I have a 70 m2 outleader for my 35 ft cat. Once you get the hang of launching the kite, it works great.
Launching is fairly simple,i have about 4 x 50 metres of stripped 6mm dyneema
, so it is around 4 mm. A small block is hoisted up the mast
on the normal spinnaker halyard
,with the "top line" for the kite. The kite is hoisted up, and the the 2 side lines tightened,the sail fills, and you can start slowly letting the lines out. When flying correctly, it flies about 20 metres forward of the boat and 5-10 metres above the mast
, so you can use your normal sails
For sailing in "bumpy" conditions, i use bungee cord attached across the boat to act as a shock absorber, so depending on wave height and wind strength, I use anything between a 6mm bungee up to a doubled 12 mm. Use the whole beam of the cat to give the necessary compensation.
Regarding loads on the lines: My 4.5 ton cat sailed at 8 knots in 8m/s with a load of around 20 kgs on each of the 3 bottom lines. This was for a 70m2 kite.
To retrieve the kite, just let the side lines go, and pull the top line in. In a strong breeze, you can even take your time, as the whole sail is just like a big flag folded double.
Rob Denney said you need more than 8 knots of wind, I agree, but have used it in 4 knots as well, the problem is launching,because when it starts pulling, a cat will quickly accelerate, and the kite collapses. My technique is to slowly reverse the boat with the engines until the kite is flying well, and then slowly reduce power as the boat accelerates.
My biggest regret is that I didn't go for a bigger kite, I reckon at least 50% larger than your normal spinnaker.
You need to experiment
a bit with the attachment points to the hulls, but for downwind I have the blocks far forward, and as the wind comes from one side, I move them back to around where the mast is, to minimise side pull.
You can mail me directly if you want more information