Raising Centerboard

Discussion in 'Laser Talk' started by StinkBug, May 30, 2011.

  1. StinkBug

    StinkBug New Member

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    Someone brought this up in another thread, and I thought it would be better to start another discussion about it instead of getting that one off topic.

    I guess I'm just a little lost on exactly what is going on here and how it works. Can anybody elaborate on this a little? I'm pretty light for a full rig and it's not uncommon to be rather overpowered.
     
  2. sailin in Eire

    sailin in Eire Member

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    if yur overpowered id think about geting a radial rig
     
  3. 49208

    49208 Tentmaker

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    The centerboard works with the sail to drive the boat forward. It's providing resistance against the water sideways and once the boat starts moving forward, generates lift. One of the results of this is a heeling force.

    The best way to visualize (or do it in actual conditions) it. Sailing upwind, pull the board up all the way - the boat will slip sideways, loose forward drive, and require much less weight to keep the boat upright. So now we know that we can reduce heeling force by raising the centerboard - But, we also learn that we loose some lateral resistance, ie we slip sideways instead of going forward.

    So, the idea of raising the board 2-4 inches when you can no longer hold the boat down is an attempt to reduce the lateral resistance enough to get keep the boat flatter. You will point a little lower, but won't actually won't be slipping sideways thru the water as much as when you heeling a lot.

    In the steps of depowering, it's usually the last step, ie you are maxed on cunningham and vang, sheeted out, and still overpowered and heeling a lot.
     
  4. laserxd

    laserxd Member

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    Raising the daggerboard for upwind sailing is not a good idea, it might it harder to keep the boat flat and you'll slip sideways losing distance. Think about the boat being flat, the deeper the blade the more force is required for the blade to displace the water. If you sail with a leeward heel, the blade has less "grip" because of this principle. Even if you are planning upwind its still not a good idea.

    Your best bet is to use enough vang so that the blocks don't rise (or rise much) when you ease the sheet, use the cunningham, and ease the main when you're overpowered. You'll sail much faster using the vang and easing the sheet, if its really windy you can be eased more than you'd think, remember you can't get height without speed.

    lastly, when you become overpowered you need to hike as hard as you can
     
  5. 49208

    49208 Tentmaker

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    Huh ? Your first paragraph is wrong and confused. Raising the board makes it easier to keep flat by reducing lateral resistance.
     
  6. laserxd

    laserxd Member

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    I don't know I was trying to think about it in a different way, you're right about it taking less of a moment to right the boat with a raised daggerboard, and that would probably be a good idea in survival conditions where you're at a complete loss of control, my theory is that when the boat is flat and the daggerboard is all the way down, the daggerboard must displace more water for the boat to heel either way, making it harder to right but providing more stability and more lift assuming you can keep the boat flat. Essentially when the blade is all the way down and the boat is flat, it requires a greater force to displace the water to either side then when it is up because there is more water surrounding the blade when it is at its deepest point.

    Imagine the boat fixed at the bow and stern dead center allowing the boat to rotate side to side, the daggerboard will act like a paddle. if the boat is heeled it will be harder to right the boat with the board down because of this, if the boat is flat there is more water that has to be dissplaced in order for the boat to heel to leeward while sailing to windward.

    Maybe I'm wrong, but the top guys always say to keep the board all the way down on the windward leg
     
  7. wessel

    wessel Member

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    If you watch the video, Ian Bruce mentions that the daggerboard for the Laser is too long. So for sailors at the *designed weight* it may be better for them to raise the daggerboard up thus sailing faster and then generating more lift over the foils to keep from loosing height.

    The daggerboard is like an airplane wing, when it is going too slow it cannot *fly* or generate lift. so if you are overpowered and not moving upwind fast enough your foils are no better than a canoe paddle. if you raise the board then the boat moves faster and the foils begin to work generating *lift* countering the leeward drift. That's the idea that Ian bruce was talking about.
     
  8. wessel

    wessel Member

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    The Laser daggerboard is a symmetrical airfoil or and airplane wing stuck straight down from the bottom of your boat. when the boat moves, water flows over the *foil* and creates lift to one side or the other depending on it's *angle of attack* resulting in higher pointing ability. I know a good Finn sailor and he says the the Laser can point higher than the Finn because of the Laser's symmetrical foil daggerboard (Finns have flat metal blade for a centerboard so it isn't considered a foil). So if your boat isn't moving fast enough the foil effect will not work.

    Symmetrical airfoils are the same on the top and bottom, they don't have a flat bottom that you see on regular planes. You have to keep the foil's angle of attack in a certain range and speed too keep the foil flying, so just by changing the angle of attack to the other side the foil lifts on the other side. Stunt planes have symmetrical airfoils this is why they can fly-upside down really well.

    http://en.wikipedia.org/wiki/Airfoil

    I think 505's have a special centerboard that allows the skipper to change the angle of attack of the foil in the centerboard case. This would help pointing in different wind conditions depending on how fast the water is moving over the foil.
     
  9. Strangler

    Strangler Member

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    No! Any of us involved in the old days of windsurfing know this is not what happens. Beating upwind on a windsurfer with a big centreplate in strong winds the hull would often 'kick up' ie capsize if your technique was not right. With a universal joint at the mast base this is obviously not caused by the rig. It is the increased efficency of the foil with the faster speed, but the effect is there all the time unless stalled. So the laser centreboard is trying to capsize you as well as the rig, both independently.
    Displaced water is a red herring [do you have them in USA?]
    My board lifts a bit by itself, I let it stay there in strong winds.
     
  10. Strangler

    Strangler Member

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    You are referring to gybing boards. Used in many classes but their popularity comes and goes. If anyone is interested I can email an article I wrote for another class explaining them in simple language. PM me. Irrelevant for Laser sailors.
     
  11. Strangler

    Strangler Member

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  12. wessel

    wessel Member

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    Thanks Strangler ... I love the 505

    I do think it is relevant to the Laser! we are trying to understand how to use the centerboard and looking at other boats and how they work in comparison is a good way to learn.
     
  13. jonathan_twite

    jonathan_twite Member

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    In simple terms: The wind puts a sideways force on the sail (as well as the forward forces) which pushes the boat sideways. If you have no centreboard, this will not cause the boat to heal, only slide sideways. By putting the centreboard down, you are resisting this sideways movement. However, the wind is pushing the sail - the top of the boat, the centreboard is resisting at the bottom of the boat causing it to rotate.

    Think about a person tripping over. Their momentum going forwards is centred on the upper half of their body, as their foot hits something, the resistive force is applied to their feet. They rotate forwards.

    The centreboard reduces sideways movement at the expense of needing to correct more heel. You need to find the balance for the conditions.

    (N.B. this does not apply when running where the centreboard does give some more stability)
     
  14. Nipper

    Nipper New Member

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    I'm not convinced any other boats board issues are relevant to the Laser, because the Laser is a slow una sailed boat with a dagger board.

    There are a lot of aerodynamic, hydrostatic and physical things going on in 505's & Fireballs etc, because they can adjust everything. They use centreboards for a start (often with gybing boards) and as the wind gets up, they raise the board a bit, which stops it gybing, but more importantly it moves the centre of effort back, which matches the movement of the C of E in the sail plan which happens when they rake the rig back. This then allows the boat to go faster, so the foils do generate more lift, so the raised centreboard produces less drag, and the VMG is increased over and above the loss of lateral resistance. (I said it was complicated!)

    In 49ers and INT 14's they do raise their dagger boards, but then their speed increase is huge compared to a laser, so they ae desparate to lose drag and increase VMG

    In our Lasers, raising the daggerboard when beating does nothing to move the centre of effort, and the speed of a Laser to windward hardly changes as the wind increases, so there is probably minimal extra lift going to be generated through extra speed.
    However, like Strangler my board tends to raise itself by an inch when it is windy, and I leave it alone when I'm sailing in waves, as I find the boat slightly easier to sail over the lumps because I need to bear away so I do not get stopped and get blown sideways. However in flat water I always have it right down, as you can afford to luff in the gusts and not lose too much speed, and hence increase leeway.

    However more important that all my theorising, the are a lot of men and women out there spending their lives sailing lasers to get to the Olympics, with coaching, videoing etc. Whilst they are all perfect physical specimens that can hike for hours on end, I have never heard that someone has found a radically new way of using different amounts of board for different conditions upwind. As far as I know, they spend ages getting a perfect finish on the board, and making sure the brake is brand new to try and stop it slopping around, and then sail with the board right down.

    Maybe they do raise the board when it is really windy, but I have never read that.
     
  15. 49208

    49208 Tentmaker

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    Pulling the board up is the last item/resort in depowering - It's not that often that racing takes place in conditions where the top guys need to consider it, they just keep on hiking hard and working the sheet. For the weekend warrior though, it can be effective, especially for the lighter folks. Long before the advent of the radial rig, this was used a lot, as you had 130-150 pounders sailing the full rig (now they move to the radial)

    I can't tell you if it started with Sunfish (a class that came before Lasers), but that's where I was first made aware of it and they are similar in that both are daggerboard/mono sail.
     
  16. laserxd

    laserxd Member

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    so which is faster upwind in overpowering conditions?

    1. easing the sheet to keep the boat flat with lots of vang and the board down

    or

    2. raising the board a few inches making it easier to keep the boat flat
     
  17. 49208

    49208 Tentmaker

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    To repeat, raising the board is only done once you have exhausted the other depowering methods.

    However, if you are overpowered to the point where you are sheeted out 3-4 feet or more constantly, you don't need full board down.
     
  18. wessel

    wessel Member

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    According to Ian Bruce, if you are a lightweight then pull up the board a bit and sail faster and let the foil work to gain height.

    I saw an article somewhere on the net where someone did a test on this and tried all wind conditions with the board at different settings I will try and find it.

    Most people that I see slam the board down and grunt it out.
     
  19. manateerichard

    manateerichard Collegiate Sailor

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    hmmmm new idea to me, I'll try it next week when the wind picks up. But the theory behind it seems right, especially for the lighter people sailing full rigs like myself where we find ourselves heeling a great deal even when we are hiking full out.
     
  20. Nipper

    Nipper New Member

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    Simply questions, but no easy reply, as it does depend a lot on the water/wave conditions.

    I'm 180lbs (82kg) but when I started in Lasers with a ful rig I was 165lb, and I sail on the sea. If I was fit enough I could be described as a weekend warrior.

    In flat water overpowering I use board right down, max. cunningham, max vang. normal clew outhual and a combination of feathering to windward, and easing the main. You can use the boats' momentum to keep moving in the gusts.

    In overpowering with waves the boats momentum is killed off by the oncoming waves, so you have to keep the boat moving forwards.
    I use max cunningham, start with max kicker, and normal outhual, and board maybe 1 inch raised (only because it seems to want to raise itself to this point). If it gets really windy, I ease the kicker, tighten the clew outhaul, and let the board up another inch.

    The reason for this, is that without the kicker on, I find the rig can "pant" when the gust strikes allowing the wind to spill off the leach, and this panting is much quicker than I can react.
    With the kicker on as the gust strikes, you heel over, you ease the sheet, the end of the boom catches in the waves, so you heel over more, the boom in the water makes you luff up, so you sail straight into a wave, which slows you down even more, which then means that there is more load in the sail, so you either capsize or go into irons. (also having the kicker off makes tacking a lot easier in waves)

    Works for me, and I think that theer is a Rooster DVD or article, where Steve does say letting the kicker right off in survival conditions is the way forward.
     

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