center of effort of a gaff sail

Discussion in 'Sailboats' started by EAP, Sep 2, 2006.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    Cliff, I would expect the tall narrow sail to more closely model the results I got from aerofoils. I don't have the theory but the center of lift was more or less at the same place for low and high aspect aerofoils that I used; however by sailing standards these were all narrow. I don't get heel on my tiny boat's experimental sailing rig because it has a Bruce foil instead of a regular keel. I am using an equilateral triangular sail which is kept as flat as possible and I was assuming the COE is independent of sail/wind angle but there are times when it does not steer well so maybe the COE is shifting.
     
  2. Tanton Yachts
    Joined: Jul 2005
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    Tanton Yachts Yacht Designer

    Gaff Main.

    946SPL.jpg Location of Center of Effort of a gaff rigged mainsail by measuring 2 triangles is a rule of thumb.
     
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  3. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    I would say the rule of thumb is using the geometric center as the center of effort. Measuring two triangles is just a practical method to find the geometric center with pen and paper (no cad).
     
  4. globaldude
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    globaldude court jester

    OK, I'm gona cut my throat now !!

    To quote Crag Cay;

    "Case 3: I built a large model yacht with internal ballast but with a high aspect vertical keel that could be slid backwards and forwards over about 25 percent of the LWL. It sailed perfectly in all positions with no detectable change in helm input.

    Case 4: In the late seventies I crewed an Admiral's Cup 2 tonner (44ft) that was a complete pig. It had terrible weather helm and would broach at the sight of a gust. No one ever tried to go through our weather as it was just too dangerous. Eventually the keel was brought back 6 inches which transformed its handling, but way out of proportion to the change in lead."

    Right or wrong, I have a hull , 50', that was built some years ago and it's not a recognised design .
    I don't have any plans , there were none I know of, and I'm right now ripping out my hair trying to figure out the positioning of the Dagger boards so as to give the right LEAD to the rig --- which I've drawn like Brians " aft mast" design .
    Add to that the centre of gravity - which some saY SHOULD BE THE SAME AS THE clr
     
  5. globaldude
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    globaldude court jester

    OK, I'm gona cut my throat now !!

    To quote Crag Cay;

    "Case 3: I built a large model yacht with internal ballast but with a high aspect vertical keel that could be slid backwards and forwards over about 25 percent of the LWL. It sailed perfectly in all positions with no detectable change in helm input.

    Case 4: In the late seventies I crewed an Admiral's Cup 2 tonner (44ft) that was a complete pig. It had terrible weather helm and would broach at the sight of a gust. No one ever tried to go through our weather as it was just too dangerous. Eventually the keel was brought back 6 inches which transformed its handling, but way out of proportion to the change in lead."

    Right or wrong, I have a hull , 50', that was built some years ago and it's not a recognised design .
    I don't have any plans , there were none I know of, and I'm right now ripping out my hair trying to figure out the positioning of the Dagger boards so as to give the right LEAD to the rig --- which I've drawn like Brians " aft mast" design .
    Add to that the centre of gravity - which some saY SHOULD BE THE SAME AS THE clr [ woops, caps lock !] My heads spinning .:confused:

    So I goes on line to read what fellow yachtys are saying on the subject and reads the quote above !!! .
    Well dam it, I think I'll go have a beer and watch the All blacks kick the english rugby bums.
    Tomorow I might feel better, less defeated, It's looing like I might have to hire a designer to give me the numbers / positions, but they seem to have differing opinions which worries me ! .
    At 49 years and limited budget I only get one shot at this and don't wish to work my butt of for the next two - three years to end up with a pig !!. Yeah yeah, I should have started with a recognised designer etc, but I'm 20 grand down the road now !! .:rolleyes:
     
  6. Tad
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    Tad Boat Designer

    dude,

    Not to panic...time out to reconsider is always good.

    Most careful yacht designers have a few sets of sections for which the behavior is known, and they scale these and modify them (in increments) to suit each new design. In this way we can predict required CE based on previous experience, not take a wild guess using questionable fudge factors. That is the road to ruin.

    In a prior life I worked designing major sailing yachts with various rigs and appendages. As these were multi-million dollar projects we were nervous about sailing balance and helm pressure. So we tank-tested almost all these boats, mainly for balance when heeled. The results were sometimes surprising. In my experience very small changes in underwater section shape (particularly forward), profile depth, and heel angle could produce huge changes in balance.

    Make a model of your hull, put it in the water and give it a push. With no heel it should go in a fairly straight line. You should include at least half the rudder and skeg in the model (fixed on centerline). Now heel it ten degrees and give it a push, do the same at 20 and 30 degrees heel. The hull will travel in three different circles, sometimes in different directions! Now try it with your daggerboard installed...move the board fore and aft. You are learning the balance characteristics of your hull form.

    In general terms if your hull turns a tight circle into the wind (high side) when pushed, you want the CE forward. You'll find that moving the board aft makes the hull run straighter when pushed. This will suit the aft CE position of your "aft mast" rig. If your hull is anything close to "average" and the trim is reasonable, the daggerboard's leading edge will be directly below or a little aft of the geometric CE. But that is a vast generalization and is to be taken as such.

    One of your problems is that you don't (apparently) have a complete design. Until you do have it complete you do not know where the CG might be, thus you do not know the trim (where she will float). If she is afloat you could estimate how the additions are going to change trim and floatation, but otherwise it remains unknown. CG and the mythical Clp are totally unrelated.

    You have an unknown hull, coupled to unusual rig and appendages. Thus you are setting up a high-risk scenario, high-risk that it won't work well. Most designers avoid this, and will require suitable payment to take responsibility. You might find one that will give an opinion but not take responsibility. Try to find someone with experience and judgment, rather than a guy who knows all the answers.

    Or it may be that you just have to build something and try it, then change things to make it work better. This is not the end of the world if you consider it a little now.

    Best of luck to you, Tad
     
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  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    dude,

    I'm not a sailing expert but... If the design permits it could be easier to move the mast than the centerboard inorder to balance the boat. Or, if the hull's launchable you could try sailing it with a leeboard until you get the location right then do the centerboard conversion later.
     
  8. globaldude
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    globaldude court jester

    Everything is "movable" right now, as I only have a bare steel hull [ excluding the rust on it ] So the mast is - well while the rake is ajustable, I don't think the "step" position is alterable a lot - because it alighns with a frame and needs the strength in my opinion.
    Tads comments have caused me to rethink my options/direction, mainly to take a breath, sit back and slow down.
    I'm just so keen to crack on with it, not getting any younger & all that !!.
    So no it's not "launchable" yet, but interesting you talked of "leeboards".
    I'd originally thought to use assemetrical leeboards, out board as they generally are, but mainly due to asthetics, opted to build in dagger boards.

    The Dutch, I'm told, sometimes had their leeboards set up on slides along the sides so as to be able to balance whatever sail plan they were useing on the day !!. Tha'd take all the [ fun!?] guesswork out of it eh !?.

    The advantages were no internal keel/board boxes, but they did " hang " out board and well maybe they looked ok . Probably the best advantage was they would swing up if they hit something -- likley when in shoal water .
    Internal outward slanting, foil shaped dagger boards would only retract strait up & so couldn't swing back if they hit the putty.
    My thinking is though, that I will be investing in [ a year or so from now] the latest forward looking sonar, which, from accounts I've read, is getting very acurate [ enough I'm told to be able to view other boats anchors & moorings] and with the alarm systems they have, should [ yeah right ! ] not hit the bottom with the board.
    I'll build them like out houses so in the event -----.
    Dam !!, wrote all that then see you were saying to use the leeboards just to get the position right !!.
    Surely I could get the position within ??, well good enough to be able to finnish trimming with sail balance eh.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Quick solution may be to go Dutch with a sliding leeboard on both sides as a temporary measure to establish the optimum size position for balance, then go for the daggerboard in the off season if you have one in NZ (we definitely have one in Canada caused by seasonally hard water). Make the DG a shade oversized and adjust its position if necessary by trimming the appropriate edge. Sounds like a plan, although the model approach might be easier.
     
  10. globaldude
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    globaldude court jester

    If my boat was in the water , just before the "off" season, I'd be sailing up to Fiji or Tonga, that's what most of the yachty's do here. They have an eternal summer !! .
    I've had to make decisions re the layout so as to be able to do a weight study.
    Which needs me to make a decision as to the placement of the boards.
    I got talking with a dutchman on the weekend, he's built & owned several yachts and said he felt , having looked at my "latest" draft plan, that my dagger boards needed to come foreward " a bit" , but that if I didn't want to, ajusting the rig , or retrofitting a bulb to the bow could/ would move my CLR forward enough to balance the boat.
    He's had one fitted to his yacht - the bulb thing - and as it is cylindrical, even with the boat heeled it still "held" the water in the V between the hull & bulb. " Another trick us Dutch do --- "
    I can imagine that working, and given it's so far forward, it wouldn't take a very big one to do the job --- if it needs doing that is .
    I don't have a scanner, so I'll get a couple of drawings scanned & post them so you can see where I'm going with this.
     

  11. naval ark
    Joined: Mar 2006
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    naval ark Member

    Retrofitting a bulbous bow to a sailing yacht?!

    And you thought that moving the leeboards 'a bit' would be too tricky? You should know that bulbous bows can give marginal gains, at the best of times, but for a sailing yacht that needs to operate throughout a range of speeds and heel angles, it's just not worth considering.

    If your boat looks like a tjalk or botter, then leeboards are the way to go. If you don't sail a barge however, a long fin or old-school centreboard would probably be best. The fin will be least sensitve to CE of sail plan, and the circular centreboard profile can alter your CLR as the board is lowered and retracted.

    This underwater profile coupled with a medium aspect rig, especially a multi-masted rig (preferably with a bowsprit for a traditional look), and it won't matter if the 'geometric lead' is 2% or 15%

    But please, forget about the bulb...
     
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