Holes in rudder?

Discussion in 'Sailboats' started by michigangeorge, Sep 24, 2012.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,866
    Likes: 299, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    As an aside, I discovered some other fascinating things about Chinese ships while researching the rudder question

    "Unlike all ancient vessels, some Chinese ships did not use oars to move; they used propellers. Soldiers spun the propellers using a bicycle like method but they were covered by the deck so any enemy would think that the ship was powered by “sprits” and most were parallelized by fear giving the Chinese the upper hand in the mental war." http://en.wikiversity.org/wiki/Ancient_Innovations


    And continuing on the rudder theme -
    "Early rudder technology (c 100 AD) also included the easier to use balanced rudder (where part of the blade was in front of the steering post), first adopted by England in 1843 – some 1700 years later. "

    http://listverse.com/2009/04/18/10-great-ancient-chinese-inventions/

    This balanced rudder doesnt sound like 'barn doors were traditional at all.
     

    Attached Files:

  2. michigangeorge
    Joined: Dec 2003
    Posts: 34
    Likes: 2, Points: 0, Legacy Rep: 27
    Location: Petoskey,Michigan

    michigangeorge Junior Member

    If I wanted a modern boat I would own one as I have in the past. I do not want anything hanging off or laying on deck any more than I would add vinyl graphics shouting C-A-T-B-O-A-T. A little respect for tradition, eh?
     
  3. michigangeorge
    Joined: Dec 2003
    Posts: 34
    Likes: 2, Points: 0, Legacy Rep: 27
    Location: Petoskey,Michigan

    michigangeorge Junior Member

    Gentlemen, I would like to thank you for your thoughts and interesting research. I've decided to go ahead with adding the toe holes. Given the nature of this 1931 design, the 3" deadwood, the three blade prop and all the attendant turbulence this rudder is already operating in I don't feel these two little holes will make a damn bit of difference in her performance or helm response. I will report back should it prove otherwise.
     
  4. upchurchmr
    Joined: Feb 2011
    Posts: 3,098
    Likes: 164, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Michigangeorge,

    I am quite happy to let everyone do their thing.
    However, I have little respect for tradition when a simple undetectable change can improve a boat.
    I did not suggest you get a catamaran instead of a catboat.

    Will you be using hemp ropes?

    I expect you would use brass for the C-A-t..... sign instead of vinyl.

    Sorry I shouldn't have baited you but the old designers would use modern materials and techniques if they had the chance. Just my opinion, I never met them.

    Have fun
     
  5. michigangeorge
    Joined: Dec 2003
    Posts: 34
    Likes: 2, Points: 0, Legacy Rep: 27
    Location: Petoskey,Michigan

    michigangeorge Junior Member

    Here I am-back already. After my last post I went out to my shop with every intention of drilling those holes in my rudder. At the last minute I decided to do a mockup before proceeding. Screwed two pieces of ply together to replicate the thickness of my rudder and cut the holes to my intended size of 2"x4", stood the piece vertically on the floor with the top held in my bench vise and tried to insert my foot into the holes. 2"x4" was neither high or wide enough for my topsiders. I determined that the smallest hole which would be useable would need to be 3"x4 3/4" and that is just too large for me to be comfortable with. So those of you concerned about my old boat loosing a quarter of a knot need not worry :)
    I do think the idea of fenestration is an interesting concept to be further explored as sometimes we don't understand all we think we know about such things.
    Who would have thought scows would be winning ocean races! (see The New Scows thread) Thanks all.
     
    1 person likes this.
  6. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 144, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    First of all I clearly stating it was a guess, but I have worked in aerodyanics for a number of years. and worked for Boeing for more years than I like to admit. they use vortex generators to keep the flow attached, tiny foils will be less costly in terms of drag than holes, but holes will have the same effects. I am speculating when i say thing, based on many years experience working in aerodynamics; it was discovered by accident most likely that putting the diamond shaped holes in the rudders it was discovered rudder effectiveness was increases by vortex formation. Yes, pressure is bled through the surface, but that lost pressure is more than made up for the large amount of free stream flow that is entrained in the vortex and curved, or accelerated to improve overall rudder effectiveness. Based on the new pic that Yipster posted, that bluff stern likely means very poor flow fields around the rudder anyway. the effect of the vortex alters flow way outside what one would think the area that would be effective. vortex flow is odd like that, only studied in detail in the last 30 years or so, and not intuitively obvious.


    the holes create more drag than using properly designed vortex generators, so when you have the ability to design something better, you would not use holes. Aircraft are very sensitive to the lift to drag ratio, so it is not a suitable solution in most applications, but it does not mean they do not work. There is no benefit other than to make the rudder more effective, but it does increase drag. Another way to improve the rudder would be to make it bigger and fix the stern design.

    Holes have been used on lots of surfaces for similar reasons, but it is an obsolete way to improve flow conditions.
     
  7. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 144, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    the idea that the holes were placed to reduce shock loads is just silly. if that was the case would it not be easier, and more obvious, to make the rudder smaller? The people who wrote that are speculating and are no more expert than anyone else guess.

    putting a lot of tiny holes in the a large rudder is way more work than necessary if the only desire was to reduce shock loads.

    Also sharpi, the sand analogy is flawed. You will get flow through the holes, no matter the angle of the free stream flow when ever you have a pressure difference from one side of the rudder to the other. that happens when it is deflected to the free stream flow direction, deflecting (curving, or accelerating) the free stream flow that creates the lateral pressure at the stern to turn the ship. when the flow is highly disturbed and separated off the surface, the effect is limited. If vortexes keep the flow attached, you have more rudder effectiveness.
     
  8. Frosty

    Frosty Previous Member

    I once made a rudder from pipes. Just pipes welded along the length approximately the same size as conventional but no flat metal at all.

    I didnt work very well and I threw it away. I hope no one finds it and starts a thread about a pipe rudder thinking it was some long lost technological breakthrough.
     
  9. rwatson
    Joined: Aug 2007
    Posts: 5,866
    Likes: 299, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    No you cant "make the rudder smaller". You would need the full size of the rudder under high sail loads, and severe pitching on a 'banana' shaped boat that exposes a lot of it when the bow dips. That article I quoted also indicates that the large rudder also is an important leeway reduction component, along with the quite common daggerboards that many large junks had as well.

    True, we are just guessing, but the 'reduction of load' is the only scenario that makes any sense, as sharp square holes would be useless for any other purpose. The idea that they have could have any 'flow' characteristics is even sillier.
     
  10. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 96, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer

    Creative Frosty! When you had persued the idea of a pipe and had put a single foiled coil around the prop you would have had that ancient technological hole in rudder breakthrough
     
  11. Frosty

    Frosty Previous Member

    You reckon!!! not without some diamond cut outs.

    You know formula 1 does things like that --they cover the gearboxes to make people thing the gearbox is new but it wasn't it was the skirt .,

    I think this is referred to as a 'red herring'

    A bogus misleading point of attraction.l
     
  12. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 144, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    ahem...ANYTHING placed in a moving fluid will have flow characteristics. the question is are they beneficial to your goals or not. Your statement is complete nonsense.

    We do not know the goal of putting in the holes, nor if it even worked or not, or if discovered it had some other beneficial effect. They may have been placed there out of superstition or decoration. One guy does it and he is a successful fisherman, suddenly every other fisherman in the village copies it.

    We do not have enough information to know what they were thinking at all, we do not think today the same way they did than.
     
  13. rwatson
    Joined: Aug 2007
    Posts: 5,866
    Likes: 299, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Yes, you are right in all you say, and I already stated that. Those references I provided also were very uncertain about their reasoning.

    But due to the other innovative developments that the chinese had in the rudder area, I reckon its a good bet to assume they had some fiendish oriental purpose behind it, and not just making work for themselves.

    Dont forget, the problem of moving large wooden appendages has been a problem for large wooden boats for a long time, especially using rope and chain. With the advent of the big steel rudders, the problem would have been even worse if Mr Flettner had not been thinking about the problem.
     
    Last edited: Oct 2, 2012
  14. Frosty

    Frosty Previous Member

    I think that sums up decoration nicely --unnecessary work for nothing of any mechanical benefit and was'nt the Chinese good at that?

    Probably the most decorative nation the world has ever known. India would be of similar ilk.

    I believe thats all they are. As are the Dragons on the side that as far as I know give no extra speed or any mechanical benefit. Possibly a modicum of spiritual comfort to those on board.

    Knowing you have diamonds cut in the rudder may offer similar comfort in times when it was thought you could fall of the end of the world.

    With beliefs such as this we need not an engineer to fathom the holes but a psychiatrist.
     

  15. Harry Josey
    Joined: Jan 2012
    Posts: 26
    Likes: 1, Points: 0
    Location: South Africa

    Harry Josey Junior Member

    Now that Michigangeorge has decided not to cut holes in his rudder we are only left with why the chinese did it. As they don't seem to know we will never be sure. No matter how erudite the aeronautical types get they can't explain flow over slab sided hunks of wood with no aerofoil sections. They certainly couldn't be used as wings!
    rwatson.
    I have never heard that the Chinese had propellers. They must have been pretty big screws to move a fort. They certainly had man powered sternwheelers. The coolies were sheltered under the poop. The walking beams were connected across the inside of the poop and the coolies only had to row that connecting beam like an oar to produce the rotary motion to drive the paddlewheel.
    Incidentally if what your source said about the Romans is an example of their accuracy
    they may not be the best people to quote.
    Regards Harry
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.