Lifting Spinnakers:does it lift the bow?

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

  1. Doug Lord

    Doug Lord Guest

    This subject was so off topic under the "Planing Trimarans" thread I moved part of it here to try to get more input. Other than my rc models I've had very little spinnaker experience but from my reading I'm convinced that certain spinnakers will lift the bow-particularly on skiffs. I'd like to hear anyone's experience and any references on the subject.....

    from the "Planing Trimarans thread:
    AP, Dave Culp is dead wrong at least on RC spnnakers such as these:
    Address: Changed:8:47 PM on Friday, November 24, 2006
    And I strongly suspect that on several of the skiff classes the spins lift the boat.
    When I develped these models they were the first fully gybable rc production spinnakers and in development we made hundreds of tests to ASSURE that the spin would lift the bow.In fact, the boats can sail in stronger wind dead downwind with the spin up than they can with just the main and jib! The spins are relatively small and flat but add tremendous power to these models; when a gust hits you can physically see the bow lift.
    105 see para 5:
    Address: Changed:12:24 PM on Wednesday, September 20, 2006
    "The power in the big spinnaker is incredible and the remarkable thing is it lifts the bow and actually make the boat easier to steer in big seas and big...."
    Correct URL is-scroll down to box-"What the 505 is"-this is an english site:
    The "Five-Oh" has verve!
    Address: Changed:4:53 AM on Thursday, April 1, 2004
    Dr.Ian Ward 07 March 1999 (his second letter 3.0 Longitudinal Stability) Spinnaker provides Lift
    Address: Changed:10:46 AM on Monday, January 30, 2006
    I e-mailed Dave Culp and invited him to participate in this thread.(11/25/06)
    Last edited by a moderator: Nov 25, 2006
  2. RHough
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    RHough Retro Dude

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    I'm not sure but I'm pretty sure this doesn't say anything about lifting bows?

    I hope Dave Culp jumps in here ...

    It ought to be fun.

    I'm making popcorn ... :cool:
  3. usa2
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    usa2 Senior Member

    Spinnakers almost always like to drive the bow down, not up.
  4. gggGuest
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    gggGuest ...

    Lifts the boat, but presses down the bow. Show me a video of the crew scrambling forward to keep the bows down in a gust and I might believe it lifts the bows!

    I suppose it might just lift the bow if the dimensions were extreme with the luff averaging less that 45 degrees to horizontal.

    With more angled luffs then the sail will have a considerable effect of stabilising the boat in pitch - as the bow pitches down the lift from the sail increases and the total pitch down force decreases. as the bow goes up the reverse. This effect would appear to be apparent in twelve footer videos.

    I think that, purely with emprical observation, it is very dificult to separate what happens purely due to the sail pressing down or not on the boat from the hydrodynamic effects caused by increased speed and the effect of the sail lifting the boat.
  5. Kiteship
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    Kiteship Senior Member

    "always" not "almost always." We settled this, believe it or not, in Sept 1995 in a long discussioin on The thread is here:

    Briefly, a spi may offer a net lift to the boat (it often does not), but as the major lift is either vertical at the stern (via guy and sheet) or forward at the masthead; there is *always* a strong positive pitch (down) at the bow. Always. Some boats are optimized for this drive, some are not, but all experience it. It is only necessary to replace the spi with an OutLeader kite to instantly experience the profound difference.


  6. RHough
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    RHough Retro Dude

    Thanks for finding that link. I was pretty sure that I'd been through this discussion at least once before (in space and time long ago and far away).

    It boils down to any driving force acting higher than the drag force of the hull pitches the bow down. Since the drag is at or below the waterline and the sails are above it, the drive force cannot lift the bow.

    If the sail is rotated so the force that was heeling the boat is now lifting the boat out of the water (heave), the whole boat lifts, but the drive force is still pitching the bow down.

    Reducing bow down pitch is not the same as creating bow up pitch.

    Look at the fore and aft crew position of a boat that is sailing with the bow out of the water with a spinnaker up. Try placing the crew in the same position fore and aft at the dock. Will the bow be higher or lower?

    Doug thinks that you and I are wrong, let's see if he can prove it. :)
  7. foilr
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    foilr Yes I've sailed one.


    I'd say the spinnaker does... but in a roundabout sort of way.

    In a skiff (18, 49er, 29er etc), the addition of the kite would provide the extra speed required to bring the apparent wind forward, thereby reducing the breeze that would've been pushing the rig forward/down.

    The bow-up aspect that you see on skiffs has a lot to do with hull shape and crew-position as well.
  8. Tim B
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    Tim B Senior Member

    ummm, ahhh, welll errrr, yes, sort of, maybe.

    In theory, if you can get the lift vector from the spinnaker to point directly away from the CB, yes, it will lift the bow.

    In normal circumstances, the lift from the spinnaker is more or less forward, and so you have a considerable force and lever. This generates a massive bow-down pitching moment.

    You can certainly reduce the moment, but what's it doing to the efficiency, and what was the trim angle anyway?

    The only way to find out is to run it through a VPP which will predict pitch, based on a pitching lever. Even a back of the envelope sum will show that the change in trim is very small.

    Tim B.
  9. Kiteship
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    Kiteship Senior Member

    You do not need anything as complex as a VPP program. Draw a sailboat. Note where its center of buoyancy is (very approximately is fine). Guess (within 10' and 30 degrees) where the resultant of the spinnaker is (a resultant is a line of force). Unless said resultant passes under the CB, the boat will pitch bow down. This is a physical certainty, not subject to reader opinion (not being snotty; just pointing out which part of the post is actually mathematics. ;-)

    Try every boat you can think of. Any combination of sails. Feel free to cheat. Unless you use a free-flying kite, you will get a bow-down pitching moment. Period. This isn't a close thing, not by a very long shot.


  10. Tim B
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    Tim B Senior Member

    You missed my point. The bow-down pitching moment is due to the driving force of the rig multiplied by a lever. So, unless there is no driving force, the bow goes down.

    However, you can change the amount of bow-down moment, but it's unlikely to get the most out of the rig, and therefore the boat.

    Do you know what the typical trim angles are upwind/downwind? except for very small boats I think you'll find it's a very small number.

    Tim B.
  11. QC3
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    QC3 Junior Member

    It seems any affect on the bow would depend on the force the chute applies to the vessel itself. The spinnaker only has three attachement points where it could influence the boat. 1.) The halyard - it would seem on most sloop rigs with the mast mostly forward that any force outward from the top of the rig would tend to drive the bow down. 2.) The sheet - which is tyically lead way aft. It would seem any force applied way aft would also tend to drive the bow down. 3.) The afterguy/foreguy (tack posisiton). Afterguys are typically led midships which would not help "lift" the bow so that leaves just the foreguy which is typically led to the foredeck. Can the lift of the foreguy alone counteract those other aftorementioned forces to lift the bow? I have usually found the opposite to be the case which is why you need to get all of your weight aft when off the wind.
    Just my layperson's opinion...
  12. Chris Ostlind

    Chris Ostlind Previous Member

    Thanks for the very direct mechanical description, Dave. I agree.

    I posted this note back on the other thread where this argument first came about. This has to do with the practical realities, but follows closely on the description as provided by Dave.

    The above quote was subsequently supplemented by Mike Leneman's quote below:

    I know it sounds racy to get your head wrapped around the potential of your bow being lifted in this application, but no amount of dream scheming is going to get you over the hump of the physics when coupled with the experiential reality.
  13. high on carbon
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    high on carbon Wing Nut

    Sail a Skiff of any kind, hoist kite, trim the kite, watch the end of the pole bend skyward. Hmmmm, What was that Newton second law thingy???

    Yes the mast is bending forward too, Net combined force? About 30 to 45 degrees up from the surface of the water, net effect = lifting the boat.

    I am old enought that I was around and designing and building boats in the I-14 class when we went to A-sails. even with a kite twice the size of the symetric kite, it most certainly provided more lift than it did pitching moment.

    It is certainly influenced by geometry but for sure it provides more lift than the old kind of kites. If we had a bigger A-sail and it produced more picthing moment, we would not have kept doing it. That's why we could all of a sudden start developing finer bows again, we spent a lot less time going down the mine.

    A-sails, set up right, lift the bow, period.
  14. RHough
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    RHough Retro Dude

    And that is why when a gust hits, you have to shift weight foreward to prevent the bow from skying? :)

    How about the CE is foreward and down compared to the old kites? Less pitch down does not equal pitch up.

  15. gggGuest
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    gggGuest ...

    I suggest you read the papers I quoted above. Lift the *boat*, sure thing, absolutely they do. Lift the *bow*? Not on a 14.
    Another thing you might try is to let the kitesheet go at speed...
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