Kiteboat Looking to Set New Sailboat Speed Record

Discussion in 'Boat Design' started by BlueBell, Dec 10, 2019.

  1. fastsailing
    Joined: Sep 2017
    Posts: 100
    Likes: 9, Points: 18
    Location: Finland

    fastsailing Senior Member

    Undefined.
    Do you mean:
    1) at what angle to the wind can your craft go on average
    2) at what angle to the true wind can the string pull you on average
    3) at what angle to the true wind can the string pull you instantaneously.
    4) ? ? ?
     
  2. fastsailing
    Joined: Sep 2017
    Posts: 100
    Likes: 9, Points: 18
    Location: Finland

    fastsailing Senior Member

    No limits on time intervals. On sailing absolute speed records are on 500 meters course.
    No winch power allowed for propulsion, just for adjusting sails or appendages.
     
  3. fastsailing
    Joined: Sep 2017
    Posts: 100
    Likes: 9, Points: 18
    Location: Finland

    fastsailing Senior Member

    What allows sail or kite or wing to extract the energy from the wind while sailing upwind is the fact that motion perpendicular to wind is provided by aerodynamic forces, while motion towards wind is provided by hydrodynamic forces.
    In addition you could instantaneously extract kinetic energy from the kite. But that does not work on average, and it brakes the rules for sailing speed records.

    That is a comparison between instantaneous behavior of a kite and steady state energy production of a sailing boat including not only sails but also appendages in the water. Doesn't make sense.
    Absolutely not. Perpetual motion suggested as no energy extraction from the wind could be done in that case. You could only extract energy from the kite instead of wind.

    EDIT:
    Unless you attach a string to a sailplane doing dynamic soaring, and pull a small irrelevant object directly upwind by the sailplane. But the result is not sailing, it's still dynamic soaring.
     
  4. Will Gilmore
    Joined: Aug 2017
    Posts: 316
    Likes: 115, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member

    Fastsailing, thank you for the in depth and considered responses.

    By angle to the wind, I mean, at what angle can a kiteboarder travel to windward in relation to the true wind direction. Directly into the wind would be 0 degrees, Dead Down Wind would be 180 degrees.

    I'm not clear on what you're saying here. What I have come to understand about this is that sailing in any direction but DDW is a consequence of extracting the energy produced from wind against lateral resistance. Sailboats have keels that provide lateral resistance to the wind and give that resistance direction through the water. Hydrodynamic and aerodynamic forces are not independent, even though they can be treated independently on paper.

    Gliders, unlike balloons, are able to extract energy from the wind because gravity gives their wings the lateral resistance they need to allow for aerodynamic forces to exceed the direct wind forces. Otherwise, they would simply drift with the wind, like a balloon.

    Not perpetual motion, the energy of wind against water was best understood by me when I read an explanation of pinching a watermelon seed between two fingers. Even though the two fingers are not moving very fast, the seed can shoot out in a perpendicular direction at much greater speed than the forces driving it. This is not perpetual motion. The sum of forces between the two fingers is still greater than the force of a watermelon seed flying through the air.

    If a kite can pull a boat closer to the wind than 90 degrees, I can see that flying the kite from starboard to port, arching it overhead, from one tack to the other, can cause a boat to move directly into the wind. Maybe not that fast, but doable.

    -Will (Dragonfly)
     
    Last edited: Jan 9, 2021
  5. fastsailing
    Joined: Sep 2017
    Posts: 100
    Likes: 9, Points: 18
    Location: Finland

    fastsailing Senior Member

    Kite can not pull a boat closer than 90 degrees angle when extracting energy from the wind, neither can anything else interacting only with the air as long as the airmass is not moving relative to some other airmass, due to windshear. The angle a kite can pull at steady state is 90 degrees + arctan(drag/lift), which is always > 90 degrees. The kiteboard can of course sail much closer to wind than the kite pulls it, because the appendage in the water.
    Like I said before, there is also a case using windshear like sailplanes do while dynamic soaring, but that is not sailing. You can also extract energy from the kinetic energy of the kite, but not for very long, and while doing so the kite can instantaneously pull the vessel much closer to wind than otherwise. You need an external power source to do that, battery, IC engine, or just human muscle power. Even for the latter case it is questionable if it's still sailing.

    Steady state apparent wind angle while sailing is arctan(drag_aero/lift_aero) + arctan(drag_hydro/lift_hydro) and true wind angle must be greater than that.
    If you are trying to figure out how close to wind any craft can sail rather than how close to wind a kite can pull, that is the best starting point for analyses.
     
  6. Will Gilmore
    Joined: Aug 2017
    Posts: 316
    Likes: 115, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member


    I think I get what's going on.

    Thanks for your patience.
     
    BlueBell likes this.
  7. Squidly-Diddly
    Joined: Sep 2007
    Posts: 1,709
    Likes: 120, Points: 63, Legacy Rep: 304
    Location: SF bay

    Squidly-Diddly Senior Member

    Not to thread jack, but can any kite experts spec (links to online retailer welcome) me a user friendly, fairly cheap, packable (roll up into small size in backpack) kite for hoisting a pair of Beofeng UV-5r radios (with the XL battery and 15" antenna....figure 600grams total including something to hold them, and maybe red blinking warning light and radar reflector for safety LOL) for a airborne radio-repeater?

    Yeah, I know asking how big a kite to lift a certain weight is 99% dependent on wind, so figure 15mph wind. Plan would be to launch the kite, and get it up into hopefully decent wind above ground clutter, carrying only an extra line and tiny pulley maybe 50ft down from kite, then once kite gets a few hundred feet up, hoist the radios.

    For this application I'd want a kite that can be set for max upward force, as opposed to sideways force at slight angle.
     
  8. BrendanfromNZ
    Joined: Apr 2020
    Posts: 49
    Likes: 5, Points: 8
    Location: New Zealand

    BrendanfromNZ Junior Member

    Pauls fishing kites in NZ, made for pulling heavy fishing lines out over a kilometer. The smaller ones would handle that easily
     
  9. BrendanfromNZ
    Joined: Apr 2020
    Posts: 49
    Likes: 5, Points: 8
    Location: New Zealand

    BrendanfromNZ Junior Member

    Good technical description fastsailing.
    Wil using your description, my max upwind angle (in optimal conditions) would be 70 deg from the wind
    I think foils could improve this to possibly 60 deg
    To improve on that would need a keel
     

  10. tlouth7
    Joined: Jun 2013
    Posts: 142
    Likes: 42, Points: 28, Legacy Rep: 10
    Location: Cambridge, UK

    tlouth7 Senior Member

    Putting the cable attachment at the top of a mast just introduces a heeling moment which must be resisted. This will inevitably make the vessel slower because it will need ballast and/or form stability.

    This is impossible, as fastsailing has already explained. The pull from the kite (easily visualised as it is in the direction of the cable) must be more than 90 degrees from the wind (where 0 degrees is directly upwind). In order to gain any forwards component from this force the heading must be less than 90 degrees from the pull (i.e. the cable must be pulling slightly forwards of the beam).
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.