why twin boards?

Discussion in 'Sailboats' started by ViennaYachtworx, Aug 30, 2011.

  1. ViennaYachtworx
    Joined: Oct 2009
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    ViennaYachtworx Junior Member

    From reading all over the net and a bit of thinking I've managed to distill a few "facts" - Please be so kind to correct me if I'm mistaken.

    ups:
    - twin boards can be set to the right angle for predetermined heeling angles, whereas a single one is always a bit "wrong" when the boat is heeled, thus the area can be reduced causing less wetted surface.
    - twin boards can be made asymetrical, thus being more efficient and requiring a smaller angle of attack (~ less drift ??)
    - twin boards can be placed "toe in", to account for the change in the hulls effective centerline when heeled and therefore causing less drag.

    downs:
    - boats have to be sailed constantly at their designed heeling angle in order for the above to work.
    - twin boards have to be reset at every tack, causing more workload on the crew.
    - boards are barely movable when under load thus "new" board has to be set before tack, and "old" board retrieved after the tack.
    - during tacks twin boards slow the boat considerably, especially when asymetrical and "toe in"
    - twin boards add weight to the design.

    what I don't understand:
    do twin boards add to the staibility? does it make a difference whether the lateral force acts on the centerline or in lee? My first guess is a force is a force.. thus no difference, but if you were to see the center of the board as a pivot point the force acts upon, it would make a difference whether the boat is on top of it or to windward with according leverage... Does that make any sense? I'm confused. Can someone explain it to me r e a l l y slow?

    Also, have I missed anything in the list above?

    Thx for contributing!
     
  2. Doug Lord
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    Doug Lord Flight Ready

    -----------------------------------
    More here:

    Since you didn't say I presume you're referring to twin daggerboards as used on VOR70's and Open 60's?
    The number one reason why they are used is because the boats have a canting keel so the keel itself does not produce lateral resistance. If the boat had a fixed keel it would not require a daggerboard or two.
    Twin boards, used one at a time add nothing to stability unless you include directional stability which they are essential for. The boards are not ballasted.
    I haven't analyzed the curved foils on Open 60's carefully enough from a stability perspective but it is not likely that the vertical lift from the curved board adds to righting moment though it may be possible at certain angles of heel, but unlikely I think.
    Boards offset from the center line have zero effect on directional stability or tacking ability.
    See the "open 60" thread under multihulls for a description about curved lifting foils and mammoth daggerboards being adjusted under high load. It can be done and on some race boats it is done.
    ----
    Twin daggerboards are generally not used on keelboats unless they have a canting keel. But there are many dinghies and multihulls that use twin daggerboards despite the purported hassle of moving them. E scows, A scows, all the fastest catamarans , and many trimarans use twin daggerboards(sometimes with a central daggerboard as well if the twin boards are primarily designed for vertical lift-like a Farrier 32S or F-85S). There are new dinghy designs using twin daggerboards: see the "Dinghy Design: Open 60 Influence" thread and the "Flight One..." thread.
    http://www.boatdesign.net/forums/sailboats/dinghy-design-open-60-influence-36401.html see post 111 among others...

    http://www.boatdesign.net/forums/sa...kind-performance-one-design-dinghy-39229.html Proposed design where a single board acts like twin curved boards for the purpose of developing vertical lift and lateral resistance.
     
  3. CutOnce

    CutOnce Previous Member

    Although I'm sure you will get lots of more detailed technical responses, I'll try a little common sense. Don't be offended but it appears that you have skipped the "school" segment of yacht design and moved straight on to comparing features and benefits. This is a phenomenon I've noticed is common with some people labeling their feature-fests as "designs", as they incorporate lots of possibly good features together on the assumption that if someone else has done a particular thing, adding together all the good parts of many people's designs is better.

    Each boat is a collection of compromises, and each answers a specific set of desires from the owner. Optimizing upwind speed often results in penalties downwind. Reducing leeway upwind may increase demands on crew precision during maneuvers. Every benefit incorporates liabilities as well.

    Before you can discuss the benefits and liabilities of twin board designs, you have to understand the venue the boat is to be used in. You have to know if the boat is designed for offshore work, or short course racing on flat water. You have to decide if the level of complexity you are heading for is within the skill level of the intended crew and also if this fits the intended use of the boat. Cruisers will quickly learn to hate added work on every tack or gybe, while top flight racers don't mind. Racers however won't even try a boat that doesn't have a ready-to-go race fleet of competition. So basically, if you are asking about twin foil designs and considering asymmetrical boards, my quick read is that this would be a new racing design most likely doomed by the difficulty in getting more than one built and racing on the water. I've learned this lesson the hard way, building an amazing high performance single hand trapeze skiff for which there is no fleet and no resale value. So you see I'm not being "smarter" than you - I've already made these mistakes.

    Many of your areas you admit not understanding are basic core fundamentals of naval engineering for which lots of solid answers lie in textbooks and education. Without putting the effort into getting yourself a good grounding of these issues, you are best to work with a good professional naval architect to make sure all the basics are right before you spend money on materials or time building. It is much cheaper in the long run, and your chances of success are far better if you use professionals where needed. Saving money up front by skipping this step will cost much more in the end, and you may end up with a non-functional, poorly performing lawn ornament that you've worked hard to build and spent a lot of money on.

    I know this is not what you want to hear, but I would not be doing you any favors by explaining detailed answers to your questions. If you are like all of us at a young age, you will probably ignore my well-intended words and forge ahead anyway. Good luck.

    --
    CutOnce
     
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  4. michael pierzga
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    michael pierzga Senior Member

    Regardless of the race boat angle, Twin bilge boards, or lee boards, conserve valuable interior space, dont comprmise the fore and aft backbone of a boat, are efficient and allow for extreme shoal draft sailing.

    Herreshoff, Oughtred and centuries of Dutch designers used them.

    http://www.leeboards.com/
     

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  5. ViennaYachtworx
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    ViennaYachtworx Junior Member

    Dear Doug Lord,

    actually I was referring generaly to the pros and cons of twin boards, to get a better idea.

    I thought I understood but then I got confused on why a- and e-scows use twin board setups as the benefits seem to be marginal if any. (the boats centerline move to lee when heeled but hardly changes direction, the angle of heel is not more than on an average keelboat and the boats are racing on short courses with frequent maneuvers)

    I've read both of the mentioned threads but, they couldn't answer my questions, and I didn't want to hijack them, by asking them.

    Dear CutOnce,
    I was asking about the pros and cons of a single feature - far from the feature fest you refer to.
    This is exactly why I asked..

    Nevertheless I truly appreciate your input, and can assure you, that I would never build a boat without consulting a professional (or several) first. This however shouldn't inhibit my desire to ask quiestions in order to learn something.
     
  6. Landlubber
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    Landlubber Senior Member

    Cut Once,

    Your response was one of the best posts I have ever read on this forum, well said mate.
     
  7. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ==================
    A and E scows are designed to be sailed upwind at an angle of heel. If they had a single board that board would most likely come out of the water near where the board joins the hull drastically reducing the efficiency of the single board.

    ----
    Don't hesitate to ask questions and welcome to the forum!
     
  8. CutOnce

    CutOnce Previous Member

    I should have been a little more direct. Tell us more about the intention of the design, where it is to be used and how, as well as who (and how many) will be using it. These things form the basis for understanding the purpose of your project. Without knowledge of this information, any information people supply you regarding one specific feature may be inappropriate.

    Answering your question more specifically, each slot on the bottom of a hull adds drag, maintenance, leak potential and the need for structural reinforcement. Using asymmetrical boards makes putting boards up and down every tack mandatory, and the timing of moving the boards is critical to avoid side loading of the boards which will make moving them difficult to impossible. Asymmetrical boards also are more expensive to build, and you are doubling the cost of the foils. One or two bad tacks can eliminate the benefits of the improved leeway on a typical race course, therefore the benefit of the boards only comes into play at the very top end of the fleet (if there is a fleet).

    Adding weight to anything that is intended to go fast is something to really, really consider before doing. Adding a lot of structure, a lot of expense and a lot of increase to build time is something really worth thinking hard about. If you can get the same leeway benefit from a single board gybing design, why add the weight and expense?

    I personally hate weight and complexity. But that is me. As you can see there are people here willing to advocate features without understanding the intended purpose of the boat. I want to know more about the application before I even think about the features to be incorporated.

    Best of luck.

    --
    CutOnce
     
  9. ViennaYachtworx
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    ViennaYachtworx Junior Member

    Dear CutOnce,

    That's the main point. There is no project. I was just not understanding why A- and E-scows use twin boards. Since both classes result from a box rule, there must have been a point when the evolution showed it would add a benefit in boatspeed which outweighed the drawback of the additional handling - which I couldn't see from the pros and cons I mentioned in my opening post - I therefore concluded that there had to be something I had not seen and suspected it would add to the stability.

    According to Doug Lord, the point I have not seen is the missing endplate effect of the hull - which comes as a bit of surprise to me as most dinghies have transom hung rudders which have no endplate either.

    Thus I conclude that either I'm still missing a piece of the puzzle or it's due to a measurement rule effect where people squeeze the last bit out of a design making huge concessions (in this case ease of handling) only to gain a tiny bit of advantage you otherwise wouldn't even think about - kind of like the pointy ends of a skerry cruiser for added unmeasured waterline length or the infamous IOR-bumps.
     
  10. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    twin boards

    ==================
    VY- it's more than that: it's the reduction in foil efficiency due to the top being ventilated in a highly loaded area. To make up for that a single foil would have to be larger which is more area, more wetted surface. Also, most scow foils and other twin foils are toed in and that improves VMG to windward by reducing the hull drag caused by the boat crabbing sideways. A single foil could make up for that by being installed as a gybing foil but the area would still have to be greater.
    You mention that most dinghies have transom hung rudders-they do but not A & E scows which have rudders under the hull. As do most boats with twin foils.
    I'm not sure you're missing anything-you have made very good points showing an above average grasp of the technical issues with board/foil design. I think there are all sorts of design compromises made for all sorts of reasons-not necessarily based on technical reasons- cost, ease of use etc as I am sure you are aware. The point about the transom hung rudder is excellent-it's been known for years that putting an endplate there as an extension of the bottom would be an improvement-there is an illustration in Gutelles book "Design of Sailing Yachts". Except on boats with gantries-and on and on...
    I think most dinghies don't require twin foils from a performance standpoint.
    Scows are different. High performance boat designers usually have all kinds of justifications for their choices. The fastest dinghy in the world has two surface piercing foils but that sub-optimum state of affairs is offset by the efficiency with which the boat sails on foils.
    -
    I think dinghy design is wide open for out of the box thinking: foil assist, and other innovations. But that doesn't mean current designs are wrong-it just means that there is lots of room for improvement based on known technology, perhaps, applied in new ways.
     

  11. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    VYW, Doug lord has imho headed you to a correct direction. You have to consider a fundamental role of the rudder in balancing the forces acting on a sailboat. So it is an imperative to allow the rudder to work efficiently under all conditions and heel angles. It means that the aeration has to be avoided as much as possible, and that its angle of attack remains inside acceptable limits as the heel increases.
    You could take look at this old post of mine, in another thread: http://www.boatdesign.net/forums/sa...ring-beat-close-reach-24628-2.html#post233863
    Though that thread is only marginally related to your question (it was about broaching), my reply back there touches some of issues which are pertinent to your question.
    Cheers!

    P.S.
    What has happened to your electric boat project? Did you make any progress with it?
     
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