weatherhelm- input needed pse.

Discussion in 'Sailboats' started by wayne nicol, Jun 9, 2011.

  1. wayne nicol
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    wayne nicol Senior Member

    hiya all,
    i sail my 15' commodore planing sailboat regulaly, and i have always noticed that as soon as one slacks off a bit on the tiller she blows right into the wind( irons!) is that correctly termed as the boat having weather helm.
    tha harder and faster you sail her, the more work it becomes to keep her on track- pretty brutal at the end of a day- now obviously a nice safety feature, when i land in the drink, well the boat is right there.
    so my question is:
    1. how is this designed into the boat( i suspect it is the relation between sail ce and cb) i have my ideas on this- but i really want to hear some expert opinions on this
    2. so how would you design this out of a new design- assuming you would want to do this.
    i have read about blue water boats being "self sailers"- and i see the risks involved.
    3. so what is the compromise- a happy medium- easy to sail- yet a margin of safety built in.
    i am not about to design my own boat- i just really want to understand this phenomenon better.
    thanks.
    wayne
     
  2. gggGuest
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    gggGuest ...

    Nine times out of ten the biggest single factor is sailing the boat heeled. If you keep her upright the boat will go faster and steer more easily...
     
  3. Olav
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    Olav naval architect

    Wayne,

    you are right regarding the difference between sail centre of effort and lateral centre of effort as the cause for weather helm (or lee helm, if the sail ce is fwd of the lateral ce). Other "sources" of weather helm are the distance between "driving force" vector from the rig and resistance vector from the hull and the asymmetry of the underwater hull as the boat heels.

    The problem is that both centres of effort are not stationary but subject to significant changes at different conditions of sail. Speed, heel angle, sheet angle, sail trim and so on are factors that influence the position of these centres.

    Unfortunately there is (to my knowledge according to well-known textbooks) no such thing as a fully developed theory how to place these both centres relative to each other. There are many different rules of thumb that are suitable for certain types of yacht and not so satisfactory for others; most yacht designers have experience with "their" method of choice and stick to it.

    It is common to use the geometrical centres of area. Some methods only take the foils (keel/centreboard and rudder) into consideration, others include the lateral area of the hull, the rudder is excluded in some cases, others rate keel and rudder with some empirical factors and so on. The difference between lateral ce and sail ce (called the "lead") is a certain percentage of the hull or waterline length; the exact values also depend on the type of yacht, the designer's experience and so on.

    Weather helm is not only a safety aspect, it also gives feedback to the helmsman and it unloads the keel (as the rudder takes some of the required lateral force) which in turn reduces leeway and thus drag (and induced drag from the keel), but it must not be too large, of course.
     
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  4. daiquiri
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    daiquiri Engineering and Design

    I second Olav's words. Designing a boat for a correct weather helm is a mix of experience, knowledge, tradition and rules of thumb.
    The fact is - there's a truly incredible lack of publicly available aerodynamic data regarding the sailboats' rigs. The ones I've come upon are all very incomplete, and the evaluation of center of pressure appears to be the very last item in the research agenda. Without the solid experimental data, one can only grope in the semi-darkness and hope that that the thumbs which were good for that other guy will be good for us too.
     
  5. MikeJohns
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    MikeJohns Senior Member

  6. Gary Baigent
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    Gary Baigent Senior Member

    If you're sailing your boat level, Wayne, and your sails appear to be set and shaped correctly - and yet you still have savage weather helm, then your centre/daggerboard has to be shifted aft. Alternatively, deepen your rudder and increase its area (that will help shift your centre of lateral resistance aft) ... but first of all, and this may be your problem, check that your present rudder is vertical, or even raked forward a little; if it is raked aft, that will produce the feel and reaction of heavy weather helm too.
     
  7. wayne nicol
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    wayne nicol Senior Member

    thanks all, a lot to think about- i always try to keep the boat as flat as possible- being a planing hull,but not always possible. last time i was out solo in a good blow- and i always try to trim the boat as best as i can- by moving forward to eliminate the excessive rake( when solo) and spill as much wind as i can, to try and keep it as flat as i can- but it does make the tiller control tough when i am a midships.
    this boat has a lot of sail area- it is by no means a doddling sail. maybe i need to buy another boat that is a bit more relaxing, so that i dont spill my wine whilst sailing:D
    but i obviously need to pay more attention to my other trim factors!!!!
    and experiment a bit more.

    so how does a designer ensure "self steering" in a design- and what are the cons with this.
    thanks
    wayne
     
  8. CutOnce

    CutOnce Previous Member

    Self steering in dinghies is kind of a misnomer. Boats with balanced helms can often be steered by boat trim - hands off the tiller and even with the rudder removed in many cases. I've seen good boat handlers sail completely under control around a race course on boat trim alone - including spinnakers. Sailing Anarchist member "Sten" has published a DVD on Musto Skiff handling which shows this.

    All that said, dinghies are NOT boats that self steer like a keelboat. Control of the boat is active and involves constant movement and much more work than sitting at a tiller. If you are looking for a hands-off, sit still and relax experience, dinghies are going to disappoint.

    Your best bet is to hunt down a local expert sailor, and get them to go out with you to assess your boat and it's handling. They'll quickly be able to determine how good or bad your set up is, and if remedial rig tuning is necessary. Hands on experience is what will get the best short term results.

    --
    Bill
     
  9. wayne nicol
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    wayne nicol Senior Member

    thanks cutonce
    not wanting to neccesarily throw my rig out, i just realized how much work it is to sail it , and wanted to understand the concept and principles better, but i hear exactly what you are saying, and i am SURE that i could always do with tweaking to improve both my boats and my performance.
    thanks again
    wayne
     
  10. gggGuest
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    gggGuest ...

    There is an awful lot of misunderstanding on this topic - well visible in the quoted thread. A modern dinghy with a flat underside and separate centreboard and rudder is a completely different animal from a traditional yacht with a rudder attached to the keel, and yet people insist on treating them the same way and think the same sums apply.

    To make matters even more complicated there are two separate phenomena which people describe as weather helm and one of them is normally benign, or even beneficial...

    If you consider a modern racing dinghy then it has two efficient foils in the water, a centreboard and a large rudder, some feet apart, which provide the sideforce which reacts against the loads from rig and crew. Both of these foils will be providing "lift". The actual percentage of the total lift provided by each one is hugely variable. You don't feel the lift from the daggerboard at all, but assuming a conventional rudder which is pivoting at the leading edge then the greater the lift the foil is generating (and the further the centre of lift of the foil is back from the pivot) then the harder the tiller will pull on the hand, and if you let go of the tiller the boat will fly into wind. Its IMHO a mistake to regard this as weather helm per se, because this lift is not slowing the boat.

    The second phenomenum is what I regard as true weather (or in reverse lee) helm, which is where the boat is permanently attempting to turn, and the tiller has to be held well offset from the centreline in order to keep the boat straight. This will kick in as soon as you let a boat heel, and the more the boat heels the worse it will get. How much worse depends on the hull shape, and modern wedge shaped hulls will tend to generate more weather helm when heeled than traditional ones which are more symettrical fore and aft.

    OK, so lets examine what happens when you move the rig about in the first situation. If you start with the boat set up so there is equal lift per square foot on rudder and centreboard then the tiller will be pulling quite hard on the hand, but whilst the boat is flat the tiller will still be pretty much on the centreline. If the rig is moved forward then the centreboard gets a higher and higher percentage of the lift, the rudder does less and less, and it pulls on the hand less. However it will still essentially be on the centreline. There are the odd 4 or 5 degrees about angle of incidence of rudder depending on loading, but no more than that. In order to get lee helm you'd have to move the rig ridiculously far forward - in practice this only happens if you sail with just the jib up.

    Similarly if you move the rig aft the rudder will get more and more heavily loaded compared to the centreboard, the tiller will pull harder, but the tiller will still be on the centreline.

    This is quite different from a traditional yacht with combined keel and rudder, where all the load is on a single foil. If you like the centreboard and large rudder on modern dinghies is balanced like a man standing with his legs apart, so it needs a very big push to send him off balance. A traditional yacht, on the other hand, is like a man standing with both feet touching, and he can be pushed off balance much more easily.

    On modern racing boats they move the rig forward and aft spectacular amounts in different weather conditions, and the tiller will still be essentially on the centreline. If it were significantly offset it would be causing huge amounts of drag and be slow.

    So the first observation is that as soon as the rudder is taking up sideload then the boat won't self steer. This is just the way things are I'm afraid, you do want the rudder foil to be taking load: its more efficient.

    OK, so your aim is to make the boat easier to handle. Moving the mast and rig back and forward per se won't make that much difference because you are just fine tuning the load between the two foils. Now if you were sailing with a top notch racing forewardhand you'd find he/she'd be playing the mainsheet continually to keep the boat bolt upright. Fine, but that's probably not what you want to do. So what can you do...

    Th first thing is a question of what you are used to. Unless you are enormously more naturally gifted than most of the rest of us then you probably reckon the boat is upright when its actually heeled 10 or 15 degrees. Just seems to be what humans are comfortable with. So try and sail the boatso its heeled slightly to windward - say 5 or so degrees on top of you. This feels horrible and unnatural until you are used to it, but if you can manage it - well, lets say you're in a boat that starts getting horrible weather helm when heeled more than 20 degrees. If you are already heeled 10 degrees then a gust only need tip you another 10 dgrees and the boat starts getting clumsy and hard mouthed. If you are heeled 10 degrees the other way though the boat must tip 30 degrees before it starts misbehaving...

    The other things is to do with the rig. There may not be much you can so about this, but a flexible mast will automatically spill wind and need less sheeting in and out to stay flat. You can make masts, to a reasonable extent, less and more flexible by playing with things like spreader lengths. But that's another topic...
     
  11. wayne nicol
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    wayne nicol Senior Member

    that explains a lot ggg.
    thanks.
     
  12. sharpii2
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    sharpii2 Senior Member

    Try pulling your centerboard up a bit.

    Since it pivots aft as it is pulled up, the Center of Lateral Area will move aft as well, reducing your weather helm

    I used this trick with my Siren 17 all the time. I even counted the turns of the lifting crank as I let the board (actually a swing keel) down.

    When I first got the boat, it was so hard mouthed it cracked the rudder cheek, as my super strong brother tried to wrestle off the wind. As I got to know it better, I could hold the tiller with just two fingers.

    I don't know how experienced a sailor you are. Is this your first sailboat?

    If so, you may be pulling the sails in too far. Try letting them out until they flutter then pull them back in until the fluttering stops. Shortly after the fluttering stops, you should notice a sudden surge of power. This is when the sails are set at their most efficient angle, relative to your course. Pulling them in further does not increase your speed.
     
  13. messabout
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    messabout Senior Member

    To further complicate the good advice given above, the condition of the sails is also a factor. Old blown out sails may very well contribute to weather helm. Good sails, not set up properly, often contribute to helm. Thats why someone above suggested the council of an expert sailor in your area.
     
  14. sharpii2
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    sharpii2 Senior Member

    Let's not go nuts with this expert stuff. Experts are expensive, often hard to find, and come often with no way to know if they have the expertise they claim. And they often come with hidden objectives (such as selling new sails)

    Why not see what the real situation is?

    Perhaps the problem can be tracked down with a few simple tests which the owner can do himself.

    1.) Make sure the sails aren't sheeted in too far. Let out the sails until they start to flutter, then pull them back in slowly until the fluttering stops.

    If the boat still insists on rounding into the wind

    2.) try adjusting the centerboard setting.

    Even with bad sails, this will probably still work, as the Center of Lateral area can be moved quite far back in relation to the Center of Sail Area.

    My guess is that the test for 'blown sails', other than them resembling open trash bags when set, is the boat refusing to sail to windward at all. This very thing happened to me with my home made sails made from plastic drop cloths. It was fixed by duct taping a 1.0 ft piece of quarter round across the top of the sail. Even then, if you would have seen those sails, you would have laughed. They had an oval airfoil section. But they did work. They drove the boat to considerable speeds, including upwind, and taught me and at least two others how to sail.
     

  15. Perm Stress
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    Perm Stress Senior Member

    Try to:
    radically increase mainsail luff tension
    let the traveler down
    let go the kicker (upwind only, for so long as traveler can remain directly below boom, than kicker will need to be tensioned)
    pull the sheet down hard so the mast bends and leech is straightened
    allow the main to "breathe"

    -this often help.
     
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