Kite, 49er, Moth: who's the fastest of them all(around a course)?

Discussion in 'Sailboats' started by Doug Lord, Dec 3, 2010.

  1. Doug Lord
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    Doug Lord Flight Ready

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    Like this: I drew a sketch of the kite guy at the angle he appeared to be sailing ,then drew a vertical line down to the water from his CG and measured the distance from the center of lift of the board to the kite guys' CG.
    I would think max RM would have to be the limiting factor.....
     
  2. Cheesy
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    Cheesy Senior Member

    Ok you are kind of right, that may or may not be the max RM as you can go as low as having your arse dragging in the water. The big difference is the lift vector from the kite and the corresponding lift vector of the board, as you can imagine the lower the rider to the water the lower the centre of effort of the kite and the lower the angle of the lift vector of the board (fins for a race board or edge for a twin tip or speed board), the lower these angles the closer they are to being equal and opposite with the remainder being the rider mass.
     
  3. Doug Lord
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    Doug Lord Flight Ready

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    The CG I used was extremely generous at 3.4'-it would be physically impossible to get the crew CG further away from the center of lift of the board unless you were way, way over 6' tall.......or wearing stilts.
     
  4. cardsinplay
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    cardsinplay da Vinci Group

    Here's the real point, Cheesy.

    You'll never get that character to admit anything if it doesn't improve his overly-hyped position in the first place.

    Example: He just loves to compare foiling boats to conventionally designed craft without lifting foils as a speed differential of monumental consequences. But, when another craft has much better performance than his favored craft and it is sans foils, suddenly all the rigorous descriptors come flying out of his mouth like so much twaddle.

    This character is not consistent in his analysis requirements and should never be taken seriously. You know the old saying... Never wrestle with a pig cause you'll only get covered in his toilet mess and the pig likes it...?
     
  5. Cheesy
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    Cheesy Senior Member

    As I said, the RM is not all that important, in fact the rider mass should really be thought of as ballast, not RM at all, as long as the rider is heavy enough to stay water borne all is good, that has nothing to do with RM or SCP.

    You need to draw all of the force vectors acting on the kite and rider to understand what is happening, then you can alter your diagram to incorporate the fact that both the kite force vector (the tension in the kite lines) and the board normal vector (assuming a twin tip board this force is perpendicular to the bottom of the board) directions and magnitudes can be independently altered.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    By the way the latest kite boarder in the SA competition climbed up to windward, I'd say that board is also acting just like an angled foil too. I mean, that was really impressive ... and I have to bite my tongue.
     
  7. Cheesy
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    Cheesy Senior Member

    Very true, it does give me a good opportunity (practice) to articulate a scientifically based argument and possibly educate others that may be interested in the topic though, as you know kites and there abilities are not well understood by the general sailing community.
     
  8. Doug Lord
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    Doug Lord Flight Ready

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    Cheesy- RM(Righting Moment) is the weight of the ballast(crew) multiplied by the distance from the center of lift of the board to the intersection with a vertical line thru the ballast(crew) CG. It is critical to evaluate the sailing performance of any sailboat and kites too.
     
  9. Cheesy
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    Cheesy Senior Member

    I know what RM is and Im still saying that it is not that important with regard to kiting, for sailing it definitely is. In fact in light of this thread I tried a couple of things this afternoon...
    With a Velocitek stuck on my board (132x39 underground) in 17kt-24kt and a 12m crossbow. First upwind as fast and high as possible, there were two factors that limited my speed, the first was the chop the second was the angle (with respect to horizontal) of the board, the limit of this was when my front leg hit the water and or my arse hit the water, the increase in drag drops you off the plane very fast. While riding upwind I could squat towards the board or stand out straight, this had no effect on speed or height, a slightly crouched stance was best to absorb the chop.

    Some other random observations; upwind only about 1/3rd of the board is doing anything, the board planing speed was about 8kts, upwind speeds of up to 13kts were easily attained, cracked off 25kts was easy in the flat water (with board flat, kite high and almost no load on it), going fast I could squat ontop of the board although the control in that position was nonexistent. I was over powered for the whole session as well so lack of power was not a factor.

    The more I think about it the whole concept of righting moment for a kite doesnt exist, SCP possibly but it is not as clear cut as for a boat.
     
  10. RHough
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    RHough Retro Dude

    It is not the RM alone but the force RM allows the boat to use. RMmax = HMmax. Heeling moment is sail force * heeling arm. With a short heeling arm you can use more force.

    The Heeling Arm on a Moth is longer and this limits the Sail Area the boat can handle.

    Just guessing that the Moth has a mast 1.5 x her length so 18ft? CE is ~40% of that above the water plane. Give the boat a 3 foot foil, the CLR will be ~40% of that below the water plane. So a Heeling Arm of 8.4 feet in "sea hugger mode". From a 7.5 ft beam and a 175# pilot, off the foils the RM is ~650#. To get 1000# you cant the boat 2 ft for. 650/8.4 = 77# of SCP. To get the 1000# of RM you have increased the Heeling Arm 2ft to 10.4 so the SCP is ~96. SCP/W then is 96/220 ~ 44%

    Give the Kite 500#/ft of RM and a Heeling Arm of 4 feet and you can see that the Kite can carry much more sail than the Moth. SCP 125# for a SCP/W of ~ 64%

    96# vs 125# is ~77% that would make me guess that the 13m^2 kite projects about 10.4m^2

    If the wings will reduce drag enough, the Moth might be back in the hunt in some conditions. But all that will do is increase the wind speed the Moth starts losing. Take a close look at that paper on Moth drag ... The sailor is 42% of the total aero drag on the Moth, so having a "boat" is more than double the aero drag compared to the kiteboard. Hulls and racks are "kind of a drag". :D

    Higher AND faster ... game over for fastest single hander?

    Paging Dave Culp? He called this right 10+ years ago.

    Heaven help the "boats" when they figure out a wing instead of a soft kite ...

    R
     
  11. Doug Lord
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    Doug Lord Flight Ready

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    The HA on the kite is much shorter-and Cheesy's observation's are very interesting.The % of drag of the kite guy as a percentage of overall drag must be quite high as well.
    Maybe a bi-foiler with variable geometry foils, solid wing and big enough for two people: convert one person to a chunk of lead sliding in a sealed slidng wing and put the other person in a streamlined cockpit?
    This is very interesting stuff-working out the physics on the kite is very enlightening. Thanks for the help guys.
     
  12. RHough
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    RHough Retro Dude

    Go back and look at the stuff Dave Culp was righting about kites in the '80's

    Dave Culp Speedsailing

    The only reason they didn't go faster sooner was they were using "boats" not planing boards as the platform. They were using stacks of traction kites and the rigs were nowhere near as good as they are today. The basic concepts were correct and many of the limitations still exist. A kiteboard has a 45m beam!

    R
     
  13. Doug Lord
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    Doug Lord Flight Ready

    Fear not, the revolution is coming: http://www.youtube.com/watch?v=FvFkqs-saB8

    http://www.youtube.com/watch?v=-y7jWp5uBVE

    PS: Cheesy have you tried a kite+foilboard? If you're right about RM-then there is no question that these will dominate once better designed foils are used.
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    editorial comment:
    For years most of the time I talk to people about hydrofoils the subject was always dominated by speed even though I have repeatedly pointed out just the plain fun of foiling. I've thought for a long, long time that the greatest development in small sailboat foiling will be on a boat or boats designed to make it easy to foil, a boat that drastically reduces the physical demands on a crew that wants to sail on foils. I still think thats where the next big development will be......
    Kite development has been extraordinary but there is a whole lot of development left in applying foils to sailboats-the story ain't over.
     
  14. Cheesy
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    Cheesy Senior Member

    In practical terms the HM can be reduced to almost zero. For a kite sail carrying power is influenced by the board angle and the kite position (angle to water), it is these two variables that you seem to have missed.

    These pictures are roughly to scale (direction and position of vectors not magnitude) the red arrow is the force from the kte, the blue is the board reaction and gray is gravity. Now all of these configurations are possible and can be used to control the kite/board in varying conditions.

    Now in this picture it is quite clear that the kite lift force is acting so low that there is almost no healing moment, infact it is likely that this could be bought even lower so it was acting through the centre of lift of the board
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    You can also see that moving the centre of gravity closer to the board will have very little effect on the power that can be used, ie gravity isnt doing a lot in this system...
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    So the kite board is really a foiler with massive beam and low coe, hmmmm!
     
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