Lifting Spinnakers:does it lift the bow?

Discussion in 'Sailboats' started by Doug Lord, Nov 24, 2006.

  1. Doug Lord

    Doug Lord Guest

    lifting spinnakers

    The info provided by Jim and Dave of past discussions is interesting and I appreciate it.
    I have 16 years development in rc spinnakers and there is no question that the microSAIL! symetrical spinnaker lifts the bow-it's clearly visible to anyone that's ever sailed one in any breeze above 10 kts.. There may even be video on the site. RC boats lose a lot of pitch stability in comparison to full size boats and if these sails didn't lift the bow they simply wouldn't work. Further, we've done hundreds of demonstrations in strong winds- with America One and the S50 to illustrate that without the spin in winds over 15 both these boats would nosedive and with the spin they don't. A specfic "set angle " has been determined that allows this kind of symetrical spin to lift. These sails are set at a fairly big angle off of twin poles and in lighter wind it is possible to sail as high as a beam reach since the sail behaves a bit like an asymetrical because the windward pole tightens the luff and leeward pole opens the leach-and tacking downwind pays in racing. In stronger winds(over 12-15) tacking downwind does not pay because the boat tends to get overpowered and lift from the spin is observable approx. 30° either side of dead downwind In over 15- 20k the boat can be sailed dead downwind WITHOUT REEFING only because of the lift from the spinnaker; dousing the spinnaker in those conditions results in almost an immediate pitchpole.Sailing higher than dead downwind with the spin set in those conditions knocks the boat flat until the sail is doused.
    There are very few that have experienced this -maybe 55 worldwide+ 14 or so prototypes but it is an absolute fact.
    In trying to come to a realistic appraisal of what is happening on full size boats your theory should be able to explain what is happening on these models as well.
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    a1a.jpg
    Address:http://www.microsail.com/images/a1a.jpg Changed:3:32 PM on Sunday, November 26, 2006
     
  2. Kiteship
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    Kiteship Senior Member

    Anecdotes versus mathematics, Doug. Show me the math and we can talk. (When you refer to "America One" and to "S50" are we talking about scale RC models or full size boats? I'm not knocking models, as you know I have no prejudice against models; just need to know the scale fo the experiment)

    Dave

     
  3. RHough
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    RHough Retro Dude

    It absolutely does Doug.

    Find the CE of the two sail configuration and the CE of the three sail configuration. I'll bet that the CE of the of the 3 sails is forward and possibly lower (probably in the case of fractional hoist spinnakers). No argument that there may be LESS nose down pitch from the forward/lower CE. Thus compared to the same hull with 2 sails in the same conditions the bow will be higher. That is not lift, that is less pitch down.

    The reduced pitch down and greater power may even allow the boats to sail fast enough to plane, on most rockered hulls, that is a bow up attitude.

    Here is a simple model sized proof:

    At wind speed 15 the boat appears to be bow up.

    If the bow up trim is due to lift from the spinnaker, at wind speed 21 the lift from the spinnaker should be double. Is the bow lifted further?

    At wind speed 30 there is 4 times the lift, is the bow higher still?

    At some point the "lift" from the spinnaker will exceed the displacement of the boat and it will be lifted clear of the water by the bow.

    If the sail is lifting the bow, a video of the boat sailing down wind should show the bow rising before the boat speed increases. From what I have observed this is not the case.

    What happens to the models in very high wind? Do they fly, broach, nosedive, pitchpole or plane?
     
  4. Doug Lord

    Doug Lord Guest

    lifting spinnakers

    In strong winds -20-30- you can only sail dead downwind with the spinnaker. If you do so without reefing the main and then douse the spinnaker the boat nosedives; if the spin is left up no nose dive period. One other thing: in rc sailing it's easy to see gusts moving and watch as they hit a boat: without a doubt and with no speed change a gust will cause a noticable lifting of the bow.
    I don't know how much rc sailing you've done but I'm 100% sure you've never sailed one of my boats in 10-12 and above.
    But try this as a mental excersise: if this is actually happening on these boats what does it mean for your theory?
    I'm not trying to be smartass- I'm curious about what you think could explain this behaviour.
    --------------------
    Note: "high on carbon" in post 13 is Magnus-a princible in the Canadian C Class catamaran team and an extraordinarily skilled sailor with years of experience in high performance sailing.....
     
  5. rayk
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    rayk Senior Member

    I can imagine a boat getting some lift out of a spinnaker. The stern in particular.
    Until the hull begins to plane the bows are dragging in the bow wave.
    Scampering aft in a boat unlikely to plane is instinctive for the crew.
    In a boat that is planing under spinnaker sitting aft doesnt seem to help trim and the crew could probably shuffle forward little.(Back to work you scurvy dogs!)
     
  6. Kiteship
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    Kiteship Senior Member

    This is not the case, in actual sailing. Fortunately for us, there are a number of over-powered sportboats these days, and a plethora of over-testosteroned crews with which to over-sail them. The limit of stability is when the bow is driven under--typically a monohull will broach before this happens, but not always (cf: going down the mine). The crew always stays back--and wishes for more "back" to go to--as the boat is more and more overpowered. I've been sailing for a long time, and I've yet to see a boat pitchpoled over the stern, due to too much lift forward from the spi.

    Despite assurances of the crew(s), *every* photo of a skiff running fast under asym has the entire crew standing on--or behind--the transom, out on the racks. Do this at the dock sometime, and see where the boat trims.

    Multihull power, thus speed, is *always* limited by burying the lee bow. Both upwind and down. Again, where crew weight is sufficient to do any good, you *always* see the crew trying to get further back--and going to depower when the lee bow buries.

    It's not a theory, Doug, but simple geometry. Draw the picture, chart the lines of force, calculate the moments. It holds for *all* waterborne structures. From your statements, you only see this "phenomena" on model boats, never full size? If so, I strongly suspect a scaling or froude number factor. I don't know what it is, but as *you* are the only guy who's replicated it, I'm thinking it's up to you to demonstrate why the geometry--and physics of the thing says it cannot be done--yet you can do it. (FWIW, it is fairly well known that skiffs with 3 sails up are more stable off-wind than identical boats with only two--it's been said here why--the effective center of drive is lower with the asym up. Lower the C of E and you will definitely reduce the pitching moment. Reduce, not reverse. I'd also note that removing the asym reduces the brute power of the system, resulting in lower boatspeed--and a marked shift aft of the apparent wind. Is this where you're getting your nosediving, without the asym up?)

    Dave
     
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  7. Doug Lord

    Doug Lord Guest


    See the Skiff picture on the cover of High Performance Sailing-crew about midship.
    I'm not sure but you may have something about the increase(not shift of) in apparent wind from behind when the sym spin is dropped on the model. But that doesn't explain the obvious rise of the bow when the boat is hit by a gust.
    ----------------
    PS-and when the chute is dropped the CE rises-that and the apparent increase may be an answer...
     
  8. RHough
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    RHough Retro Dude

    Here are a few more ways to test the theory of lifting spinnakers.

    Take the boat with spinnaker rigged and gimble just above the CG it so it is level, but free to pitch up or down. Mount the gimble on the hood of your car and drive down the road. If the spinnaker lifts the bow as you claim, the boat will pitch up in the gimbles.

    Take the boat and set it in a pond and attach a spring scale to the bow. Note how much tension it takes to lift the stem 1", now double the tension, then double it again. The pitch change curve you get is exactly what to expect if the spinnaker is lifting the bow. See if that trim comes close to what you have observed.

    As I asked before, when the "lifting spinnaker" boat finally crashes, What is the nature of the crash? Does the spinnaker lift the boat out of the water? Are you saying that no matter what the wind speed the boat never lifts out of the water and never nosedives? That would mean that the spinnaker produces less lift at higher wind speeds, but continues to produce enough lift to prevent a nosedive? That would be a very neat trick.

    You know very well that the lift force from the spinnaker will increase exponentially with wind speed. Therefore the boat should lift exponentially higher with each gust. Are you claiming that is what happens?

    You designed it Doug. You tell us how it is possible for the spinnaker to lift the bow at 10 knots yet not pull the boat out of the water at 30 knots.

    I once made the mistake of giving serious consideration to an idea that I know won't work. I won't waste my time thinking that spinnakers lift bows while driving the boat, it cannot happen.
     
  9. gggGuest
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    gggGuest ...

    I'd be wary about cannot: given a long enough pole and a short enough hoist I imagine it could happen, but I doubt such a rig would be very practical or desirable! Best to use one of Dave's kites - I bet they lift the bows like crazy!
     
  10. Chris Ostlind

    Chris Ostlind Previous Member

    Turbulent Layer

    How free of shoreline obstructions are these ponds where you observe these "lifting bow" phenomona?

    Due to the close proxitimity of the sail to the water, and the likelihood of shore clutter to make the surface turbulence worse, how do you know that you are not actually seeing the sails reacting to swirling surface turbulence?

    Since you don't mention it, I assume you have not considered the potential.
     
  11. DSmith
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    DSmith Junior Member

    You can't beat the physics but I have never cartwheeled the 49er with the kite up. 2 sail reaching on the other hand...... ahhhhh!....splash.
     
  12. Crag Cay
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    Crag Cay Senior Member

    All the 'definitive' examples for the lack of lift in a spinnaker have been on boats with mainsails up as well, ie the total combined effect has been quoted. I think the improvised 'car top' wind tunnel, or even a real wind tunnel test, would be needed to isolate the component forces.

    However some observations:
    The 'everyone aft - we're going to nose dive!' cry was more common when deep running on non spinnaker boats such as Albacores, Enterprises, etc. Right from early Cadet racing as a kid it was noticeable how much more longitudinally stability the spinnaker gave in a blow.

    I have always been surprised how much more stable even large keel boats are when running if you reef their mains first. It seems to dramatically reduce bow down pitch and therefore bow rudder. This has helped the handling of all sorts of over pressed boats from big IOR through to more recent big boats.

    I think long bowsprits and asymetrics have altered the dynamic again. There must be an some uplift component because it bends the long carbon 'lever' upwards in excess of the deflection I can get from two of us hanging on it, when back on the trailer. (400lbs). The tack to Centre of Flotation (rotation) distance (lift lever) is about 80 percent of the height to the small heavy weather spin hoist. This must be a different picture to that of a conventional spinnaker where, as was said earlier, only the foreguy has any attatchment forward of the mast.
     

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  13. PI Design
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    PI Design Senior Member

    For what its worth I agree with Doug, Crag etc on this one. Imagine a boat with a vertical mast and no heel. How can the manisail produce a force down (or up)? It can't, but due to the CofE being above the Centre of Flotation a moment is produced, which does trim the bow down. Please note, this is a moment, not a force. Now, any sail with an aft raked luff e.g. jib, asymmetric etc will produce a suction lift force with a forward component and an upwards component. If I could post pictures I would draw it for you, but its fairly easy to imagine and at any rate the upward pull on the bow sprit demontrates it. However, two opposing moments are produced. One is the result of the upward component being forward of the LCF, the other is the forward component being above the VCF (as per the mainsail example). The two moments oppose each other. Which ever is greater will determine whether the net moment is bow down or bow up. But, either way, the force component is upwards, not downwards - hence a reduction in displacement (even if the bow is trimmed down).
     
  14. water addict
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    water addict Naval Architect

    Totally dependent on the specific geometry of whatever boat. Sum the forces and moments created by where the spinnaker attaches to the boat (halyard, pole, sheet) and the CG + hydro drag. If the summed total moment causes pitch up or pitch down, then the boat will do that until forces and moments are in equilibrium.
    Of course you would need some pretty sophisticated sensors to determine this and a lot of data analysis. Better to just go sailing and have fun!
     

  15. PI Design
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    PI Design Senior Member

    And another thing...
    The only way a spinnaker can reduce the bow down pitching moment is if it acts in the opposite direction to the moment from the main and jib. Raising the kite may lower the lever, but it increases the force, so that the net result would be to trim the bow further down (if you're statement was correct). Therefore, if you admit that hoisting a spinnaker lifts the bow (compared to not ahaving a spinnaker), you must be agreeing that the spinnaker has a lifting moment.
     
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