Effect Rudder

Discussion in 'Hydrodynamics and Aerodynamics' started by sottorf, Jul 10, 2012.

  1. sottorf
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    sottorf member

    I am looking into designing a so-called "effect rudder" that is supposed to create thrust rather than drag when located behind the propeller. For example see http://servogear.org/productrange/effectrudders.html for more information.

    Can anybody refer me to some good technical literature on these rudders and how to design them?

    Do they really work as claimed?
  2. daiquiri
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    daiquiri Engineering and Design

    The idea is imho not bad, though it's not new either. If I've got it correctly, it consists of a rudder blade either twisted or with asymmetric airfoils arranged in such way to best conform to the rotational flow coming out of the rudder. Furthermore, the rudder and the prop hub are joined together into one streamlined body. Both features serve to diminish the rudder drag and to delay an onset of cavitation.

    These two concepts are not a new idea, see for example these two cases:
    1) http://techlinkcenter.org/summaries/twisted-rudder
    2) http://www.rolls-royce.com/marine/products/propulsors/promas/

    What I find really curious is Servogear's description of the system, which imo denotes a lack of knowledge in the field of hydrodynamics by the person(s) who wrote it. Citation from that site:
    "Servogear’s special designed «effect rudder» is shaped like a vane and follow the angle of the water leaving the propeller. It transfers some of the rotating engergy into positive thrust at no loss of effect on the steering.
    I think that the choice of the words is wrong, because this type of rudder arrangement can, at most, reduce the drag from the rudder blade, but will not produce thrust. A propeller will.

    Furthermore the cross-section of the rudder blade is designed as an aero plane wing giving a lift in the water flow which again produces forward thrust."
    Apart the pure linguistic issue of the term "aero plane" (incorrect) vs. "aeroplane" (more correct but obsolete - the contraption is called "airplane" nowadays), any person with a basic formal training in hydrodynamics would know that a cross-section of a rudder blade cannot be shaped as a wing. A cross-section is a 2-D shape, whilst a wing is a 3-D shape. Both a wing of an airplane and a blade of a rudder are 3-D shapes (they have a finite span or length) and their cross-sections are called airfoils.

    I am sorry for being a smart-*** here, but I firmly believe that words are important, in technical as well as in non-technical communication.
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  3. Luc Vernet
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    Luc Vernet Senior N.A.

    The person who wrote the description may not be expert in hydro/ aerodynamics, but the people at Servogear are, you can take that for granted!!!

    So, what seems to me strange is the pretended "extra-thrust" given by the rudder..!!??!? If I can very well imagine that a plane/ foil at a certain angle in the spiraling water behind a propeller CAN produce forward thrust (usually twisted rudders are designed basically for reducing the rudder drag) I would also imagine that, if designed so that it produces a thrust, would it be minimal, and not to just improve the flow, it might then create sort of a "back pressure" behind the propeller that would impede the correct flow around it, be it "en amont" (French; literally= "uphill"). The end result would be a loss, I am afraid. If not, why would every single boat not be fitted with twisted rudders ???

    I would not be surprised if it was no more than another load of it from some commercial MBS (Master of Bull-****), where a single, minimal and perhaps only theorical aspect is described as beneficial, while hiding the fact that this arrangement is/ would be prejudicial as a whole (which I would not expect from Servogear...:confused:!)
  4. Silver Raven
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    Silver Raven Senior Member

    Probably - 1/ - that steers the vessel -
    2/ the best - most efficient - with the least - 'slow-down' to forward motion ! ! ! !

    Where do you 'little kids' get such questions ?????

    Ciao, james - Forget the answer - cause to me this question is just - not on the agenda - not at all. jj
  5. CDK
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    CDK retired engineer

    There was this children's story about a kid pulling at his hair to lift himself from a swamp; I think it was from "the baron of Muenchenhausen".
    A rudder of any shape producing forward thrust falls in the same category.

    Unless it is powered by mechanical or electrical means, an object in the flow of water can only create drag. The only objects behaving differently are those near the propeller that can improve the efficiency, like a Kurt nozzle. They also create drag but the sum of all effects may be positive.
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  6. daiquiri
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    daiquiri Engineering and Design

    It is imho a bad choice of words, like the other parts of that description. A twisted rudder reduces the rudder drag through an alignment of the spanwise airfoil sections with local water flow, which has both axial and tangential (rotational) components. Less drag means more speed for the same engine power (everything else being equal). But more speed means again more drag from both the hull and the rudder, which must be equal to the thrust produced by the prop. So at the end a new equilibrium between the propeller's thrust and the vessel's drag is established, with somewhat higher vessel speed and perhaps somewhat higher prop thrust.
    The miswording in Servogear's website consists in describing the above process in such a way to make it seem that the thrust comes from the rudder, when it actually comes from the prop.

    I see basically a well-known twisted rudder in those pics. Molland an Turnock, in their book "Marine Rudders and Control Surfaces" say that various authors have reported a measured reduction of drag and cavitation levels for known twisted rudder configurations. Which is also rationally an expected outcome (imho). So twisted rudders probably do deliver an improvement in performance, though not in the way Servogear's story describes it.

  7. Ad Hoc
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    Ad Hoc Naval Architect


    I think your excellent critique and appraisal of the their poorly chosen words and dubious claims have scared them. The link is now broken. :)
  8. Servo Gear
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    Servo Gear Junior Member


    The text quoted is taken from the old web site that should no longer be online, see www.servogear.com for our current web site.
    The text was written more than 10 years ago by our marketing department facing the old problem of explaining technical features to non technical people, hopefully we do a better job explaining ourselves today.
    Servogear’s first effect rudder was tested in 1984 on M/S ”Sunnhordland” and it was reported that she went 0.3 knots faster with rudders than without. She is still running today with the same rudders.
    I also invite you all to come our stand at SMM to discuss the finer details of the effect rudder and other features.

    Best regards,


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  9. Luc Vernet
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    Luc Vernet Senior N.A.

    Absolutely! These not too well chosen words were giving a bad feeling about the whole description!

    Twisted rudders are quite good, yes, and logical. From what I know, they are patented - or used to be, by the US Navy I think. Damen shipyard, here, who are not for spending money for nothing, use them on many of their vessels.

    But Servogear has now replied and straightened things up!

    One problem , however, remains the cost of their equipment. We had been approached for fitting their props/ rudders on our 33m. catamaran motor-yacht, that is presently being built ="http://www.curvelle.com/blog/", but if their product is perfectly valid on commercial vessels, the gain of speed for the cost makes it debatable on a yacht!
  10. BMcF
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    BMcF Senior Member

    Worked closely with your propulsion gear during the construction of some 38m catamaran ferries at Batservice, Mandal many years ago.;)

    To the original topic..frankly I didn't see anything to take issue with in the older verbiage on your site; only the OP's interpretation and question. I read it to mean simply that the rudder helped recover some energy in the rotating flow from the prop, as thrust, that would otherwise be lost.
  11. jehardiman
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    jehardiman Senior Member

    Lets call them by thier true name....contra-guide rudders, and yes they are old hat...at least from the early 1930's (the 1936 tanker SS R. P. Resor was famous for hers) and widely fitted on MARAD ships in the 1940's. Works best with a contra-guide sternpost also. Abandoned because the increase costs in fabrication and maintaince to prevent fouling didn't justify the fuel savings. They need to be repainted often because of wash edge effects.
  12. sottorf
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    sottorf member

    So after a lot of comments (some really stupid ones) we have established that these rudders:
    - do provide benefit (drag reduction rather than thrust)
    - were (possibly) invented by the US NAVY (David Taylor model Basin?)
    - have been proven in service by Servogear who are hydrodynamic experts as anybody in the business will know. Their product is only only the most efficient high-speed craft propulsion system on the market today!
    - One reference for design: Molland an Turnock "Marine Rudders and Control Surfaces"

    Can anybody provide additional design references? Any of the US Navy work in public domain?

    Petter: will Servogear supply an "effect rudder" for a FFP installation (25m catamaran ferry)?
  13. daiquiri
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    daiquiri Engineering and Design

    Hello Petter,

    It's ok, I have never doubted the technical capabilities of your Company, just the technical competence of the guy who wrote that text. That old description was really looking non-professional, I'm sure you will come up with another simple to understand description which will not sacrifice the technical credibility. :)

    I am also a bit puzzled by the graph showing a comparative efficiency of your drive vs. the all the others. It is in this page: http://www.servogear.com/index.php?pageID=50
    The claimed difference is remarkable, so remarkable that it would be interesting to understand where are those numbers coming from. It shows 75% efficiency of your drive against 65% of a conventional prop at 30 kts. Or an 80% efficiency of your drive vs. 65% of a waterjet at 50 kts.

    Now, ideally it could be possible to get up to 70-75% from a conventional prop at 30 kts - though it is practically pretty hard to get from the off-the-shelf products, for small crafts and for surfaces subject to marine growth. A lots will also depend on a wake fraction, for example, or on the prop loading.

    So the question is - are the data for your propeller installation coming from similar "ideal" conditions, calculated from open water diagrams, or are they coming form full-scale tests on a sea-going vessel? The graph in question apparently looks like a case of comparison of apples to oranges (or ideal vs. practical conditions) - though I might be dead wrong on this, of course.

    In order to sort these things out, could you please comment a bit on that graph and back it with some hard data, perhaps by pointing us to relevant technical papers or other publications where those figures are taken from?

  14. Petros
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    Petros Senior Member

    the web site description was likely put up by either their marketing department, or their IT department (or an IT subcontractor). My experience with such none technical types is they will make notes from the chief engineer's description of the new design/technology, and than go write something up for the web site/advertizement, randomly sprinkling the engineering terminology in the text without regard for accurate use of the words.

    The few times I attempted to correct the inaccurate use of the words, the marketing types were very indigent of an engineer criticizing "their" work. Apparently for them, accuracy has nothing to do with selling the product.

  15. PAR
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    PAR Yacht Designer/Builder

    I've found the opposite to be true, where as marketing teams took lead or allowances, that wouldn't get approval from the technicians looking over proofs. Most firms continuously bounce proofs around, from IT, marketing, technical, corporate and legal departments until they've refined and honed the messaging, the way it reads, the way it looks with the graphic department (in house or out) making sure it also fits the "family" look and feel of the rest of their marketing efforts. This level of coordination is what I've found typical in all but the smallest or inexperienced of firms. Of course, it's time consuming and tedious, getting an offering through all the departments and committees, but part of the process to insure the company stays on messaging within the focus and imagery of similar marketing.
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