Team NZ Kite Report

Kites for AC class yachts.

There's no doubt that in any open rule class, kites(that is, the type that fly), can pull yachts downwind faster than will ever be possible with conventional spinnakers or gennakers.

When tethered to an immoveable object, a kite's maximum speed through the air is equal (near enough) to the kite's lift to drag ratio times the true wind speed.  When it is tethered to a yacht going straight downwind, the kite's max. speed is the kite's L/D times the difference between the true wind and the yacht's speed through the water.

When using long lines (say>20 times the kite's wingspan) and a kite flying in a horizontal figure eight, the average kite velocity will be quite close to it's max.

With average traction kites now attaining an L/D of about 5.5, the pull generated by this apparent wind speed gain is so great that even displacement style yachts can be hauled almost entirely clear of the water every time the kite passes through the centre of the wind window.

For shorter lines, (say down to 4 or 5 times wingspan), appreciable apparent wind speed is lost in the turns but even then no conventionally sailed yacht (not excluding 18ft skiffs, windsurfers or multihulls) has any chance of staying in touch, (upwind is a different story).

For AC class yachts, rules appear to impose such restraint on a kite's area, L/D, use and handling that it's not clear whether kite power can be faster.

Area:  For equal projected area, I don't believe there will be any significant difference between the lift coeff. of the type of kite envisaged and a spinnaker or gennaker ditto, but a major handicap for kites built to qualify as a spinnaker/gennaker under the ACC rule is measured area.  Because kites will have to use aerodynamic forces to hold their spanwise form (rather than compressive loads in the mast and spinnaker pole), for the same measured area they will have less projected area- by my estimate, about 15% to 20% less.  

L/D:  My view is that the restrictions of; just three line attachment points (that is no multiple bridling), no battens, no stiffening, and no double skins or ram air inflation will eventually limit the L/D of such kites to just under 3.0

Evidence for this?

*The kites we have developed within the AC class rule so far have max. L/D a little more than 2.  But, this was after just one week's work so I would be surprised if there isn't 30% more to be had there yet. 

*The current mainstream traction kite that is nearest to meeting the AC class rule is the NASA wing style.  It would conform except for having multiple bridles.  After 20 years development NASA's now get to an L/D of 3.5.

*the NASA's leading edge bridles should be able to be replaced with a flare of fabric each side- which then require only one line each- without much loss of L/D. 

*An early NASA variant without centre bridles had an L/D of approx. 2,5

An L/D of 3.0 will limit the kites max. pull to approx. 9 times that available from a spinnaker or gennaker of the same projected area or about 7 times by measured area.

Use: But if "surging" is the only apparent wind improving mode permissible, other techniques such as "figure eighting' being prohibited under the "no pumping" rule, what will the nett gain be?  Any estimates of this depend on too many assumptions to be useful- some tests will be necessary.

Handling:  There are three main aspects to handling:  Steering, launchability  and reliability( that is, resistance to collapse).  Steering, and launchability generally improve as L/D increases, but collapse resistance usually gets worse. 

Steering:  If the "no pumping" rule prevents any controlled 'figure eighting' as a means of generating apparent wind then there is no advantage in having fast steering.  The kite must be steerable though- at least to the extent that it can be positioned where it needs to be. 

Launchability:  Although we were not able to launch the scale prototype kite from the Etchell with the mainsail up during the 29th December '02 test, I'm sure that this was more to do with technique than a fundamental problem.  The (now) obvious approach would be to launch the kite from the rear half of the boat while reaching with the mainsail out at 90degrees.  We never tried this.  Nor did we rig the kite's main lines other than at deck level- and therefore the kite had to be out a long way before it was high enough to clear sail interference. 

Reliability: Luffing is the main problem because a kite that luffs while a boat is on a downwind course often collapses to the water.  Kites that will fly in very light apparent wind are inherently more collapse resistant and there are other design features that also contribute to luff resistance.  Attentive trimming is the final defence.

My view; getting satisfactory handling will be a challenge, but probably not an insurmountable one.

What other advantages may kites be able to offer for AC class yachts?

they offer?

*Kites can be flown higher, in stronger wind- but then the kite's lines will not be horizontal and the vertical component of pull will be wasted.  Therefore, at this stage it seems prudent to ignore any wind gradient effects, but if other advantages appear to be worth having this assumption should be checked out.

*It should be possible to leave the foresail up while using the kite- and if so this is will by itself offset most of the projected area loss.

*Are there advantages for kites inherent in the present right of way rules for sailing boats?- this proved to be the case when kitebuggying first became a sport and attempts were made to race under IYRF sailing rules.

er an L/D of 3.0 will be enough to allow the available apparent wind gain to make up for the measured area loss.   

Accepting that the pull from a kite built to conform to the AC Class rule will be down 20% relative to a spinnaker or gennaker ditto in the same apparent wind speed, how could a kite make up for this and provide something additional?

Thirdly, a kite's apparent wind speed can be much greater than that of the boat.  Although volitional "figure eighting" is probably prohibited by the "no pumping" rule, I'm guessing that "surging" (vertical or horiz.) and "weaving" may be allowed if this is a natural pattern of flight for the kite rather than something caused by steering inputs.  Such harmonic kite behaviour can be contrived, but the amplitude and frequency tends to be speed sensitive.  tends to be speedis line length changes rather than caused by some continuing control inputsand doesn't require peret Pumping: circling etc

Other rules effects: Even without a rule change, could some umpire ruling be made, such as 'yes kites are allowed under the spinnaker/gennaker rule but only if their normal position is sheeted hard to the mast or etc' which would effectively preclude the uses of kites?.