Good kite for boat

[Munti]

the gruntest kite I know of is a flexifoil blade3 10.5 , and it pulls like a train , but you might be talking lei I assume

I think I'm talking LEI or closed foil, as it has to be water launchable. But hey, I'm here asking questions because I don't really know what I'm talking about.

[BWD]

You need help from a naval architect, it seems.
If you are certain to spend thousands on something, go on and take a year to learn kiting.
Then apply your knowledge to the problem.
As a benchmark, consider the power a human can hold from a kite or sail is about 4-6 hp.
All kitesurf and windsurf gear is designed around this.
What can you do with that amount of power?
Semi-planing in a boat that carries several passengers would be a tough ask (likely impossible ). Of possible, it might be along lines of the seagull racers in bermuda. I would guess you also would face serious limits on sailing angles except in stronger winds, which would tend to fairly soon destroy gear meant to propel about max. 100 kg, not the 300-1000kg of most small boats.
A small cat is likely the most viable platform that could take 2 crew instead of just one, probably.
It could give a chance to go upwind even, with luck.
There are some intermet videos around of people having success with hobie cats and medium to large kitesurf kites...
Good luck

[bragnouff]

In south of France, I've seen people experimenting successfully with kite boats, and they seem to use standard kitesurfing gear, in bigger sizes obviously. The people doing it are very competent kiters and instructors, and it seems like some level of active flying is involved, which makes it fun. Yet, this is wayyyyyyy more mainstream and accessible than the cutting edge projects of Don Montague, on another planet altogether.
Their boat platform is called the KiteTender 400 (https://business.facebook.com/kitetender/).

There's one photo on that page showing a "NASA wing". And that actually wouldn't be a bad option in terms of grunt and stability, light winds performance and cost. Pretty easily scaled to size. Poor upwind performance though, but I guess by design your motor boat would suck at it anyway even if coupled to a high performance kite. Not too sure about the relaunch, as it's a single skin. Maybe they'd need to be reeled in and re deployed.

Lots of fun ahead!

[Regis-de-giens]

Hi

In my video the wind was about 12-15 knots (I thought it was less than 10 when I choose this kite size) and the boat speed was about 10 knots I think ( not sure I have little experience in boat speed estimation); there are some driving specificities like upwind turns that are more tricky than gibes (on a small boat, if you loose the dynamics, since there is a (impressive) risk to be pulled rearward by the kite, then hard to drive the boat and risk to damage the boat), but overall it is quite easy if you know how to drive a kite and a sail boat (the passenger had his first (memorable) trip on a boat on this video ... ) ;

It is quite open to the kite design choice, LEI or closed cell kite, everythink works (like on snowkiting) ; having to optimized it for a slow and heavy boat, I would agree with this :

Grunty kite is kite with profile optimized for low flying speed - so it has "grunt" even while flying slow. Typically this means deep heavy cambered profile.
Looping the kite can solve low boat speed - although only downwind.

... and indeed go for a grunty kite rather than high L/D ratio kite like race foilers, in order to get more “torque” as it has been explained, but also stability and relaunch; relaunch requires to have a good keel and put the boat perpendicular to wind (but certainly less sensitive for a >1 ton boat); maybe a nasa wing is however a bit exaggerated if you want to keep an acceptable wind rang and upwind angle (and not relaunchable);

For a little boat like 420 or the Lynx model on the video, the kite size should be approx the same as for kiteboarding; it can be even smaller (since you do not have the limitations of both waterstart and upwind planning), it works but just involve less boat speed; I tried with 15m and 12m foil kites and also 9m and 14m LEI (inflatable kites) ; so the bigger the best , but I would not go for a far larger kite than kiteboarding (depending on wind range off course) since the higher force may lead to accelerated wear if you use a serial model of kite;

I sometimes attach the kite to myself because a fixed point will give you almost no signs of over-power (and kite/line damage) . At least , when attached to you, you are ejected from the boat in a high gust, rather than damaging kite or boat ( better for a test but not in your application) ; but when you are in the suitable wind range, having a fixed point is faaaar more comfortable (and you can move more easily walk around the boat without changing the pull point) ; attachment point needs to be close to the keel to avoid impacts of kite directions and wind gusts on the boat directions; the lower it is , the less boat inclination you get , so I prefer a lower attachment point, for passenger comfrots and keel efficiency; be careful it should withstand all traction directions from horizontal to vertical, but also 360 degree angles ; overall you get a good stability of the boat, almost comparable to a standard sail;

Now, about automatic driving : having a full auto-pilot system seems tricky, but … I am quite sure (not tested however) that you can get a kind of auto-stable kite , in the altitude you want (so enabling the actual pulling of the boat), by taking advantage of the large width of your boat : indeed , by routing the rear lines through pulleys at portside and starboard extremities of the boat (i.e. left and right extremities, at keel level approx) and then after the pulley, connect them at the boat center to a kite bar or a trolley ; indeed, (during a travel perpendicular to the wind will be the most remarquable), when the kite starts to go down to the water, it will pull on the upper rear line of the kite which will force it to get back upward ( and reversally); you will even be able to set this neutral balance altitude point by just relocating the trolley closer to starboard or portside;

Be carefull, kite will be more tensioned than kitesurfing, so it may wear more rapidly ; you need strong lines, and strong kite.

A (old or recent , whatever) Peter Lynn arc model could be a good candidate , as it is solid , robust in time, and has an auto-zenith ability on front line that could ease the start of the kite in the middle of the water with less operators ( but not tested) ; and you can find very cheap ones for your first tests ( I think it is not worth spending the price of a Soul at the start …)

Hope this enlight a bit your research ...

[tomtom]

Very good points...

Imo forget about Soul - its too racey too power spikes etc - simply too high performance...
I will use either low aspect LEI or FS peak. - IMO peak but from stronger fabric will be ideal. I explain:

IMO you dont want relaunchability - it can be dangerous - you want to stop if kite hit water - to have deal with sudden relauch etc. Especially as beginner.
You need to have some boat launch technique. So you can use this for restart too. Peak will be easiest kite to boat launch and also land. I will be easiest to pack and to wind lines.

I will work on some boat attachment point but with some fuse with defined release to not overload kite. I will not attach kite to boat without fuse.

LEI will be maybe more easy to you if you dont have lines and bridle management skills.

With peak i can easy imagine some reel system to launch and land kite. LEI are not stable enough for this.

Upwind will be very much dependent from boat tracking ability - you will need something like sail boat keel or deep V bottom and/or very big rudder

[Munti]

Some very good inputs there
As I said before, I'm not worried about speed. If I want action sailing, I can use my sailboat for that. So I intend to stay way underpowered. And as tomtom mentioned, I will use a fuse when attaching kite to boat. Should be easy to make one. In my early experiments it's ok to attach to keel because it's easy to take the boat out of the water. For bigger boat it needs to be above waterline. Interesting point about taking advantage of the width of the boat for auto-stable kite. Defenitly something worth experimenting with. Probably also means the boat would fly the kite very actively, thus producing a lot of power, in choppy waters. Need to factor that in.
Very good point about the increased wear when using kite for boat. For experimenting a little it probably doesn't matter to much, but for regular use I would probably need to either reinfoce existing kite, or have one specially made.
I intend to start with water launchable, so will try to learn more about Lynn's arc kites. I expect I will have bought many kites (and ruined a few) before I know what I'm doing, so cheap used ones is clearly a good thing. I am however intrigued by what tomtom said about launch system for the peak. I'd love to play with that as soon as I have learned more (or at the same time), so if anyone has input on a simple launch system, please tell.

[tomtom]

use you imagination -i will make simple reel wheel maybe with foot brake and control kite directly on back lines while releasing reel. On reel kite should be depowered. /correct relationship front to back lines/

Execution will be chalenging but doable - no other kite i know is stable enough to pull this. Imo stability is more important to you than waterelaunchability. Peak is stable while depowered - all other kites are the more depowered the less stable... Also you will probably need more than one kite size - for different wind strenght.

But of course there are different options - and a lot of things actually can work. This will be long road a you definately needs to be a good kiter

[tomtom]

also look at ozone alpha.

[Munti]

Does it matter what number peak it is? I'm asking because there happens to be a secondhand peak3 6m available in my country now.

[Regis-de-giens]

if you are ready to buy a new kite, I would also consider Conceptair Wave that now exists in big size (12.5 meters) ; it is grunty, extremely stable and excellent depower, almost autozenith; it is IMO at least as stable as peak, and very easy water relaunch, almost automatic. I think such launch method also works with the wave.

relaunch is a ggod point IMO ; but rolling the kite to put back in the boat from sea will be more difficult than peak (which cab be rolled very rapidly even on board of the boat