Lifting Spinnakers:does it lift the bow?

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

  1. PI Design
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    PI Design Senior Member

    The conclusion is only valid for that particular boat. Okay, so I used the spinny lift data which is also only applicable to that boat :eek:, but I'm using it to make a point, not present absolute accurate calcs. The fact of the matter is that the force from the Cherub kite is acting at and angle of 41% to the horizontal. If you use that angle on the 12' Skiff dimensions, you get a bow up moment. If you use them on a Cherub (which has a shorter pole, I think) you get a bow down moment. I've only read the Cherub precis of the analysis, not the WB report itself as I can't access it. Any assumption on the location of the LCF is critical - can anyone who can access the WB site tell me what LCF value was used?
  2. water addict
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    water addict Naval Architect

    The world is Flat!!!! The world is Flat!!! Why don't you understand!!!
  3. PI Design
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    PI Design Senior Member

    You're right of course. I'm going to give up now and move on to proving that the earth rotates around the sun. It should be easier...:rolleyes:
  4. Baronvonrort
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    Baronvonrort Junior Member

    I dont know where you got your 12 ft skiff data and the mast heights,pole lengths and other dimensions are a bit out.Try >8m for #1 mast.
    In all fairness they do have 4 rigs of varying size to cover the 2-37 knot windrange.
    Sail area along with mast height and pole lengths are UNRESTRICTED in the 12 ft skiff class.
    A Cherub does measure as a 12 ft skiff although it will be much slower and i do not agree that they are very similar and many other dinghy's that are 12 ft long will also measure as a 12ft skiff.Not many rules with the 12ft skiffs!

    The skipper trapezes with both feet aft of the transom on a 12ft skiff when both are on the wire under kite.The crew would have his back foot in line with the transom.Depending on how fat the skipper is this gives a pretty large bow up moment.

    Carry on
  5. PI Design
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    PI Design Senior Member

  6. Baronvonrort
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    Baronvonrort Junior Member

    There was absolutely no interest in the restricted 12 ft skiff down here and it was originally an idea to reduce costs for beginners.

    There were 0 boats registered as restricted 12 ft skiffs.

    The cricket is going well for
  7. Andy P
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    Andy P Junior Member

    In the real world, in a cherub, the kite does stabilise the boat ( ie it's easier to get downwind in a blow with the kite up - apart from the hoist and drop) - there is noticeable lift from the kite so the whole boat is lifted, and it damps out the pitch.... it feels like up.. up ...up...up then DOWN then up.. up etc.
    2-sailing there is much more frequent down pressure.- with the ups and downs reversed.
    But the test is the flap whilst whizzing along - a flappy kite means the bow goes up ( and the boat sinks - the transom definitely goes down - well it would with both the crew at the transom. )
    Some AUS cherubs have footloops along the transom - not the gunwales ( ie the crew stands right over the back )

    With mods to the UK boats for bigger kites / masthead kites a few yrs ago, it was the feeling that masthead kites were faster, but more nosedivey, and longer pole short hoist kites were slower but easier to sail without nosing in.
  8. CT 249
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    CT 249 Senior Member

    I have no idea what this pic means, but I have never seen a Heron this bow up at such speeds.

    The skipper was sailing alone and is a generously sized lad.


    (For the benefit of most readers, I should say that the Heron - a '40s (?) cruising dinghy - doesn't normally carry any spinnaker here in Oz, much less something that looks like it was found in the beer trough after a 12 Foot Skiff party).
  9. Doug Lord
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    Doug Lord Flight Ready

    Lifting Spinnaker?

    It's been a while but I don't think this is settled.....(don't ignore the humor):

    Attached Files:

  10. bgulari
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    bgulari Junior Member

    the moth does not count

    that is a computer generated image
    1 person likes this.
  11. Paul B

    Paul B Previous Member

    Why even reply to his nonsense?
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    I missed this thread earlier so it's good to have it come to life however briefly to allow those like myself another opportunity to take a shot at it.

    Doug asks "lifting spinnakers: does it lift the bow?" That a spinnaker lifts the boat seems beyond question, and it is, after all attached well forward. No doubt a kite sail can lift the bow if attached sufficiently far forward and flown with the tether at a sufficient angle. But a spinnaker is also attached high at the mast which would simultaneously depress the bow.

    I'm sure a spinnaker can be designed to lift the bow in the absolute sense like a kite. It would have to be attached well forward certainly using a bowsprit and have a great deal of rake. Those shown in Doug’s latest pics do have those features; if they are real photos that is, but it’s still a maybe.

    Maybe the question should be reworded: “when the spinnaker is deployed is the bow higher than it would be motoring at the same speed?”

    Does that give us a testable theory?
  13. Chris Ostlind

    Chris Ostlind Previous Member


    I'm assuming, then, that you have read the entire thread to this point and still have some questions that have not already been totally mashed into the ground?
  14. Doug Lord
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    Doug Lord Flight Ready

    Terry, they are real photos except the Moth, of course-which is why I said: "Don't ignore the humor".... On the models mentioned at the beginning of the thread there was no question that the bows were lifted-you could watch a gust hit the boat and see the bow lift up. And there is no question that the right spinnaker can reduce the displacement of the boat(as can a main working with a spinnaker)-see below. I hoped that by bringing this up three years down the line there might be some fresh thinking added.......

    Spin 50 and America One racing(first production gybable RC spin boats):

    Attached Files:

  15. Doug Lord
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    Doug Lord Flight Ready

    Heres a very interesting comment from 2002 on the multihulls list:

    "I haven't followed the thread Dave mentioned in his post but I _did_ have
    enough Math and Physics experience to say the following:
    - a boat sailing at a constant speed under whatever headsail , be it spi,
    genoa, blade jib or reacher fulfills 2 basic (vector) equations, 1)- the
    sum of all forces is ZERO and 2)- the sum of all moments is zero . That's
    it, neither more nor less .

    The relative difficulty comes from quantifying the said forces and moments.
    The forces are also interdependent . First is the sail lift vector (which is
    projected along the horizontal x axis - thrust- and its magnitude is equal
    but has opposite direction with drag , and the vertical component plus the
    boat displacement plus boat buoyancy equals zero) . Lift unfortunately is
    dependent on apparent wind , which is dependent on true wind and boat speed
    which is dependent on boat drag. Hard to put into equation and solve .

    The forces that generate moments are the sail lift, boat drag, boat buoyancy
    and boat weight . The sum of all these moments w.r.t. an arbitrary point is
    zero. Some of the moments are acting as follows:

    -boat drag tends to rotate the bows down .
    -headsail lift vector would tend to rotate the bows either up or down,
    depending on the vector's angle and sail center of lift position. The moment
    is the distance between the sail center of lift and the hull center of
    rotation (this would be the arm) times the lift vector. The only thing that
    matters here is if the lift angle is more or less than the hull center to
    sail center line and horizontal . If it's above, the sail lift vector has a
    net effect of lifting the bows.

    For Dave, who tends to treat kites as a very special case, be assured that
    mathematically the same above mentioned equations don't give a rat's a*s
    about what specific numeric values some of the variables might have. For
    instance, gradually lower the sail center , gradually enlarge the sail area,
    increase its tilt angle and the forces resemble your kite boats. Increase
    the sail center hieght, reduce its tilt angle and gradually you will get a
    picture which will represent a traditional rig. It's not good kites vs. bad
    rigs, it's just a matter of quantity . Take a large enough headsail, a low
    enough aspect rig, a long bowsprit, and you'll see the bows lifting. Take a
    small enough kite, attach it too high above deck and you'll see the bows
    bury . The main thing is the same equations apply. If you want to optimize
    things, you may do so with the equations (provided you wrote them first
    } ). There's absolutely no reason for which headsails would be prohibited
    from lifting the bows and kites would be allowed to do so . If your goal was
    to lift the bows you could optimize for that. I'm pretty sure the overall
    drag wouldn't be optimized at the same time, or the thrust would be
    maximized , so your optimization might be local . Sailboat desing seems to
    be today at about the stage airlpanes were before WW1 , a lot of gut feeling
    and seat of the pants and very little science , a little voodoo if you

    Best Regards,

    Matt Tudor , MSEE "
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