I've a 2 years exp on hydrofoil and keeping to enhance my skills. Still have a problem on going fast. I have a race wing (alpinefoil) and race kite (diabloV3) but at a medium speed about 25-26knots (naturally, for some it's just low speed ) my foil becomes unstable, I loose upwind course and can't accelerate more . What should be body position, stance on board, kite position, etc. Thanks for all tips!
If your foil becomes unstable at speed you are probably riding with too little front foot pressure. Try moving the foil forward and as a last resort shim the stabiliser to a higher angle if you are out of room to move the foil forward.
But why do you say upwind course? You go the fastest on the Downwind when you are too powered to ride any other direction but up or Downwind.
If your foil, kite, and control system are all properly tuned...
Get going fast on a reach or barely upwind, make sure you've got a ton of speed, then sheet in almost all the way at full power and let the kite "drag" the nose of your board downwind. It will look/feel like the kite is falling back in the wind window, but you shouldn't feel yourself slow down as you do this. As you start going downwind, you want to always maintain tight front lines and never let the kite go slack, you'll continue accelerating until it feels like you're leaning against the kite so hard that you're actually riding upwind, except you're going downwind.
This is called "apparent wind" and is what you're trying to generate, the feeling of going so fast you're going upwind except the wind is behind you. Once you're going downwind at a full head of steam, carefully playing the balance between going deeper downwind without losing any of your momentum or front line tension, then you can barely begin edging upwind a bit into more of a reaching heading so that you're not going maximum downwind, but maybe just slightly upwind from there. That's the fastest point of sail for any vessel...
If you need more than that, I'd recommend watching as much YouTube footage of the Kitefoil Gold Cup, Formula Kite, and the Hydrofoil Pro Tour as you can dig up. Watch what the top racers down when they go from hard upwind to hard downwind at the windward mark. The bear-away process and how they accelerate downwind will more or less show you this perfectly, especially the Bridge brothers who are really tactical sailors at mark roundings.