February 2007

Newsletter, February '07

I'm harbouring a major grudge- happens every year about now.

The weekend before last we went to the Nelson kite festival, our favourite, and we DROVE to it, sure it's 500km's from here, but this takes about the same time (well almost twice as long but you'll be getting the drift) as driving to our airport then stuffing around for hours with parking, checking in, endless security procedures and the inevitable drama's associated with excess baggage.  Automobiles, and more particularly that subset called Toyota Granvia 4wd vans, are a wonderful invention.  Biff all the junk in, as many kites as you like, say yes to 'oh and can you throw in a whatever or two for us please', no packing and re-packing kite bags to maximum compression, no having to cut back to less pairs of undies at the last moment for that final 100gms saving, and definitely no humourless security bastard stealing my post-check-in gifted bottle of wine.  Not only all this, but the Granvia makes the best possible on-the-field cabana and kite anchor, not least because nobody takes the keys away 'for safekeeping' just before there's a 180deg wind shift.  And then for coming home, it's totally untroubled by the mysterious process that causes all our stuff to double in size and bulk during kite events. 

Last weekend I did it again- 500km in the opposite direction.  This time with 2 tonnes of old engines and a replica of the first ever motor car (Carl Benz's of 1886) on a trailer behind as well.  Wonderful time; running circles with the Benz around 30 or so steam traction engines all weekend while shouting "Here Comes the Internal Combustion Engine" and other insulting things.  A confession though, I never took the kites out of their bags- bad wind, no room, all those excuses that I deride others for.  Oh well penance comes next week (Pasir Gudang in Malaysia) and so on for the rest of the year, but my batteries are most definitely re-charged.  Check-in and security apparatchiks beware- oh, and I'll flatten anyone I even vaguely suspect of being likely to remove keys from anchor vehicles!

And now, as promised, the first month's testing and development of the 7.5m x 0.47m monohull kitesailing boat.

We call it the "Longboat"; Dave Culp calls it a monomaran. 

It's surpassed all my expectations.

I'm astonished that such a small-scale version of this layout is useable, especially because it has a person sitting on top-, rather like having an elephant as deck crew on a 12m yacht by comparison.  The Longboat is far too small to be a 'fair' test, but was made this size so we could compare it with the KiteCat, a developed mutihull kitesailing boat that uses a similar size of kite.  Generally I believe the comparison will scale conservatively- that is, monos will perform even better than multis, as they are made bigger.

It is stable, and goes so fast I think that every other style of larger kite boat just became an historical footnote. 

We haven't yet had it out in strong wind or surf but I have few doubts now.  Doing wild manoevers with the kite has no noticeable effect except that it goes faster and becomes more stable.

The Longboat needs less pull (smaller kite) than the KiteCat to go the same speed and it's currently more than twice as heavy (on account of the hull I used has taken up a lot of water into the foam core over the years).  Even with a smaller kite it outpoints the KiteCat-I assume because of its lower aerodynamic drag.  It clearly has much less drag than anything we've ever tried before.  In tow testing, at 12km/hr a KiteCat had 4kgms drag, the Longboat 3kgms.  At 18km/hr the KiteCat drag was 8kgms, the Longboat's 5 kgms.  Even when paddling, the Longboat goes like a rocket.

Sitting statically, I can lean it to more than 45degrees before it will tip, with no-one on it, it goes to 135 degrees before tipping and is self righting even from 180degrees with the smallest perturbation.  It now weighs about 80kgms, of which 19kgm is the keel bulb, 0.4m below the hull.  At 160kgm total for boat and flier, this keel weight is only 12% of the total weight.  For performance orientated conventional yachts the keel can be50% or more of the total weight- and a scaled up version of the Longboat won't also scale the person whose centre of gravity position is very adverse at this small size.

The Longboat has fore and aft opposite turning rudders to eliminate rudder induced heel.  This is absolutely crucial.  So sensitive is heeling to the relative area moments of the fore and aft rudders that even quite small changes are noticeable.  Final tuning of this balance has been by trial and error rather than basic calculation as there are far more factors at work here than first thoughts suggest.  This steering system has the added advantage of making turning much quicker, which is why it's used on the KiteCat.  The Longboat's turning circle is not much larger than the KiteCat's- which surprised us a lot as it's 7.5m long compared to 4.3m.

Heeling effects initially seem to be more alarming than they actually are because the righting effect of the keels increases with lean angle- resistance to heel is soft initially but increases rapidly with angle.  The greatest virtue of keel stability is that it's self righting- as against multihull stability that's rock solid until some limit- then you're upside down and requiring rescue. 

The Longboat's kite attachment system is a boom that's pivoted at it's forward end to a sliding track on the foredeck and attached just rear of it's half point to a cranked swiveling link that is pivoted at the bottom of a well in the deck just forward of the steering bar.  Because it's front attachment point is at deck level and it's rearward one is against the bottom of the hull, the kite pull line is actually about 150mm below the bottom of the hull when hard on the wind- and it needs to be, no heeling at all, ever.  The other key thing is that the kite attachment point is outboard of the gunwale by about 1m on this boat and is able to go to and even below the water surface at this point when the kite is low- and it especially needs to do this as for best upwind performance the kite lines don't clear the water until at least this distance from the side of the boat.  The swiveling link works a treat, means that we don't have to cut the boat in half where the link is attached.  It swivels automatically to crank over the appropriate gunwale.  The linkage is set up so that the kite pull position automatically slides to a suitable longitudinal position depending on whether you're going upwind, downwind or even backwards.  The pull 'envelope' is rather like a section from a tapered spherical roller bearing.  Attached is a photo of an earlier version - on a 6m monohull-, which shows how it works more clearly.  There is zero tipping by kite pull except when the flier pulls on the lines to keep the kite up in zero wind- then the load goes through the flier's shoulders which are 0.75m above the deck rather than through the attachment system which puts it about 150mm beneath the hull.  On larger versions the pull will always go through the boom system, and then it just doesn't heel at all either way, not even with the hull stalled and being pulled sideways or backwards, not during kite pull surges, nor kite under looping, - no effect at all, it's eerie.  I don't know of any other attachment system that does these things- maybe it's the key to kitesailing- (especially in conjunction with fore and aft steerable fins).

Kite types for the Longboat:  I think we're seeing that the higher the boat L/D (hydrodynamic drag angle), the less important it becomes to have a high kite L/D (aerodynamic drag angle).  This figures, it's just Marchaj's course equation as the sum of the aero and hydro drag angles.  The cross over point in performance for the KiteCat, using a 20m Nasa versus a 19m Venom is close to 10 kn.  For the Quadsurfer (fast but high drag planing style hulls) there is no usable window for the NASA style kite at all, no upwind and no planing is possible using any size Nasa until there's enough wind to fly a 19 Venom, which is than always faster and points higher.  I expect that for the Longboat (very high hydrodynamic L/D), the crossover may be at 12kn or more and that it might perform satisfactorily, even upwind with relatively low efficiency kites.

We have all noticed, again, that having less direct contact with the kite makes flying in light winds much more difficult.  In marginal wind, it takes a high order of kite awareness to keep the kite up- much more difficult than for hands on flying- but it can be done.  I've been re-learning this skill, but still have some way to go compared to what we all managed with the kites and systems we used 20 years ago. 

When there's enough wind to allow the kite to park at the edge, it all gets easy.  For larger boats, the kites will always be attached to the boat so the relative difficulty of remote flying is only a problem in comparison with boats small enough to permit hands on flying- i.e. single person craft that are, say, 50kgm and less.

None of us here think that this type of design will be a challenger at the small boat end of kite sailing- the versatility and user friendliness of the KiteCat is way ahead, but for larger boats, there's nothing else that's even remotely in the same game.  I'll stick my neck out here and say that in larger sizes, no multihull or any other style of kiteboat I know can be competitive.  The only possible remaining advantage for multihulls in larger sizes is to provide a kite handling platform- but there has to be other answers to this seeing as the performance difference is clearly so great.

Peter Lynn, Ashburton, 1 Feb '07