Rudder Design

Discussion in 'Boat Design' started by Norman Brown, Jul 11, 2009.

  1. Norman Brown
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    Norman Brown Junior Member

    Whilst looking at rudders for a new yacht I thought how poor the idea of bending the rudder at one point is ! The water has to stall on the upper side of the rudder when you push it beyond a few degrees. This is when you want it to work as a verticle wing. the upper surface should be working for you and it isn't.
    If the rudder was broken into strips (verticaly) and worked from the rear with the leading edge pinned. The flow would stay attatched. The efficiency goes up and as a consequence the rudder can effectively be made smaller !
    Has any one tried this ?
     
  2. MikeJohns
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    MikeJohns Senior Member

    Norman
    Rudders pivot on an axis (they don't bend on a point :) ) and the reason they are foil shaped with a carefully considered planform is to give good lift to drag ratios. The stall characteristics are dependant on the reynolds number and the planform.

    Not quite sure what you mean by the upper surface ? But the system you propose has been tried with a variety of articulated rudder systems, but only to advantage really for powered vessels. that require higher manouverability at low speeds, then they are directing the prop thrust and they can even do that better with other setups.

    Whilst you might keep laminar attached flow on one side you also need to consider what the flow does on the other side and there you will quickly see a bleaker picture.

    On a sailboat you will find this will only add to turbulence and drag over a simpler foil or skeg rudder combination which is a single articulated foil.
     
  3. tspeer
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    tspeer Senior Member

    Some boats have the rudder hinged to a fixed skeg, which is like the simplest form of your idea. There is a rudder for commercial vessels that has two hinge lines to it.

    Making the rudder smaller may not be required, as maximum lift may not be the limiting factor for sizing the rudder. The rudder also serves as a directional stabilizer as well as for maneuvering, and the area required for this function cannot be reduced even if the maximum lift is increased because the rudder needs to produce a minimum yawing moment per degree of leeway angle.

    Drag is also important. The drag due to lift from the rudder depends on the depth of the rudder, so to avoid increasing this drag component means reducing the rudder area by reducing the rudder chord, which also means reducing the thickness of the rudder, reducing the strength and stiffness of the rudder stock.

    But I think the biggest difficulty with what you propose is the large range of flow angles that the rudder experiences, especially when maneuvering at low speed. A spade rudder can maintain its angle of attack when turning, despite the fact that the rotation of the stern makes the flow at the rudder come from the side. On my boat, for example, steering is via a tiller in the center cockpit connected by cables to the rudder, and maneuvering is limited by the rotation of the bellcrank on the tiller. A rudder hung off a fixed skeg, even a segmented rudder, cannot rotate to follow the flow and still maintain a fair shape. The skeg would become a spoiler instead.

    A segmented rudder would provide a higher maximum lift for trimming out yawing moments when sailing straight ahead, but if that is required, then the boat has bigger problems than an oversized rudder.
     
  4. Submarine Tom

    Submarine Tom Previous Member

    Yes!

    Norman,

    It has been tried before and is in use today on commercial airliners.

    The wing has sliding flaps that extend and change the curve of the wing.

    There are many reasons for this but one of them is to provide more lift

    at lower speeds. A rudder has many functions and demands placed on it

    and yet has a fixed design. Your idea is great. Implimenting it a challenge.

    Get a hold of an aerodynamics book and you'll learn a ton, if you're

    interested.

    Keep thinking outside the box.

    Tom

    P.S. How much flow is there behind a spinning propeller???
     
  5. MikeJohns
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    MikeJohns Senior Member

    Read books on hydrodynamics its quite a different field to aerodynamics. But those books can be heavy reading unless you have a solid maths/physics background.
     
  6. Submarine Tom

    Submarine Tom Previous Member

    Disagree

    Mike,

    I disagree.

    Tom
     
  7. Submarine Tom

    Submarine Tom Previous Member

    Attached Flow

    Norman,

    In my P.S. earlier, I meant "how much attached flow is there behind a

    spinning propeller."

    I should also add that of course there would be value in referencing

    hydrodynamic books too, but this was not my direction of study when I

    came through school so most of my theoretical understanding came from

    aerodynamics which, as I understand it, is very similar.

    All the best,

    Tom
     
  8. MikeJohns
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    MikeJohns Senior Member

    Tom Not sure what you are disagreeing with but I'll give it a shot.


    "Theory of wing sections" is the standard intro to foils and it is definately worth reading but many aerofoils do not give the same L/D characteristics when used as hydrofoils (foil in water ).

    Aircraft designers use a variety of techniques to reduce vorticity because the vortices are the principal cause of drag in a gas. For example the slots and guide vanes used on an aircraft to reduce vorticity would actaully increase drag on a hydrofoil.

    Boat foils operate in the flow and pressure field that surrounds a hull moving on a bounded surface which is quite different to an aircraft fully enclosed in its fluid.

    The academic texts on fluid dynaimcs quickly dive into vector fields and introduce a lt of material that would be of questionable use for a yacht designer wanting to design a foil which is why I'd recommend reading something like theroy of wing sections and then Marchaj, or Gutelle (or Hoerner or Blevin )

    cheers
     
  9. Norman Brown
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    Norman Brown Junior Member

    I can see where the Tom's are in this (you should never have Tom Tom's beaten by a john :eek: Pun!).
    As a design engineer I can see that I am beating myself with my own stick. But I do remember people (unsuccessfully) trying bending keels and moving nosecones. They all make pressure drops and increases. What I am trying to do is keep that flow attatched all the way to the back of the rudder. No stall characteristics ! We are all familiar with unbalanced rigs creating drag on the rudder.
    Stepping the rudder seems one possibility. Its how ? With different characteristics at dif. speeds. I have a notion so I will lay out some sketches in the next few days. Limiting the bends according to load seems to be part of the solution and having a maximum for each joint. How many joints is another trial and error.
    I can try various developments on the new boat with stern hung rudders. - I feel more development coming my way !
    What thinks ?
     
    Last edited: Jul 13, 2009
  10. philSweet
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    philSweet Senior Member

    Dave Gerr's book "boat mechanical systems handbook", Ch 13, deals with all sorts of special ruuder designs, including the Deflector Rudder, which is a commercial product. I think that chapter can be sampled on the web. Herreshoff built one for a race boat, but I can't remember her name.
     
  11. rwatson
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    rwatson Senior Member

    This was tried on one of the Australian Americas cup challengers.

    It was a three or four section rudder degined just to do that.
    It was controlled with an 'inner wheel' on the main 'Steering' Wheel.

    It wasnt continued in later designs due to the complexity of operation.
     
  12. MikeJohns
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    MikeJohns Senior Member

    RW The Americas cup boats have tried everything looking for the edge in that case they were probably chasing a faster tighter turn, they make compromises too. Its like most modern innovations, many were tried in one form or another already, even 100 years ago.

    Here's some pics of the sort of articulated rudders common on power vessels and motor-sailers. It can be mounted behind a skeg too. The mechanism is often on the top and has several forms some very robust.

    But if you are chasing low stall characteristics at lower speeds then you would still be better changing the planform IMHO.

    What speed and what rudder angle did you want before stall?
     

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  13. rwatson
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    rwatson Senior Member

    Mike - I dont think thats what that articulated edge is intended to do.

    Is it not the Anton Flettner invention that aided turning heavy rudders by use of the trailing surface ?

    It actually works in reverse of the fairing process - turn the trim tab to port, the Rudder is pointed to Starboard by the trim tab, and the boat is turned by the major rudder surface.

    eg

    http://en.wikipedia.org/wiki/Servo_tab

    and

    see http://www.rexresearch.com/flettner/flettner.htm

    and another good one

    http://www.trimtabsolutions.com/company/what_is_trimtab/

    and

    http://pdf.aiaa.org/preview/CDReadyMASM04_665/PV2004_902.pdf
     

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    Last edited: Jul 14, 2009
  14. MikeJohns
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    MikeJohns Senior Member

    RW

    Small trailing foils altering the pressure distribution over a lager foil are common, you will see a lot of them in steering systems both as self steering and trim and correction tabs, but I doubt that approach would work for this application since the application is reversed i.e. you put the tab to stbd to turn the ship to port.

    I read it as wanting the rudder to curve in a nice hydrodynamic shape to get max lift at low speeds and high turn angles.
     

  15. rwatson
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    rwatson Senior Member

    I am not sure what you are saying Mike.

    Those pictures you posted are not 'articulated rudders', to improve laminar flow, which is what the orignal poster was proposing.

    I dont think the pictures you posted are anything to do with improved laminar flow - they seem to be either trim tabs or servo tabs to assist with the difficulty of turning a big rudder.
     
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