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Rudders, sails with holes

  1. Jul 27, 2015 #1
    I have read articles regarding Chinese junks which utilise rudders made of rubber, and they usually have diamond shaped holes in them. These articles claim that the holes provide better handling of the junks compared to a full rudder without holes in them.

    What are your thoughts regarding how sails with holes would affect the speed of the sail craft? Apparently the wind cannot maximise the work done on a sail when it has no holes on them, as the wind currents will rebound back onto oncoming wind currents. Is there any real physics explanation behind these two phenomena?
     
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  3. Jul 27, 2015 #2

    Baluncore

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    Welcome to PF.
    You ask an interesting question.

    Firstly, the fluid dynamics that makes sails work requires air have viscosity, which also restricts flow through small holes.

    The flow of air over a sail or airfoil wing is optimum at some air speed and angle of attack. There are many ways of adjusting the lift when outside the optimum by having slots or flaps on the airfoil section. Slots can reduce the speed at which a stall takes place.
    Likewise, a slot in a rudder may give better steering over an extended range of attack angles that may be present when sailing in a confused sea.

    The effect on a boat of a jib sail is actually to improve the efficiency of the mainsail. The gap between the two sails is a virtual slot in all the sail area present. That virtual slot is conveniently where the mast is when changing tack.

    A parachute works better if some air can pass through the cloth. That may also be true for some sail material where the airflow cannot be optimised. “Junk rig” sails would probably fall into that category, and so benefit from a slightly porous material.
     
  4. Jul 27, 2015 #3

    anorlunda

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    I think you can be sure that sailboat racers, have explored everything that has been imagined to make them go faster. Volvo Ocean Racers, and Americas Cup challengers in particular seem to have nearly unlimited budgets, and probably huge numbers of suggestions.

    If holes would make them go faster, they would be doing it.
     
  5. Jul 27, 2015 #4

    Baluncore

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    It is more important to be safe and reliable than fast. Racing places emphasis on high performance airfoils that need to be continuously trimmed to operate at their best. Performance falls off rapidly if that trim is not maintained. Suits of sails with slots between them work better together to multiply performance.

    There are many more relaxed ways of sailing that do not take so much effort to squeeze out the last few percent of speed. As an example, a Junk Rigged boat is much easier to handle. https://en.wikipedia.org/wiki/Junk_rig

    Winning races requires significant funds to support the materials and technology. Racing employs a handicap system that pushes the design of sailing boats to be unstable and dangerous. The Fastnet, The Sydney to Hobart and the Round the World Races have all demonstrated that situation. https://en.wikipedia.org/wiki/1979_Fastnet_race
    https://en.wikipedia.org/wiki/1998_Sydney_to_Hobart_Yacht_Race

    Holes in rudders and sails will almost always hurt their tuned peak performance, but holes will often improve the worst case performance under difficult conditions. I consider survival to be far more important than the slim chance of winning a race coupled with the high risk of becoming a candidate for the Darwin Award.

    Being last is better than being a total failure = DNF.
     
  6. Jul 28, 2015 #5
    If we consider a simple case: the sail boat has the wind directly behind itself, and the sail craft is moving upwind. How would holes in the sail make a difference?

    I'm not exactly familiar with the terms used to describe a sailboat, which is why I decided to bring up this simple case. I agree that parachutes require some air to pass through to work well. If I recalled correctly, parachutes have an air vent at the top to stabilise the chute. Would this stabilisation help in making the sail craft faster?
     
  7. Jul 28, 2015 #6

    Nidum

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    Rudders with holes require less effort to move than solid rudders .

    This was important when junks had to be worked entirely manually and in a geographic area where wind and sea conditions could change frequently and rapidly .

    The holes do not impair the steering function very much .Neither do they enhance it .

    There are modern versions of rudders with holes .

    Sometimes known as fenestrated rudders .
     
  8. Jul 28, 2015 #7
    If there is less effort needed to move the rudders, shouldn't it enhance steering? Maybe I don't understand how steering is enhanced (or how it works).
     
  9. Jul 28, 2015 #8

    Nidum

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    Depending on its design a rudder at a set angle relative to a vessel may make the vessel turn rapidly or more slowly .

    The rudder with the hole causes vessels to turn at about the same rate as a similar rudder with no hole . So the effectiveness of steering of the vessel is about the same in both cases .

    The real advantage is that crew members could move rudder to desired position more easily . This made it just a little easier for them to adapt to fast varying conditions of wind and tide .

    I cannot say for certain but I think that the rudder with the hole was not only easier to move in simple sense of having lower drag but probably kicked less as waves hit it as well .
     
  10. Jul 28, 2015 #9
    I really appreciate the contributions thus far, how about the idea that holes in a sail would allow a sailcraft travelling directly upwind to move faster? My (non-scientific) explanation is that the work done by the wind on the sail is reduced as it gets deflects off the sail onto the oncoming wind currents.

    Can anyone refine my explanation in a better way or are there any better ideas than mine?
     
  11. Jul 28, 2015 #10

    anorlunda

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    There is something called the Circulation Controlled Wing which is vagely similar to a sail with holes. It injects air to the top side of a wing to prevent or delay "separation" of laminar flow. The idea has been reaserched and (I think) discarded. I assume that holes in sails have also been researched and discarded. Modern racing sails have a layer of mylar which has near zero porosity compared to woven fabric.

    Your made your sailing preferences very clear. I am like you. I live and cruise aboard a Westsail 32; a boat that many consider the most seaworthy sailboat ever made. But even I admit that there is room in this world for racing boats as well as cruising boats.


    No no, that simple case is the least appropriate. Old sailing ships had square rigged sails. Modern boats use fore-aft sails that act like airfoils. When the wind is directly behind, one can achieve only a fraction of wind speed (0.1 to 0.2) because as the boat speeds up, relative wind speed approaches zero. At any other angle, the wind flows over the sail turing it into an airfoil. Only drag limits boat speed; there is no theoretical upper limit less than c. When sailing into the wind, apparent wind apeed is boat speed plus wind speed. Iceboats with very little drag can sail almost directly into the wind and achieve 5x wind speed with records on the order of 120 knots.

    Polar_40810_TW.gif

    No no. Ideally, one balances the sails so that no rudder deflection is needed to steer the boat. When that is achieved (as is often the case with ocean crossing passages) then the rudder does nothing at all other than be dragged through the water slowing you down. If holes in the rudder make you go faster, they must reduce drag.
     
  12. Jul 28, 2015 #11

    Baluncore

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    mupp, being new to sail theory and terminology you must learn to walk before you can run.
    google “Review of Modern Sail Theory” Gentry
    Get the .pdf file and study it carefully. Work through it until you understand the slot effect between the jib and the mainsail.
    You will then have a better understanding of sails and be ready to consider the complexity of the fluid dynamic changes that occur when you put holes in surfaces.
     
  13. Jul 29, 2015 #12
    I have tried making a foot hole in the rudder of a small sailing boat, to act as a climbing step, and it had little effect on the performance. Maybe the stalling angle was improved but not sure. I suppose that holes in a rudder would help when turning the rudder when stationary.
    One of the tall ships has/had a large hole in each of its square sails, but the idea does not seem to have caught on. In the case of a parachute, there is no flow and the hole helps stability by establishing a small airflow. But there seems to be no problem with flutter of square sails down wind. May I mention that a square sail can be used for upwind sailing, similarly to a triangular sail - the usual limitation being that the mast stays obstruct the angle to which it can be braced up.
    Regarding the action of two sails working together, a jib and mainsail for instance, some work was done by Dr Patrick Couser at Curtin Univ at Perth (now deleted from the web, I think), and he found that the two sails in isolation are about equal to the pair when working together and optimised. However, when working together, the jib does much more work and the mainsail much less. One feels, therefore, that a single large sail would provide equal power if it could be successfully handled.
     
  14. Jul 29, 2015 #13

    Baluncore

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    In a third world fishing village on the coast, there will be a tradition of sailing boat design and handling. That tradition will have developed to make sailing safer and less demanding on the fishermen. At a competition yacht club, a different and more critical boat design and handling procedure will develop. Pores in sail fabric and holes in rudders will probably not optimise peak performance at a yacht club, but they may well improve the stability and robustness of sailing technology under adverse conditions near many fishing villages.

    The analysis of deviation from perfection requires first the identification, definition and study of perfection.
    C.A.Marchaj, a scientist and competition sailor wrote “Sailing Theory and Practice” 1966, and later “Aero-hydrodynamics of Sailing” 1979, revised 1988. In those books he applies scientific analysis to the peak performance of yachts. They are well worth reading if you can find a copy. Try bookfinder.com to locate a copy.
    ISBN 0396077390 “Aero-hydrodynamics of Sailing” 1987, is usually available second hand for about US$50.
    “Sailing Theory and Practice” 1966, is usually available second hand at a significantly lower cost.
     
  15. Aug 2, 2015 #14
    I'm sure the rubber composition and holes in the rudder are there to preserver rudder integrity, in, as Balancore put it, a confused sea. (nice turn of phrase.)

    Though it might be difficult maneuvering and maintaining the tack of a junk without a rudder, it could be done in gentle sea.

    wind vents in banners
    wind-vents-21-e1302518372366.jpg
     
    Last edited: Aug 2, 2015
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