March 2010

                                                      Kite-on-a-Stick.

Insanity is inherited- you get it from your kids.  But in this case it seems to have worked the other way around.

I caught the "make-kite-sailing-work" virus from John Waters (Spin sock inventor, kite traction pioneer) at Lincoln City in 1987.  But John probably wasn't entirely to blame; I was just a Petri-dish-in-waiting for this one.

And, unfortunately it might be a genetic predisposition- one of our children has now come down with a bad case.

He's also Peter (called Pete), which name choice might seem to have been a bit uncreative on our part as parents, but we did arrange a distinction.  I'm Peter R Lynn and he's Peter S Lynn (and the generation to follow can have a Peter T Lynn and so on )- clever huh?!

Anyway, Pete has a solution to the kitesailing problem which, unfortunately, appears to be a complete answer- unfortunately because I didn't think of it first (forget Cain and Abel, father/son rivalry is the big one) and also because once it's solved, what will we do to pass the time?

And the kite sailing problem is-------- what?

Pre-European Hawaiians used kites to pull canoes from island to island, and George Pocock is said to have taken a group of family and friends out for a three day sail on the Bristol channel under kite power before 1830.

So kite sailing has been do-able for quite some time.

Kite traction's big advantage is that the kite attachment can be contrived so that no heeling moment is generated.  Ideally the kite's pull should be aligned with the craft's underwater centre of pressure (where whatever is resisting side forces acts).  By contrast, the forces generated by conventional sails act at a point about one third of mast height and are therefore always trying to tip the boat over.  To resist this tipping moment, conventionally rigged sail boats use either form stability (catamarans and trimarans) or hang a thumping great lump of lead weighing about as much as the rest of the boat, in a keel below.

Both of these cause significant extra drag.  For multihulls, there is the wave drag interference between hulls plus extra aerodynamic drag and the extra drag inducing weight of connecting structures.  For monohulls, there is the extra hydrodynamic drag from having to push twice the weight through the water, and additional surface area drag from the keel.

Kite rigged craft can avoid these penalties completely, so have the potential to be faster- not just in top speed, but across the wind range.

But there is a problem- isn't there always!- or else kite sailing would now be ubiquitous, masts and sails long since consigned to history.

The problem with using kites is not the difficulty of launching and retrieval- there are quite a few adequate solutions to this in use now.

The problem is that when there isn't enough wind, kites fall into the water- causing no end of trouble- whereas conventional sails just hang off their masts flapping disconsolately.

So, don't go sailing when there isn't enough wind then?

This is the solution used by kitesurfers- they chase wind like ski bunnies chase snow.  But it limits the places and times suitable for kite surfing to a tiny fraction of what is available to the wider sailing fraternity. 

To get any significant share of the wider sailing market, kite powered craft need to be able to function satisfactorily in the same range of conditions as conventionally rigged sailboats.

And, contrary to anecdotal experience, periods of zero or very light wind are much more frequent than squalls.  When the wind is below, say 10km/hr, kites don't fly at all, but conventionally rigged sail boats still move along well.  It's frustrating watching a fleet disappear over the horizon while you're still trying to get your kite out of the water and dry enough to re-launch.  Other sailors notice this also- and as a consequence are not in the market for kite rigs.

Pete's development solves this.

Instead of flying the kite from lines, he attaches the kite to the end of a flexible ( and usually tapered) pole, or more often, two poles.

These poles are just strong enough to support the kite's weight,-so launching is as simple as attaching the kite and lifting it into the wind- and the kite is held aloft through any lulls.

In the absence of sufficient heeling stability, the attachment geometry of these flexible poles can be arranged so as to have a, below the water line, 'virtual centre' to align the kite's pull with the craft's hydrodynamic lateral resistance centre.

A disadvantage of the "kite-on-a-stick" system as compared to kites-on-lines is that the kite cannot be figure eighted to build up apparent wind speed when sailing 'off the wind', for more rapid acceleration or for jumping.

But conventionally rigged sail boats do not have this facility either- and they own the market now.

And it's not much of a disadvantage for larger boats anyway- an extra minute getting up to speed doesn't make much difference over trans-oceanic distances.

And, because all courses become upwind courses (from an on-board perspective) for efficient sailing craft, this inability to 'work' the kite independently does not ultimately cost performance. 

Because of the reduced drag from not requiring heel resistance, "kite on a stick' rigged boats will have the advantage over conventionally rigged fast sailing craft on every course. 

Their evolved form is almost certain to be long narrow low monohulls;-"monomarans"- there being no reason to have more than one hull nor any superstructure to add aerodynamic drag, nor more than just enough extra weight in the centreboard and rudder to ensure self righting in case of a tip over.

Ultimately there is no reason why their kites cannot be winched out to fly on longer lines either, if it's found that this is sometimes beneficial (to capture more wind up higher for example).

Two of the "kite on a stick" tests Pete has tried so far were buggy based.  These worked well, but were more for testing convenience than from any intention of commercialising a land based version because increases in the vertical component of kite pull reduces the weight on the tires and hence their ability to resist side pull. 

But, for fins engaged in water, the ability to resist side pull increases with the square of velocity (up to 75km/hr or so), matching the aerodynamic driving force available all the way.  Also, for water craft, the vertical component of kite pull acts to increase speed by taking some of the weight off the hull.

A third test using an original monohull kitesailer hull proves the system's excellent on-the-water potential.

The next step could be a midsized purpose built monohull- perhaps about 15m on the water line.  This would have a fair turn of speed, handle reasonable waves, and be big enough for the crew to get inside out of the wind and spray, while still trailer able.

I tried retiring a few year ago but somehow it didn't stick, but maybe now the time has come*.  Maybe this could be a good retirement project.

Peter Lynn, Ashburton New Zealand, Feb 28 '10

PS  Also from Pete, the kite shown in the second buggy based test above is an overpressure 'foil.  This uses a small battery powered electric fan for boosting internal pressure to a few percent above what is available from ram air inflation alone.  The advantages are reduced bridles, smoother leading edge form, push button initial inflation, and excellent collapse resistance.

*But nah; I reckon the dual mode Arc approach may yet give Pete's solution a run for it.  Could be yes to the boat building though- it'd work well with a DM Arc also.