If I were to possibly to a long downwinder 500km +-
What sort of training would I need??
What kite would I use with the least strain on my arms?
How many people would be a safe group?
Would I be able to go straight downwind for so long or will I have to go at an angle??
And what other questions should I ask??
Please reply as this has been a dream of mine for a long time and I wanna start preparing etc.
It is hard to say what the limit might be for a days riding. Some have estimated something approaching 200 miles or about 320 km. We seem to average around 10 to 12 knots with some long distance runs, in waves of course. If you undertook your run in sheltered, calm waters with some good wind you might be able to knock it out in a couple of long fairly grueling days. Then again, planning for a longer duration run would be more realistic.
As far as training, I would say progressive longer kiteboarding runs for starters. Supplement that with cardio and some weight training focusing on building endurance. If you have any current healing injuries or weaknesses in effected areas, I would try to work them out.
I would try out the smallest kite that keeps you going at a reasonable clip using a board that doesn't require you to edge much. I would make sure your control bar has a powerlock or at least a fixed harness line to reduce arm loading. A longer board with directional characteristics could help. I would pay attention to the foot straps to make sure they fit very well, are well padded and comfortable. Riding powered up will not be likely to help you keep riding through 8 to 10 hour days.
A couple of the greater challenges include using the same muscles in all probability for a very long time. In transit injuries or even irritations could create some real problems. Actual kiteboarding and some specific work in the gym could help to improve endurance.
I would target having 2 to 4 riders and winds for a beam reach or riding slightly off the wind. The winds would need to be predictably stable for the planned duration of the run and a bit longer. Some places have fairly reliable consistent winds independent of cold fronts and other migrating weather systems. I would put someone knowledgeable and experienced in charge of weather/sea condition planning and monitoring. You don't want to have to tack much IF you are going for distance in a reasonable period of time. You should plan for at least one reliable chase boat, some communications would be good, signaling gear, impact vests, good GLOVES and hydration packs. Depending on the temperature you may need to fill up your packs with fluids regularly. Diet is important, consisting of ...? I would talk to some people that are familiar with that for athletes in distance events.
Mental conditioning is important as well if you are going to do this run fairly quickly as an endurance trip. Nothing breeds success like confidence built through experience, i.e. progressive training distance runs. Doing something because you know you can do it with reasonable preparation helps as well.
There is more than this to plan for but here's a start anyway.
Have fun and post pictures!