not pass acceptance testing, a review I was involved in found out that they assumed that there were no waves with a period greater than 7 seconds in the required open ocean Sea State.

    3) The person who makes the final decision is rarely the hydrodymancist and rarely is the final decision based solely on engineering performance. More often the customer who makes the decision is presented differing and often conflicting criteria. Ok, all designers raise their hand if the customer has told you "I want it done this way" and you did it even though there was a better way.

    4) Fashion sells. This is true if you are spending 10k or 1 billion. Nobody wants to be too far from the crowd so they (customers and designers) all bunch up around a design that may not be the best but is currently winning. Often a good helmsman who had a good race sets a design trend when people believe that it was a piece of hardware that beat them, not that they themselves were out performed. And the design brief and/or rules plays into that also. Australia II's winged keel was not hydrodynamicly more efficient than Liberty's but the manipulation of the chain girth measurement gave it a sail area advantage. Same thing with IOR bustles; they didn't make the boats go faster.

    5) Pretty pictures sell even better. I watched GD-EB present a project to the US Navy with a manifestly flawed flow analysis...the PowerPoint pictures were pretty though. Same as the shark fin keels of a decade ago...the humpback flipper fins of today… or elliptical planforms. No real performance advantage, just look “better”.

    And finally...

    6) The only thing a Naval Architect has to sell is his opinion, because I can guarantee you that if you have two NA's analyze the same shape you will get two different answers....
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    "...What do you think should be best choice for a fast sailboat?.."

    In addition to what has been said above...whatever works for you!..and define 'fast'.

    I've designed some very very basic 'spade' type rudders on boats which would be considered "fast" by sailing the fraternity, ie flat plates. So long as the rudder provides the yaw/moment required, without being overly draggy, the final shape is almost irrelevant.
     
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